18.1 Studying Viruses and Prokaryotes

How are bacteriahelpful to humans?colored SEM; magnification unknownThese bacteria live in the lining of anesophagus, the tube that leads fromyour mouth to your stomach. Only a fewyears ago, it was thought that nothing couldsurvive in an esophagus, but the entiredigestive tract is home for many types ofbacteria. Being a home to bacteria isn’t bad,though. Some doctors hypothesize thatthese bacteria may protect us from somethroat and stomach cancers.ConnectingCONCEPTSBiotechnology Hundreds ofbacteria can fit on the tip of apin, as shown at left. About thesame number of bacteria willfit on a silicon chip. Scientistshave made a device that cancollect signals from bacteriajoined to silicon chips. Thesebacteria have been speciallyaltered to glow when near pollutants.The chips then measurethe amount of light emitted—themore light, the morepollution present. (colored SEM;magnification 1100)Chapter 18: Viruses and Prokaryotes 543

18.1 Studying Virusesand ProkaryotesKEY CONCEPT Infections can be caused in several ways.MAIN IDEA• Viruses, bacteria, viroids, and prions can allcause infection.VOCABULARYvirus, p. 544pathogen, p. 544viroid, p. 544prion, p. 545Reviewprokaryote, archaeaConnect Bacteria are everywhere, including in and on your own body—such asthe bacteria that live in our digestive tracts. The relationship between you and themicroorganisms in your body is usually mutually beneficial. Under certainconditions, however, normally harmless microorganisms can cause disease, andsome types of microorganisms are particularly nasty—they always make you sick.FIGURE 18.1 Prokaryotes, such asthis Escherichia coli bacterium,are single cells that have all of thecharacteristics of living things.(colored TEM; magnification 6000)MAIN IDEAViruses, bacteria, viroids, and prions can all causeinfection.You are probably familiar with the terms virus and bacteria, but you may notknow exactly what they are. A virus is an infectious particle made only of astrand of DNA or RNA surrounded by a protein coat. Bacteria, on the otherhand, are one-celled microorganisms that can also cause infection. Any livingorganism or particle that can cause an infectious disease is called an infectiousagent, or pathogen.In Chapter 1, you learned that all living things share certain key characteristics:the abilities to reproduce, to use nutrients and energy, to grow anddevelop, and to respond to their environments. They also contain geneticmaterial that carries the code of life. Prokaryotes—such as the bacteriumshown in FIGURE 18.1—are clearly living things, since they have each of the traitsof life. But are viruses living things? Like living cells, viruses respond to theirenvironment. Viruses have genes and can reproduce. Unlike cells, however,viruses cannot reproduce on their own. Instead, they need living cells to helpthem reproduce and make proteins. Viruses are also much smaller than mostcells, as you can see in FIGURE 18.2. While viruses have key traits similar to livingcells, they also have many differences. In fact, viruses are not even given a placein the Linnaean system of biological classification.A viroid has even less in common with living things than do viruses.Viroids are infectious particles that cause disease in plants. Viroids are made ofsingle-stranded RNA without a protein coat. They are passed through seeds orpollen. Viroids have had a major economic impact on agriculture because theycan stunt the growth of plants.544 Unit 6: Classification and Diversity

FIGURE 18.2 Relative Sizes of Cells and Infectious ParticlesAlthough eukaryotic and prokaryotic cells can be microscopic,they are large in comparison to viruses, viroids, and prions.1 nanometer (nm) = one billionth of a meter100 nmeukaryotic cells10,000–100,000 nmprokaryotic cells200–10,000 nmviruses50–200 nmviroids5–150 nmprion2–10 nmInfer Why are viroids and prions sometimes called subviral particles?At the boundary between living and nonliving, perhaps the strangest entityof all is the prion. A prion (PREE-ahn) is an infectious particle made only ofproteins that can cause other proteins to fold incorrectly. When proteinsmisfold, the protein will not work properly. Prions are unusual in that they areinfectious yet have no genetic material. They play a part in certain diseases ofthe brain such as mad cow disease, known to scientists as bovine spongiformencephalopathy, or BSE. Humans may become infected with BSE when theyeat meat from animals that are infected. Food safety laws in the United States,however, try to reduce the risk of infection. Creutzfeld-Jakob (KROYTS-fehltYAH-kawp) disease (CJD), another brain disease that affects humans, is alsoassociated with prions. Prion diseases can incubate for a long time with noeffect on their host. However, once symptoms appear, they worsen quickly andare always fatal, because the body has no immune response against a protein.Synthesize Why are viruses, viroids, and prions not included in the Linnaean systemof biological classification?TAKING NOTESUse a two-column chart totake notes on viruses,viroids, and prions.Main IdeaVirusViroidPrionDetail18.1 ASSESSMENTONLINE QUIZClassZone.comREVIEWINGMAIN IDEAS1. What are the main differencesbetween living cells andviruses?2. Viruses, viroids, prions, and somebacteria can all be consideredpathogens. What do all pathogenshave in common?CRITICAL THINKING3. Infer Prions were not widelyknown to be infectious agents untilthe 1980s. Give two reasons whythis might be so.4. Apply An RNA-based diseasespreads through pollen. Is it likelydue to a virus, viroid, or prion?Explain.Connecting CONCEPTS5. Medicine To multiply, virusesmust take over the functionsof the cells they infect. Whydoes this make it difficult tomake effective antiviral drugs?Chapter 18: Viruses and Prokaryotes 545

DATA ANALYSISTrends in Infectious DiseaseCollecting data on the spread of infectious disease in a populationis an important part of monitoring trends and determining a courseof treatment. Different methods of displaying data, such as bargraphs or line graphs, often convey different information. Recallfrom page 497 in Chapter 16 the difference between discrete andcontinuous data.CHOOSING DATA REPRESENTATIONDATA ANALYSISClassZone.comTABLE 1. STATES WITH MOSTTB CASES IN 2005TABLE 2. RATE OF TB FORU.S. RESIDENTSLungs infected by tuberculosisState Number of cases Year TB Cases per 100,000 PersonsCalifornia 2900 1955 46.6Texas 1535 1960 30.7New York 1294 1965 25.2Florida 1094 1970 18.1Illinois 596 1975 15.7Georgia 510 1980 12.2New Jersey 485 1985 9.3Virginia 355 1990 10.3North Carolina 329 1995 8.7Pennsylvania 325 2000 5.82005 4.8Source: Centers for Disease Control and PreventionCHOOSE DATA REPRESENTATIONThe tables above show two sets of data describing tuberculosis (TB) infectionin the United States.1. Connect For each of the tables above, identify whether the data arecontinuous or discrete.2. Graph Data Determine which type of graph would best represent each setof data and construct the graph for each set.3. Analyze What trend did your graph show in rates of tuberculosis cases inthe United States between the years 1955 and 2005?4. Analyze Which table gives a more complete picture of TB infection in theUnited States? Explain.5. Predict What trend do you expect the rate of TB cases to show in 2010?546 Unit 6: Classification and Diversity

18.2Viral Structure andReproductionKEY CONCEPT Viruses exist in a variety of shapes and sizes.MAIN IDEAS• Viruses differ in shape and in ways ofentering host cells.• Viruses cause two types of infections.VOCABULARYcapsid, p. 547bacteriophage, p. 549lytic infection, p. 551lysogenic infection, p. 551prophage, p. 551Reviewendocytosis, lipidConnect Just like the computer viruses that you hear about in the news, virusesthat affect living things pass from one host to the next. While computer virusespass through networks from one computer to another, human viruses pass fromperson to person. Also like computer viruses, viruses of living things can be simpleor complex in structure, and have several different ways to get into their hosts.MAIN IDEAViruses differ in shape and in ways of enteringhost cells.Healthy leafInfected leafFIGURE 18.3 These picturescompare a healthy leaf and aleaf infected by tobacco mosaicvirus (TMV). TMV was the firstvirus identified by scientists.The idea that infectious agents cause certain diseases was a fairly new conceptin 1892 when Russian scientist Dmitri Ivanovsky made a surprising observation.He was studying tobacco mosaic disease, named for the scar pattern lefton affected leaves of tobacco or tomato plants. Mosaic disease, shown inFIGURE 18.3, was thought to be caused by a bacterium. But so far no one hadbeen able to prove it. Ivanovsky passed extracts of diseased tobacco leavesthrough filter pores small enough to strain out bacteria and found that theextracts could still pass on the disease. Was this a new bacterium? Or was itsome unknown type of organism?In 1898, Dutch microbiologist Martinus Beijerinck built upon Ivanovsky’swork. He showed that the disease agent passed through agar gel. He proposedthat tiny particles within the extracts caused infection, and he called theparticles viruses, from the Latin for “poison.” The observations of Ivanovskyand Beijerinck laid the groundwork for more discoveries. Scientists beganfinding that many diseases of unknown causes could be explained by viruses.The Structure of VirusesViruses have an amazingly simple basic structure. A single viral particle, calleda virion, is made up of genetic material surrounded by a protein shell called acapsid. Capsids can have different shapes. In some viruses, the capsid itself issurrounded by a lipid envelope. A lipid envelope is the protective outer coatof a virus, from which spiky structures of proteins and sugars may stick out.Chapter 18: Viruses and Prokaryotes 547

FIGURE 18.4 Viral ShapesSome viruses attach to host cells by these spikes. The spikes are such anobvious trait of some viruses that they can be used for identification.Viruses can only reproduce after they have infected host cells. Viruses aresimply packaged sets of genes that move from one host cell to another. Unlikebacteria and other living parasites, a virus has no structures to maintain—nomembranes or organelles needing ATP, oxygen, or glucose. All it carries intothe cell is what it needs to reproduce—its genes.The structure and shape of viruses play an important role in how theywork. Each type of virus can infect only certain hosts. A virus identifies itshost by fitting its surface proteins to receptor molecules on the surface of thehost cell, like a key fitting a lock. Some viruses are able to infect several species,while other viruses can infect only a single species. Common viral shapes areshown in FIGURE 18.4.The different proteins that make up a viral capsid give viruses a variety of shapes.ENVELOPEDHELICALPOLYHEDRALEnveloped viruses, such as thisinfluenza virus, often have spikes.The envelope is shown in orange.(colored TEM; magnification 255,000)capsidnucleic acidlipidenvelopesurfaceproteinsSome viruses have a long, narrow,coiled shape called a helix. Therabies virus is an example of ahelical virus that also has an envelope.(colored TEM; magnification65,000)surface proteinscapsidnucleic acidlipidenvelopePolyhedral viruses are many-sided,like the one shown here thatcauses foot-and-mouth disease inanimals. (computer illustration)surfaceproteinscapsidnucleic acidCompare and Contrast What are the similarities and differences between the threetypes of viruses shown above?548 Unit 6: Classification and Diversity

In some viruses, capsids form a 20-sided polyhedral. Rodshapedand strandlike viruses often have capsids shaped in coils,like a spring or helix.In contrast to prokaryotes and eukaryotes, in which DNA isalways the main genetic material, a virus can have either DNA orRNA but never both. The genetic material of viruses can be singlestrandedor double-stranded, and linear, circular, or segmented.Viruses that Infect BacteriaOne group of viruses is the bacteriophages, often called simply“phages.” Bacteriophages (bak-TEER-ee-uh-fayj-ihz) are virusesthat prey on bacteria. One example is the T-bacteriophage thatinfects Escherichia coli, the bacteria commonly found in the intestinesof mammals. The T-bacteriophage shown in FIGURE 18.5 has a 20-sidedcapsid connected to a long protein tail with spiky footlike fibers. The capsidcontains the genetic material. The tail and its spikes help attach the virus tothe host cell. After attachment, the bacteriophage’s tail releases an enzyme thatbreaks down part of the bacterial cell wall. The tail sheath contracts, and thetail core punches through the cell wall, injecting the phage’s DNA. The phageworks like a syringe, injecting its genes into the host cell’s cytoplasm, where itsDNA is found.colored SEM; magnifications:large photo 25,000; inset 38,000FIGURE 18.5 The SEM aboveshows bacteriophages attackingan E. coli bacterium. While injectingtheir genetic material into thebacterium, the protein coatsremain outside the cell (inset). Theunique structure of a bacteriophageis shown below.capsidViruses that Infect EukaryotesViruses that prey on eukaryotes differ from bacteriophages in their methodsof entering the host cell. For example, these viruses may enter the cells byendocytosis. Recall from Chapter 3 that endocytosis is an active method ofbringing molecules into a cell by forming vesicles, or membrane-bound sacs,around the molecules. If the viruses are enveloped, they can also enter a hostcell by fusing with the plasma membrane of the host cell and releasing thecapsid into the cell’s cytoplasm. HIV is a virus that enters cells in this way.Once inside the cell, eukaryotic viruses target the nucleus of the cell.Summarize Describe how the structures of a bacteriophage are well-suitedfor their functions.DNAtail fibertail sheathMAIN IDEAViruses cause two types of infections.The ways in which viruses enter and leave a cell may vary, but two basicpathways of infection are similar for all viruses. These pathways are shown forthe most studied viruses, the bacteriophages, in FIGURE 18.6.Once inside the host cell, phages follow one of two general paths in causingdisease. In one path, the phage behaves like a bad houseguest. It takes over thehousehold, eats all of the food in the refrigerator, and then blows up the housewhen it leaves. The other path of infection is somewhat more subtle. Insteadof destroying the house, the phage becomes a permanent houseguest. Neitherpath is good for the host.Chapter 18: Viruses and Prokaryotes 549

FIGURE 18.6 General Pathways of Viral InfectionA lytic infection results in the lysis, or breaking apart, of the host cell andrelease of new viral particles. A lysogenic infection does not destroy thehost cell.BIOLOGYExamine the twoways in which virusesinfect host cells atClassZone.com.host bacteriumLYTIC INFECTIONThe bacteriophage attachesand injects its DNA into ahost bacterium.The host bacterium breaks apart,or lyses. Bacteriophages are ableto infect new host cells.The viral DNAforms a circle.The viral DNA directs the hostcell to produce new viral parts.The parts assemble into newbacteriophages.The virus may enter thelysogenic cycle, in which thehost cell is not destroyed.LYSOGENIC INFECTIONThe prophage may leave thehost’s DNA and enter thelytic cycle.The viral DNA is called a prophagewhen it combines withthe host cell’s DNA.Many cell divisions produce acolony of bacteria infectedwith prophage.Although the prophage is notactive, it replicates along withthe host cell’s DNA.CRITICALVIEWINGWhy are no capsids or tail sheaths made duringa lysogenic infection?550 Unit 6: Classification and Diversity

Lytic InfectionA lytic infection (LIHT-ihk) is an infection pathway in which the host cellbursts, releasing the new viral offspring into the host’s system, where each theninfects another cell.• When the viral DNA enters the host cell, it takes over control of the host’sown DNA, turning on the genes necessary to copy the viral genes.• Under direction of the viral genes, the host’s DNA undergoes transcriptionand translation, and produces capsids and enzymes. The enzymes thenhelp in the copying of the virus’s DNA.• Using energy from the host cell, the capsids and viral DNA assemble intonew virions. Viral enzymes dissolve the host cell membrane, releasing thenew virus particles into the host’s bloodstream or tissues—and destroyingthe host cell in the process.Lysogenic InfectionIn a lysogenic infection (ly-suh-JEHN-ihk), a phage combines its DNA intothe host cell’s DNA.• After entering the host cell, the viral DNA combines with the host’s DNA,forming a new set of genes called a prophage. A prophage is the phageDNA inserted into the host cell’s DNA. In organisms other than bacteria,this stage is called a provirus.• The prophage is copied and passed to daughter cells, with the host’s ownDNA, when the host cell undergoes mitosis. Although this process doesn’tdestroy the cell, it can change some of the cell’s traits.• After the cell has been copied, the prophage faces two possible paths. Atrigger, such as stress, can activate the prophage, which then uses the cell toproduce new viruses. Or the prophage can remain as a permanent gene.Connect Using the analogy of viral infections resembling houseguests, explainwhich describes a lytic and which describes a lysogenic infection.VOCABULARYThe term lytic comes from theGreek word lutikos, meaning“able to loosen.” The word lysisis often used in biology todescribe a cell breaking apart.For more information on viruses,visit scilinks.org.Keycode: MLB01818.2 ASSESSMENTONLINE QUIZClassZone.comREVIEWINGMAIN IDEAS1. Name and describe the main partsof a typical virus.2. What are the differences between alytic infection and a lysogenicinfection? Include the effects ofeach type of infection on thecells of the host organism inyour answer.CRITICAL THINKING3. Apply Researchers studyinginfection can often grow bacteriamore easily than they can growviruses. What conditions mustscientists provide for virusesto multiply?4. Classify A wart is caused by a virusthat may lie dormant for yearsbefore any symptoms appear. Doesthis resemble a lytic or lysogenicinfection? Explain.Connecting CONCEPTS5. Evolution If the virus is aforeign invader, how is itpossible for the proteins of itscapsid to match the receptorson the host cell’s surface?Consider natural selection inyour answer.Chapter 18: Viruses and Prokaryotes 551

18.3Viral DiseasesKEY CONCEPT Some viral diseases can be prevented with vaccines.MAIN IDEAS• Viruses cause many infectious diseases.• Vaccines are made from weakened pathogens.VOCABULARYepidemic, p. 553vaccine, p. 553retrovirus, p. 553Connect Why do we worry about catching a cold or the flu every winter? Coldweather itself does not cause us to get sick, but spending time close to otherpeople can. For most people, winter means spending more time indoors. Coldand flu viruses then easily transfer to hands from doorknobs and other objects.That’s why frequently washing your hands can help keep you healthy.MAIN IDEAViruses cause many infectious diseases.As you have read, viruses follow two pathways of infection once they encountertheir target cells. But to enter the host’s body in the first place, the virusmust first pass a major obstacle.Connecting CONCEPTSCells Recall from Chapter 3that receptors are proteins thatdetect chemical signals andperform an action in response.In the case of a host-specificinfection, these normally helpfulreceptors provide littleprotection to the cell.FIGURE 18.7 Cold virus particles(yellow) on the surface of a cellculture (blue). (colored SEM; magnification10,000)First DefensesIn vertebrates, the first obstacle a virus must pass is the skin, but in otherorganisms it might be an outer skeleton or a tough cell wall. Viruses canpenetrate the skin only through an opening such as a cut or scrape. Or theycan take another route —the mucous membranes and body openings. It’s noaccident that some of the most common points of entry for infection are themouth, nose, genital area, eyes, and ears.Once inside the body, the virus finds its way to its target organ or tissue.However, the targeted cells don’t just open the door to this unwanted guest.Body cells have receptors that guard against foreign intruders. These receptorsact almost like locks. When the virus arrives at the host cell, it uses its ownsurface proteins as keys to trick the cell into allowing it to enter.Examples of Viral InfectionsViruses can cause symptoms that range from merely bothersome to lifethreatening.Below are a few of the many human illnesses caused by viruses.The common cold The most familiar viral disease is the common cold. Morethan 200 viruses are known to cause this seasonal nuisance. One such coldvirus is shown in FIGURE 18.7. With so many viruses, it’s not easy to find a cure.In fact, cold viruses can mutate as they move from one person to another.Although they’re unpleasant to have, colds usually last only about one week.552 Unit 6: Classification and Diversity

Influenza Winter usually causes concern about the influenza, or “flu” virus—and with good reason. The flu spreads quickly and can result in frequent localepidemics. An epidemic is a rapid outbreak of an infection that affects manypeople. In the United States, up to 20 percent of the population is infectedwith the flu each year.At this time, only three influenza subtypes infect humans; other subtypesmay infect horses, pigs, whales, and seals. More than fifteen subtypes infectbirds, and are all referred to as avian influenza, or bird flu. Sometimes amutation enables a virus to jump from one species to another, making thespread of infection difficult to control. The high mutation rate of surfaceproteins on viral capsids makes it necessary for a new influenza vaccine to bemade every year. A vaccine (vak-SEEN) is a substance that stimulates thebody’s own immune response against invasion by microbes.SARS Severe acute respiratory syndrome (SARS) is another viral respiratorydisease. It has symptoms similar to influenza, such as fever and coughing ordifficulty in breathing. SARS is a relatively recent concern. It first appeared inAsia in late 2002. By the following summer, it had spread to other countries.SARS continues to be monitored globally by the World Health Organization.HIV Human immunodeficiency virus, or HIV, is a retrovirus. Retro- means“backward,” which describes how retroviruses work. Usually, DNA is used tomake an RNA copy in a cell, but a retrovirus is a virus that contains RNA anduses an enzyme called reverse transcriptase to make a DNA copy. DoublestrandedDNA then enters the nucleus and combines with the host’s genes as alysogenic infection. The viral DNA can remain dormant for years as a provirus,causing no symptoms to its human host.When the virus becomes active, it directs the formation of new viral parts.The new viruses leave, either by budding or bursting through cell membranes,and infect new cells. This stage of the disease is a lytic infection that destroyswhite blood cells of the host’s immune system, as shown in FIGURE 18.9. Theloss of white blood cells ultimately causes AIDS, acquired immune deficiencysyndrome. Once a person’s immune system is affected, he or she may beunable to fight off even the commonmicroorganisms that humansencounter every day. HIV’s unusuallyhigh mutation rate has made it achallenge to treat. The combined useof several antiviral drugs—medicationsthat treat viral infection—hasproved somewhat effective in slowingthe spread of the virus once aperson is infected.Analyze How do retroviruses workdifferently from other viruses?HIV-infected whiteblood cellFIGURE 18.8 Nurses in Canadawalk outside an emergency SARSclinic, which was opened to dealwith an outbreak.ConnectingCONCEPTSHIV Certain types of whiteblood cells of the humanimmune system are targeted byHIV to cause AIDS. You will learnmore about HIV transmission andhow this virus targets theimmune system in Chapter 31.FIGURE 18.9 This scanning electronmicrograph (SEM) shows theHIV virus as purple dots on aninfected white blood cell.Destruction of white blood cellsweakens the immune system andcauses AIDS. (colored SEM; magnification:3500)Chapter 18: Viruses and Prokaryotes 553

FIGURE 18.10 Viral DiseasesVIRAL INFECTION SYMPTOMS OF DISEASE TRANSMISSION OF DISEASE U.S. VACCINE RECOMMENDATIONChickenpoxrash, itchy skin, fever,fatiguecontact with rash, droplet inhalationfor children between 12 and 18 monthsHepatitis Ayellow skin, fatigue, abdominalpaincontact with contaminated fecesfor people traveling to infected locationsand protection during outbreaksMumpspainful swelling in salivaryglands, feverdroplet inhalationfor children between 12 and 15 monthsand again at 4 to 6 yearsRabiesanxiety, paralysis, fearof waterbite from infected animalfor veterinarians and biologists in contactwith wildlifeWest Nile fever, headache, body ache bite from infected mosquito no available vaccineMAIN IDEAVaccines are made from weakened pathogens.Connecting CONCEPTSHuman Biology Vaccines helpbuild up the immune system toprepare for exposure to a pathogenby recognizing its surfaceproteins. You will learn moreabout the immune system inChapter 31.Chances are good that you have had vaccinations. In the United States, childrenare vaccinated at an early age against diseases such as measles, mumps,rubella (MMR), and chickenpox. Every year, millions of people are vaccinatedagainst influenza. How does a simple shot provide protection against disease?A vaccine is made from the same pathogen—disease-causing agent—that itis supposed to protect against. Vaccines consist of weakened versions of thevirus, or parts of the virus, that will cause the body to produce a response. Theimmune system is triggered by the surface proteins of a pathogen. In the host’sbody, the vaccine works by preparing the host’s immune system for a futureattack. Vaccines can prevent some bacterial and some viral infections, includingthe viral diseases shown in FIGURE 18.10. Whereas bacterial diseases can alsobe treated with medicine once they occur, viral diseases are not easily treated.Vaccination is often the only way of controlling the spread of viral disease.Vaccines cause a mild immune response. If the body is invaded again, itwill be able to start an immune defense before the virus can cause damage.Apply Before the chickenpox vaccination was available, children were oftenpurposely exposed to the virus at a young age. What was the reason for doing this?18.3 ASSESSMENTONLINE QUIZClassZone.comREVIEWINGMAIN IDEASCRITICAL THINKINGConnecting CONCEPTS1. Name and describe two infectiousviruses and a body’s first defenseagainst infection.2. Briefly describe how a vaccine canprevent some viral infections.3. Infer If a vaccine is in short supply,why is it often recommended thatolder adults and children getvaccinated first?4. Apply Why might getting a fluvaccination sometimes cause youto get a mild case of the flu?5. Human Biology Peopleinfected with HIV, the virusthat causes the disease AIDS,can become unable to fight offinfections by organisms thatnormally do not harm people.Why is this so?554 Unit 6: Classification and Diversity

18.4Bacteria and ArchaeaKEY CONCEPT Bacteria and archaea are both single-celled prokaryotes.MAIN IDEAS• Prokaryotes are widespread on Earth.• Bacteria and archaea are structurallysimilar but have different molecularcharacteristics.• Bacteria have various strategies forsurvival.VOCABULARYobligate anaerobe, p. 555obligate aerobe, p. 555facultative aerobe, p. 555plasmid, p. 556flagellum, p. 556conjugation, p. 558endospore, p. 558Connect Humans not only share the environment with prokaryotes—for manyspecies, we are the environment. Up to 500 types of prokaryotes can live in thehuman mouth. In fact, you may have as many as 25 different types in your mouthright now. One milliliter of saliva can contain up to 40 million bacterial cells.FIGURE 18.11 A “window” madeinto a cow’s rumen, the first of itsfour stomachs, allows scientists tostudy digestion. Anaerobic bacterialive mutualistically within acow’s stomach. The bacteria haveshelter and nutrients, and breakdown plant material for the cowto digest.MAIN IDEAProkaryotes are widespread on Earth.Prokaryotes, which include bacteria and archaea, are the most widespreadand abundant organisms on Earth. Consider that humans are one species withabout 6 billion individuals. In contrast, scientists estimate there are more than1 billion (10 9 ) types of bacteria and more than 10 30 individual prokaryotic cellson, above, and under Earth’s surface. Bacteria and archaea are an importantpart of every community they inhabit. These tiny organisms live in just aboutevery habitat on Earth, including the air we breathe. Prokaryotes have beenfound living inside rocks, in deserts, and in polar ice caps. One gram of soil maycontain as many as 5 billion bacterial cells from up to 10,000 types of bacteria.Prokaryotes can be grouped based on their need for oxygen. Prokaryotesthat cannot live in the presence of oxygen are called obligate anaerobes. Anobligate anaerobe (AHB-lih-giht AN-uh-ROHB) is actually poisoned byoxygen. As you have learned, archaea are prokaryotes that can live in extremeenvironments. The archaea that produce methane gas are obligate anaerobes.They live in marshes, at the bottom of lakes, and in the digestive tracts ofherbivores such as deer, sheep, and cows, as shown in FIGURE 18.11. Thesemicroorganisms release nutrients from plants that animals are unable to digeston their own.In contrast, some prokaryotes need the presence of oxygen in theirenvironment. Organisms that need oxygen in their environment are calledobligate aerobes (AHB-lih-giht AIR-OHBZ). This group includes severalfamiliar pathogens, such as those that cause the diseases tuberculosis andleprosy. There are also prokaryotes that can survive whether oxygen ispresent in the environment or not. This type of prokaryote is called afacultative aerobe (FAK-uhl-TAY-tihv AIR-OHB).Evaluate Bacteria are often associated with illness. Why is this a misconception?Chapter 18: Viruses and Prokaryotes 555

Connecting CONCEPTSClassification Recall fromChapter 17 that archaea andbacteria are in separate kingdomsand in separate domains as well.Both their kingdoms and theirdomains have the same names,Archaea and Bacteria.TAKING NOTESCreate a Venn diagram to comparebacteria and archaea usinginformation from this section.bacteriabotharchaeaFIGURE 18.12 The most commonshapes of bacteria are rods, spirals,and spheres. Many bacteria arenamed after these shapes. Someexamples are shown at right.(colored SEMs; magnifications: lactobacillimagnification unknown; spirochaeta5000; enterococci 7000)MAIN IDEABacteria and archaea are structurally similar buthave different molecular characteristics.Members of domain Bacteria and domain Archaea comprise all of Earth’sprokaryotes. Domain Bacteria is the more diverse and widespread of the twodomains, while many archaea are found in Earth’s extreme environments.Some archaea are even able to grow at temperatures greater than 100°C(212°F). Bacteria and archaea have many structural similarities but importantgenetic and biochemical differences.Structural ComparisonsEven under the microscope, archaea look very similar to bacteria. For example,both archaea and bacteria are small, single-celled organisms that have cellwalls and plasma membranes. Archaea come in many shapes, while the threemost common forms of bacteria are shown in FIGURE 18.12. Bacteria are oftennamed based upon their shapes. Rod-shaped bacteria are called bacilli. Spiralshapedbacteria are called spirilla or spirochetes, and spherical bacteria arecalled cocci.Prokaryotes do not have any membrane-bound organelles, such as anucleus containing double-stranded DNA. Instead, their DNA is in the formof a circle and is surrounded by cytoplasm. Prokaryotes may also have plasmids.A plasmid is a small piece of genetic material that can replicate separatelyfrom the prokaryote’s main chromosome.Most prokaryotes can move on their own. Many bacteria and archaea moveby gliding or using flagella. A flagellum (fluh-JEHL-uhm) is a long, whiplikestructure outside of a cell that is used for movement. The flagella of prokaryotesare attached to the plasma membrane and cell wall. They may be at oneend of an organism, or they may have different arrangements over the entirecell. Although similar in appearance, the flagella of bacteria and archaea arestructurally different from each other. In addition, their flagella are bothstructurally different from the flagella of eukaryotes. You will learn moreabout the flagella of eukaryotes in Chapter 19.Many prokaryotes also contain structures called pili that are thinner,shorter, and often more numerous than flagella. Pili help prokaryotes stick tosurfaces and to other prokaryotes. A typical prokaryote is shown in FIGURE 18.13.Lactobacilli: rod-shaped Spirochaeta: spiral Enterococci: spherical556 Unit 6: Classification and Diversity

FIGURE 18.13 Prokaryote StructureThis diagram shows the typical structure of a prokaryote.Archaea and bacteria look very similar, although they haveimportant molecular differences.plasma pilimembranechromosomecell wallflagellumplasmidMolecular ComparisonsIt was not until molecular analysis techniques were available that the manydifferences between bacteria and archaea became clear. Despite their similaritiesin function and appearance, bacteria and archaea are not closely related.Molecular evidence suggests that archaea have at least as much in commonwith eukaryotes as they do with bacteria. Forexample, archaea cell walls and membranes arechemically different from those of bacteria. Themembranes of archaea contain lipids that are notfound in any other type of organism on Earth, andbacteria have a polymer called peptidoglycan(PEHP-tih-doh-GLY-cuhn) in their cell walls,which archaea do not.The amount of peptidoglycan in their cell wallsis an important characteristic of bacteria. Bacteriaare often classified into one of two groups based onthis difference, as shown in FIGURE 18.14. A stainingmethod called a Gram stain is used to tell the twogroups apart. The Gram stain is important fordiagnosing infectious bacterial diseases, and itsometimes helps determine the type of medicine adoctor chooses to fight infection. Because of theircell wall differences, archaea are often not affectedby medicine used to treat bacterial infection.Contrast Archaea were first named archaeabacteria, aterm that you may still find in some books and articles.What are two differences between archaea and bacteria?FIGURE 18.14 GRAM STAININGA staining technique called a Gram stain is used to identifytypes of bacteria. This stain identifies the amount of a polymer,called peptidoglycan, that is present. The result is either grampositive or gram negative. (LMs; magnifications 500)Gram-negative bacteria havea thin layer of peptidoglycanand stain red.GRAM NEGATIVEoutermembranecellwallGram-positive bacteria havea thicker peptidoglycan layerand stain purple.peptidoglycanplasmamembraneGRAM POSITIVEcellwallChapter 18: Viruses and Prokaryotes 557

MAIN IDEABacteria have various strategies for survival.The strategies that prokaryotes use to survive and to transfer genes have madethem very successful at adapting quickly to almost any habitat.Gene Exchange in ProkaryotesProkaryotes reproduce by a type of cell division called binary fission, whichmeans “division in half.” While the chromosome copies itself, the cell grows.When the cell has about doubled in size, its plasma membrane grows inwardand divides the cell into two equal-sized daughter cells. Each daughter cell willcontain a complete copy of the parent cell’s genes.Although prokaryotes reproduceasexually, they are still able to exchangegenetic material in several ways. In oneprocess, called conjugation (kahn-juh-GAY-shuhn), prokaryotes can exchangeparts of their chromosomes through ahollow bridge of pili formed to connecttwo or more cells.Surviving Harsh ConditionsDuring conditions unfavorable forVISUAL VOCABIn conjugation, genetic materialtransfers between prokaryotes, producinggenetic variation. A conjugationbridge forms from the donor cell to arecipient cell.conjugation bridgesurvival, some bacteria can produce anendospore, a specialized cell with a thick,TEM; magnification 6000protective wall. To form an endospore,the bacterium copies its chromosome and produces a wall around the copy.This thick wall around the bacterial DNA helps it survive harsh conditionssuch as drying out, temperature change, and disinfectants. Endospores can lastfor centuries. Some have even been found in Egyptian mummies!Connect Why are disinfectants alone not enough to kill all types of bacteria?18.4 ASSESSMENTONLINE QUIZClassZone.comREVIEWINGMAIN IDEAS1. What are the three most commonshapes of bacteria?2. Why are bacteria and archaea classifiedinto different domains?3. Prokaryotes will take up foreignDNA. How is this characteristic usedin genetic engineering?CRITICAL THINKING4. Infer Scientists estimate that only1 percent of prokaryotes can begrown in the lab. What does thissuggest about our knowledge ofbacteria and archaea?5. Synthesize Prokaryotes multiply bybinary fission, which simply divides acell in two. Why are mutations andconjugation important for naturalselection in prokaryotes?Connecting CONCEPTS6. Health Bacteria in your mouthconvert foods containing sugarand starch into acids that canthen cause cavities in yourteeth. These bacteria will bepresent even if you brush yourteeth, floss, or use mouthwash.So why are these hygienehabits so important?558 Unit 6: Classification and Diversity

18.5Beneficial Rolesof ProkaryotesKEY CONCEPT Prokaryotes perform important functions for organisms and ecosystems.MAIN IDEAS• Prokaryotes provide nutrients to humansand other animals.• Prokaryotes play important roles inecosystems.VOCABULARYbioremediation, p. 561Reviewnitrogen fixationConnect People usually think bacteria in or on food are harmful, and it is truethat food poisoning caused by bacteria can be a serious problem. However, somebacteria are safe in food, and actually provide a taste or texture that many peopleenjoy. Swiss cheese, sour cream, and butter are just a few products that are madewith the help of bacteria. Eating food produced by bacteria is not dangerous, aslong as they are the right kind of bacteria!MAIN IDEAProkaryotes provide nutrients to humans andother animals.ConnectingCONCEPTSEcology Recall from Chapter 14that symbiosis is the close associationof two or more species. Aniche is a specific role an organismplays in its environment.Prokaryotes, such as the bacteria shown inFIGURE 18.15, are a key part of animal digestivesystems. A balanced community of prokaryotesin our bodies is important for ourhealth. Prokaryotes have a beneficial relationship,or mutualistic symbiosis, with thehost animal and break down food whilegetting a place to live. They also makevitamins and other compounds, and keepaway harmful microbes by filling niches thatmight otherwise be filled by disease-causingbacteria. In turn, the host animal providesthe bacteria with food and a home with astable pH and temperature.FIGURE 18.15 These bacteria, foundin human intestines, are beneficial toour health. They produce B vitaminsand keep out harmful microbes.(colored SEM; magnification: 6300)Humans can get nutrients from prokaryotesin other ways as well. Many foods thathumans enjoy are fermented by bacteria. Bacteria help ferment, or chemicallybreak down, many dairy products people eat every day, such as yogurt andcheeses. Pickles, soy sauce, sauerkraut, and vinegar also depend on fermentationby prokaryotes to produce their flavors.Summarize What are two ways in which prokaryotes that live within our bodiesare helpful to us?Chapter 18: Viruses and Prokaryotes 559

QUICK LABOBSERVINGExamining Bacteria in YogurtSome types of bacteria can ferment milk, producing lactic acid in the process.Yogurt is a product of fermentation. It is acidic and stays fresh longer thanmilk, and it is also digested more easily. In this exercise, you will prepare amicroscope slide of yogurt.PROBLEM What types of bacteria can you observe in yogurt?PROCEDURE1. Using a toothpick, place a dab of yogurt ona microscope slide. Caution: Do not eat inthe laboratory.2. Mix the yogurt in a drop of water and carefullyadd a coverslip.3. Examine the slide with a compound microscope.4. Record your observations by drawing a pictureof what you see through the microscope.ANALYZE AND CONCLUDE1. Identify Recall the terms bacillus, coccus, and spirillafrom the previous section. Which type or types ofbacteria did you observe in your slides?2. Analyze Many people do not produce lactase,which is an enzyme that breaks down the milksugar lactose. As a result, lactose-intolerant peoplehave trouble digesting dairy products. Why mightthey have fewer problems eating yogurt?MATERIALS• toothpick• dab of yogurt• microscope slide• drop of water• coverslip• microscopeMAIN IDEAProkaryotes play important roles in ecosystems.Even though you can’t easily see them, prokaryotes play important roles inevery ecosystem they occupy. Some, such as cyanobacteria, produce oxygenthrough photosynthesis. Others help recycle carbon, nitrogen, hydrogen, andsulfur through the ecosystem. The absence of prokaryotes in the environmentcan disrupt an ecosystem, since other organisms rely on them for survival.Photosynthesizing prokaryotes include purple and green photosyntheticbacteria and cyanobacteria. Whereas purple and green bacteria use light tomake carbohydrates, they do not produce oxygen. Cyanobacteria, however, aresimilar to plants in how they produce oxygen as a byproduct of photosynthesis.Cyanobacteria are named for their greenish blue (cyan) color. Recall fromChapter 12 that cyanobacteria played an important part on early Earth,supporting the life forms we are familiar with today. Fossil evidence suggeststhere was very little oxygen on Earth prior to the appearance of cyanobacteria.Some colonies of photosynthesizing cyanobacteria, as well as other bacteria,are also able to fix nitrogen. Although much of the atmosphere is made upof nitrogen gas (N 2 ), this is not in a form that plants or animals can use tomake amino acids or proteins.560 Unit 6: Classification and Diversity

Recall from Chapter 13 that nitrogen fixation is the processof converting atmospheric nitrogen into ammonia (NH 3 ) andother nitrogen compounds that plants can then use. Prokaryotessupply usable nitrogen to ecosystems ranging fromgrasslands and forests to the arctic tundra.Some types of nitrogen-fixing bacteria are free-living, whileothers live along with other organisms. Legumes, a group ofplants including peas, beans, alfalfa, and clover, have a mutualisticrelationship with nitrogen-fixing bacteria. These bacterialive in the plant’s nodules, small rounded lumps that form theroots, as shown in FIGURE 18.16. The bacteria provide usablenitrogen to the plant by capturing nitrogen gas from airtrapped in the soil. They combine the nitrogen with hydrogento produce ammonia. In return, the plant supplies food andshelter to the bacteria.Scientists have found many ways to use prokaryotes to benefit industry andthe environment. One important use of prokaryotes is in bioremediation(by-oh-rih-mee-dee-AY-shuhn), a process that uses microbes and other livingthings to break down pollutants. For example, some types of bacteria candigest oil, which is helpful for cleaning up oil spills and other industrialaccidents. Workers spray oil-polluted beaches with a fertilizer that helps thebacteria grow.Bacteria can digest almost any product that humans can make, includingpoisons. Therefore, they play an important role in recycling and composting.When you hear the term biodegradable, it often refers to the ability of bacteriato break down a material. Some of the only materials made by humans thatcannot be biodegraded are certain types of plastics.Apply When there is a toxic chemical spill, sometimes workers will spray bacteriaover the contaminated area. Why might they do this?FIGURE 18.16 Root nodules ofthis white clover contain nitrogenfixingbacteria. The symbiotic bacteriaconvert nitrogen from theatmosphere (N 2 ) into a formusable by the clover. In return, theplant produces carbohydratesthrough photosynthesis that thebacteria can consume. (inset coloredSEM; magnification 90)18.5 ASSESSMENTONLINE QUIZClassZone.comREVIEWINGMAIN IDEAS1. Describe two ways bacteria providenutrients to humans.2. What are two roles prokaryotes playin the cycling of elements in anecosytem?CRITICAL THINKING3. Connect Think of an examplein which the use of bioremedia-tion either has improved the environmentor has the potential todo so.4. Synthesize How do prokaryoteslend stability to an ecosystem?Connecting CONCEPTS5. Ecology Prokaryotes in cowintestines produce moremethane if the cow is fed adiet high in grains rather thangrass. Some scientists proposethat overfeeding grain to cowscontributes to global warming.How did these scientists arriveat this hypothesis, and howcould it be tested?Chapter 18: Viruses and Prokaryotes 561

CHAPTER 18MATERIALS• petri dish containing aselective medium• petri dish containing anonselective medium• permanent marker• 2 leaves from the sameplant• pencil with eraser• transparent tapePROCESS SKILLS• Observing• Evaluating OutcomesINVESTIGATIONLeaf Print BacteriaPPFM are pink-pigmented bacteria that grow on plant surfaces. They help plants bystimulating seed germination and plant growth. A unique feature of PPFM bacteriais that they use methanol as their sole source of carbon. Therefore, a simplenutrient mixture that contains only methanol as a carbon source will “select” forPPFM bacteria. This type of mixture is called a selective medium. A mixture thatcontains many nutrients is called a nonselective medium. In this lab, you will makeleaf prints on selective and nonselective media to study the growth of PPFM andother bacteria.PROBLEM How do populations of bacteria grown on various media differ?PROCEDURE1. Obtain two petri dishes, one containing a selective medium (methanol carbonsource only) and the other containing a nonselective, nutrient-rich medium.2. Label the petri dishes with your name, the date, and “selective” or “nonselective.”3. To make the first leaf print, lay one of the leaves on the surface of the selectivemedium. Use the eraser end of a pencil to gently press the leaf into the medium,as shown below. After making the impression, carefully lift the leaf away from themedium and discard. Close the petri dish.4. Repeat step 3 using your other leaf and the nonselectivemedium.5. Seal the petri dishes with tape. Store them upside down atroom temperature.Caution: Once the dishes are sealed, they should not beopened again. Follow your teacher’s directions for disposal ofthe petri dishes at the end of the lab.6. Write a prediction based on what you know about PPFMbacteria and the selective and nonselective media.7. Observe the dishes over a period of one or two weeks. Record your observationsdaily. These should be both qualitative (shape, color, and size of colonies) andquantitative (number of colonies). To count colonies, it may help to create a gridon the underside of each petri dish.8. At the end of the observation period, compare your results with those from therest of the class.ANALYZE AND CONCLUDE1. Analyze How many types of bacteria were present on the nonselective medium?the selective medium? What might account for differences between the two?2. Analyze Is PPFM bacteria growth the same on both types of media? Whichobservations support this conclusion?3. Analyze Do all bacteria grow at the same rate? Which observations supportthis conclusion?4. Infer Why might bacteria grow faster on the nonselective medium?EXTEND YOUR INVESTIGATIONDesign an experimental procedure to test the hypothesis that different types ofPPFM bacteria are found on different plants.562 Unit 6: Classification and Diversity

18.6Bacterial Diseasesand AntibioticsKEY CONCEPT Understanding bacteria is necessary to prevent and treat disease.MAIN IDEAS• Some bacteria cause disease.• Antibiotics are used to fight bacterial disease.• Bacteria can evolve resistance to antibiotics.VOCABULARYtoxin, p. 563antibiotic, p. 564ReviewhomeostasisConnect In the early 1900s, most deaths in the United States were caused byinfectious diseases, such as bacterial pneumonia and tuberculosis. Thanks to newmedicines, infectious diseases were among the least common causes of death bythe century’s end. In recent years, however, many diseases are making a comebackdue to a new problem—antibiotic resistance.FIGURE 18.17 Clostridiumbotulinum causes a seriousillness called botulism. Foodcontamination by this bacteriumoften comes from improper homecanning. (magnification unknown)MAIN IDEASome bacteria cause disease.Some bacteria cause disease in plants and animals by disrupting the hostorganism’s homeostasis, or the stability of its internal environment. Bacteriacan cause illness to a host in two basic ways: by invading tissues and attackingcells or by making poisons, or toxins, that can be carried by blood to sitesthroughout the body. A toxin is a poison released by an organism.The disease tuberculosis (TB) is an example of bacteria invading the host’stissues, and using the tissues for nutrients. Mycobacterium tuberculosis bacteriamultiply in the lungs, killing white blood cells that respond to the invasion.The host’s reaction to an invasion by bacteria may itself cause serious problems.In the case of TB, the host responds to the infection by releasing enzymesthat cause swelling. That swelling, in turn, damages the host’s lungs.TB is a good example of the changing ecological balance between host andpathogen in an infectious disease. A host is not usually aware of pathogensthat its immune system defeats. It is when the host’s immune system fails thatthe host becomes aware of the pathogen’s presence. Most healthy people candefeat a potential TB infection, especially if there are not many bacteria present.Bacteria, such as Staphylococcus aureus and Clostridium botulinum, shownin FIGURE 18.17, can also make their hosts sick through food poisoning.S. aureus, which normally lives in nasal passages, can be transferred to foodwhen food handlers don’t wash their hands after they blow their nose. Thistransfer can result in serious food poisoning, known as staph poisoning. Evenhigh temperatures cannot destroy a toxin produced by S. aureus. The mostcommon source of food poisoning by S. aureus, however, is from foods thatwere contaminated after they were cooked. If contaminated food is notrefrigerated, bacteria can multiply and produce a large amount of toxin.Chapter 18: Viruses and Prokaryotes 563

FIGURE 18.18 Common Bacterial InfectionsINFECTION BACTERIUM SYMPTOMS CAUSESAcne Propionibacterium chronic cysts, blackheads increased oil production in skinAnthrax Bacillus anthracis fever, trouble breathing inhaling endosporesLyme disease Borrelia burgdorferi rash, aching, fever, swelling of joints bite from infected tickTetanus Clostridium tetani severe muscle spasms, fever, lockjaw wound contaminated with soilTooth decay Streptococcus mutans tooth cavities large populations of bacteria in mouthFIGURE 18.19 “Flesh eating” groupA Streptococcus bacteriaare commonly found on skin.They are fairly harmless unlessthey come in contact with tissuesthey do not normally colonize,such as muscle or fat. This canoccur through open wounds.ConnectingCONCEPTSImmune System Although antibioticsdo not work on viruses,vaccines may work on bothviruses and bacteria. This isbecause vaccines trigger theimmune system. You will learnmore about how the immunesystem works in Chapter 31.Staph food poisoning can make you pretty sick, but botulism can kill you.C. botulinum produces a deadly toxin. Botulism is usually caused by the eatingof improperly canned foods that were contaminated with endospores beforebeing sealed. Bulging cans are a sign that C. botulinum may be present.Normally harmless bacteria can be destructive when introduced to a partof the host that is not adapted to them. Disease can result if these bacteria getinto tissues they do not usually colonize through a cut, scrape, or surgicalincision. You can see one result of typically harmless Streptococci, which wehave normally in our mouths and noses —and often on our skin—becomingpathogenic in FIGURE 18.19. These are also the bacteria that can cause what iscommonly known as strep throat.Apply Potato salad left out at a picnic is sometimes a source of food poisoning.Which bacterium mentioned above is the most likely culprit? Explain.MAIN IDEAAntibiotics are used to fight bacterial disease.If you’ve ever had a cold, your doctor may have told you that the only cure wasto let the cold “run its course.” If you had strep throat, however, the doctorwould prescribe a powerful antibiotic. Why do you get antibiotics for strepthroat but not for the common cold?Colds and strep throat are treated differently because they are caused bydifferent pathogens. Viruses cause colds, while the bacterium Streptococcuscauses strep throat. Many types of antibiotics—or chemicals that kill or slowthe growth of bacteria —work by stopping bacteria from making cell walls.Antibiotics are produced naturally by some species of bacteria and fungi.They can be used as medicine for humans and other animals without damagingtheir cells, since animal cells do not have cell walls. Because viruses also lack cellwalls, antibiotics do not work on viral infections.Antibiotics can be effective when used properly, but they should not be thefirst line of defense against bacterial infection; prevention should. Overuse ofantibiotics can completely wipe out the community of intestinal microbes,resulting in illness.Infer Why don’t antibiotics affect our bodies’ own cells?564 Unit 6: Classification and Diversity

MAIN IDEABacteria can evolve resistanceto antibiotics.Although antibiotics should certainly be used when needed,the inappropriate and incomplete use of antibiotics hasproduced a serious public health issue—multidrug-resistantbacteria. Resistance occurs as a result of natural selection, asindividuals who are more resistant are more likely to surviveand reproduce. This has led to the evolution of multidrugresistantstrains of “superbugs” that are almost impossible totreat. As you can see in FIGURE 18.20, bacteria can acquire genesfor resistance through plasmid exchange. This has happenedwith many bacteria with a wide range of commonly usedantibiotics. This problem has arisen due to various factors.Overuse The potential problem with antibiotics is that theymay create a selective pressure that favors the very bacteriathey are intended to destroy. Using antibiotics when bacteriaare not causing an illness may make some bacteria resistant.Underuse Failure to take the entire course of antibiotics prescribed for abacterial infection is one of the main factors leading to drug resistance. If yourdoctor prescribed a ten-day course of an antibiotic, you must finish the entireprescription. Otherwise, you may not have destroyed all of the bacteria—onlythe weakest ones.Misuse A large portion of the antibiotics distributed in the United States arefed to livestock. Antibiotics are often misused in agriculture to increase theanimals’ rate of growth. However, when antibiotics are added to the food ofhealthy animals, bacteria within the food—including pathogens—can becomeresistant to multiple antibiotics.Connect How can you use “superbugs” as an example of natural selection?FIGURE 18.20 ANTIBIOTIC RESISTANCEA bacterium carriesgenes for antibioticresistance on a plasmid.A copy of the plasmid is transferredthrough conjugation.Resistance is quicklyspread throughmany bacteria.18.6 ASSESSMENTONLINE QUIZClassZone.comREVIEWINGMAIN IDEASCRITICAL THINKINGConnecting CONCEPTS1. What are two ways in whichbacteria can cause disease?2. How can antibiotics stop bacterialinfections?3. What is antibiotic resistance, andhow does it occur?4. Apply Why are antibiotics noteffective against viruses?5. Synthesize Evolution is oftenthought of as taking thousands, oreven millions, of years to occur.What are two reasons that antibioticresistance has been able toevolve in bacteria so quickly?6. Ecology Pesticide resistanceoccurs in much the sameway as antibiotic resistance.How could we apply whatwe have learned about antibioticresistance to howpesticides are used in theenvironment?Chapter 18: Viruses and Prokaryotes 565

CHAPTER 18OPTIONS FOR INQUIRYUse these inquiry-based labs and online activities to deepen yourunderstanding of viruses and prokaryotes.DESIGN YOUR OWN INVESTIGATIONUsing Bacteria to Break Down OilSome types of bacteria have enzymes that can breakdown oil and are used by scientists to help clean up anoil spill. Some types of drain cleaners contain bacteriathat work in the same way to clear drain pipes. In thislab, you will test the effectiveness of different types ofenzymatic drain cleaners at breaking down cooking oil.SKILLS Designing Experiments, ConcludingPROBLEM Which enzymatic drain cleaner is mosteffective at breaking down oil?MATERIALS• 4 10-mL test tubes with caps• 6 cm masking tape• marker• test tube rack• 5 plastic droppers• 3 10-mL graduated cylinders• 2 mL cooking oil• 2 mL 0.02% tetrazoliumindicator solution• 2 mL each of three types ofenzymatic drain cleanerPROCEDURE1. Label four test tubes A, B, C, and D.2. Determine a procedure for your experiment, usingsmall amounts of the materials. Decide whichmaterials and how much of each will be placed ineach test tube. You will test the effectiveness ofeach type of drain cleaner at breaking down cooking oil. Each class group will use adifferent type of oil. The tetrazolium indicator solution turns pink when oil is brokendown.3. Identify the independent and dependent variables in your experiment. Identify thecontrols and constants in your experiment. Develop an operational definition of yourdependent variable.4. Get approval from your teacher to carry out your experiment. Wear gloves.5. Use a separate dropper for the oil, the tetrazolium solution, and each type of draincleaner. Once they are prepared, place the caps on the test tubes and gently swirl thetest tubes to thoroughly mix solutions.6. Observe your test tubes every day for at least five days. Record your data in a table likethe one below. Day 0 represents the observations you make immediately after yourtest tubes have been prepared.TABLE 1. DRAIN CLEANER EFFECTIVENESSDay Tube A Tube B Tube C Tube D01ANALYZE AND CONCLUDE1. Analyze Which drain cleaner was most effective at breaking down the oil?2. Compare Compare your results to those of other groups in the class that used differenttypes of cooking oil. Which type of oil was broken down most effectively?3. Conclude Which drain cleaner would you buy? Explain.566 Unit 6: Classification and Diversity

INVESTIGATIONModeling VirusesThe structure of a virus actually helps the virus toenter into a host cell and multiply. In this exercise,you will create a model to help understand how avirus attacks a healthy cell.SKILL ModelingBIOLOGYCLASSZONE.COMVIRTUAL LABTesting Antibacterial ProductsDo antibacterial products really kill germs asthey claim? In this interactive lab, you willculture bacteria, and then determine theeffectiveness of different germ-killing agents.PROBLEM How does the shape of a virus relateto how it functions?MATERIALS• research materials (books, scientific journals,Internet)• markers• pipe cleaners• clay• tooth picks• construction paper• white paperPROCEDURE1. In your group, select one of the followingviral illnesses:• chickenpox • mumps • AIDS• common cold • polio • West Nile• influenza • measles • hemorrhagicfever2. Research the structure of the virus that causesthe illness. State the scientific name of the virus(or virus family) that causes the illness.3. Using the provided craft materials, construct amodel of the virus based on the micrographs thatyou find.ANALYZE AND CONCLUDE1. Analyze Look at all the models. What differencesdo you see? How are they similar?2. Apply How does the specific shape of your virusenable it to attack healthy cells?3. Evaluate What characteristics of the virus are notrepresented by your model?ANIMATED BIOLOGYWhat Would You Prescribe?Is an antibiotic always the best medicine?Review patients’ symptoms and diagnoses,then determine if an antibiotic should beused to treat the patients.WEBQUESTModern agriculture uses antibiotics to keeplivestock healthy. Is there a downside togiving livestock antibiotics? Complete thisWebQuest to find out. Explore linksbetween antibiotic resistance and antibioticuse in agriculture.Chapter 18: Viruses and Prokaryotes 567

CHAPTER18@ CLASSZONE.COMKEY CONCEPTS Vocabulary Games Concept Maps Animated Biology Online Quiz18.1 Studying Viruses and ProkaryotesInfections can be caused in several ways. Viruses,viroids, and prions have characteristics ofboth living and nonliving things.Unlike bacteria, viruses cannotreproduce on their own. A virushas genetic material and aprotein coat. Viroids haveonly RNA and no protein coat.Prions are made of only protein.18.2 Viral Structure and ReproductionViruses exist in a variety of shapes and sizes.Viruses can be helical like aspring, many-sided, or enveloped.Bacteriophages, orviruses that attack bacteria,have a many-sided capsidwith a long protein tail andspiky footlike fibers. Thereare two basic types of viral infections: lytic andlysogenic. A lytic infection results in the host cellsbursting open, while the virions in a lysogenicinfection do not immediately destroy the host.18.3 Viral DiseasesSome viral diseases can be prevented withvaccines. Viruses can enter a body throughwounds or body openings such as mouths ornoses. Many infectious viruses exist. Examples ofillnesses caused by viruses include the commoncold, influenza, SARS, and AIDS. Vaccines canprevent some, but not all, viral diseases.18.4 Bacteria and ArchaeaBacteria and archaea are both single-celledprokaryotes. Prokaryotes are widespread onEarth. Archaea look very similar to bacteria, butmany of their structures are made of differentcompounds. Some prokaryotes can survive harshconditions by forming endospores. Prokaryotescan transfer genes to each other throughconjugation.18.5 Beneficial Roles of ProkaryotesProkaryotes perform important functions fororganisms and ecosystems. Prokaryotes that livein an animal’s digestive tract help the animalabsorb nutrients from the food that it eats. Animalsand plants also depend on prokaryotes tofix atmospheric nitrogen. Nitrogen is necessaryto make amino acids and proteins. Bioremediationuses prokaryotes to help break down pollutantsin the environment.18.6 Bacterial Diseases and AntibioticsUnderstanding bacteria is necessary to preventand treat disease. Although the majority of bacteriaare not pathogenic, some do cause disease.Bacteria can also cause conditions such as foodpoisoning, chronic acne, Lyme disease, and toothdecay. Antibiotics are used to fight bacterialinfection. However, through natural selection,many bacteria have become resistant to commonlyused antibiotics.Synthesize Your NotesConcept Map Use a concept map like the one below tosummarize what you know about infectious agents. Includedetails about genetic material and types of hosts theyinfect.infectiousdiseasesincludeincludeCycle Diagram Use two cycle diagrams like the one belowto summarize lytic and lysogenic infections.viral DNA entershost cellprionstraits traits traits traits568 Unit 6: Classification and Diversity

Chapter AssessmentChapter Vocabulary18.1 virus, p. 544pathogen, p. 544viroid, p. 544prion, p. 54518.2 capsid, p. 547bacteriophage, p. 549lytic infection, p. 551lysogenic infection, p. 551prophage, p. 55118.3 epidemic, p. 553vaccine, p. 553retrovirus, p. 55318.4 obligate anaerobe, p. 555obligate aerobe, p. 555facultative aerobe, p. 555plasmid, p. 556flagellum, p. 556conjugation, p. 558endospore, p. 55818.5 bioremediation, p. 56118.6 toxin, p. 563antibiotic, p. 564Reviewing VocabularyCategory CluesFor each clue, list the appropriate vocabulary termfrom the chapter.Category: Viral Infection1. protects against infection2. plant virus3. host and viral DNACategory: Bacteria4. virus of bacteria5. fights bacterial infection6. pollution digestionGreek and Latin Word Origins7. The term flagellum comes from the Latin word flagrum,which means “whip.” Explain how this meaning relatesto flagellum.8. The term conjugation comes from the Latin wordconjugare, which means “to join together.” Using thismeaning, explain how it relates to what conjugation is.9. The term epidemic comes from the Greek wordsepi-, which means “upon,” and demos, which means“people.” Explain how these meanings relate toan epidemic.10. The term aerobe means “an organism that requiresoxygen to live.” The prefixes a- or an- mean “without,or not.” How do these meanings relate to theterm anaerobe?Reviewing MAIN IDEAS11. Viruses, viroids, and prions are not considered to beliving things. Which of their traits resemble livingorganisms, and which traits do not?12. The flu virus has an envelope with surface proteins thatallow it to infect its host cells. What structures helpviruses infect bacterial cells? Explain.13. Explain the differences between the two ways virusesinfect their host cells.14. Children across the United States get “shots,” orinjections, during their physical exams. Explain whatthese shots are and why they are recommended forall children.15. The success of prokaryotes is due to special characteristicsthey have, such as the ability to form endosporesand perform conjugation. Explain how each of theseabilities helps prokaryotes survive changing environments.16. It surprises most people to learn that their lives dependon bacteria. Describe three roles bacteria play in humanhealth and survival.17. Due to their unique ability to break down an enormousarray of substances, prokaryotes play critical roles inecosystems. Summarize two of these roles.18. Doctors recommend washing hands before eating toprevent the spread of disease. What is the connectionbetween bacteria and disease?19. Prokaryotes have the ability to carry genes other thantheir own. How is this trait important for geneticengineering?20. Recently, doctors have been advised to limit the use ofantibiotics whenever possible. Why is this recommendationimportant?Chapter 18: Viruses and Prokaryotes 569

Critical Thinking21. Apply Many bacteria cause food spoilage because theyhave dietary needs similar to humans. However, somebacteria consume chemicals such as heavy metals, sulfur,petroleum, and mercury. How are these bacteria beingused to help humans?22. Compare and Contrast In the lysogenic infection, viralgenes can become a part of the host’s cell. In the lyticinfection, the host cell is destroyed. What might be thebenefit of each type of infection to the virus?23. Infer New viruses may quickly kill their host afterinfection, but after many generations viruses tend toweaken and cause fewer deaths. Why might it be adisadvantage for a virus to quickly kill its host?24. Synthesize Endospore-forming bacteria include thosethat cause the diseases tetanus, botulism, and anthrax.Endospores themselves, however, do not cause illnessand cannot reproduce. Why, then, are endospores sucha concern to the food and healthcare industries?Interpreting VisualsUse the diagram below to answer the next three questions.The host bacteriumbreaks apart, or lyses.Bacteriophages areable to infect newhost cells.The bacteriophage attachesand injects its DNA into ahost bacterium.host bacteriumAnalyzing DataUse the hypothetical data below to answer the nextquestions.TYPE OF BACTERIA AND LENGTHType of bacteriaAverage length (nm)Streptococcus 500Staphylococcus 900Vibrio 2600Aquaspirillum 2800REPLICATION TIME OF STREPTOCOCCUSTime (min)Number of streptococcus cells0 128 256 484 828. Connect For each of the tables above, identify whetherthe data are continuous or discrete. Explain.29. Calculate Assuming that nutrients are unlimited, howmany Streptococcus cells will there be after 112 minutes?Explain.The viral DNA directs the hostcell to produce new viral parts.The parts assemble into newbacteriophages.The viral DNAforms a circle.25. Apply What type of viral infection is shown above?Explain your answer.26. Apply Why is it necessary for the viral genes to enterthe host cell?27. Analyze How would the way that the virion enters thehost cell change if the virus were a type that infectedanimals, and the host cell were eukaryotic ratherthan prokaryotic?Connecting CONCEPTS30. Writing a Pamphlet Scientists agree that a form of theavian flu virus has the potential to cause a worldwide fluepidemic. This type of virus is known to mutate easilyand adapt quickly to host changes. Imagine you are arepresentative from the Centers for Disease Controland are writing a pamphlet to educate citizens aboutthe virus and how it actually causes infection. Usingyour knowledge of cells and viruses, make a detailedpamphlet that the general public could understand.31. Synthesize The bacteria in the esophagus shown onpage 543 are one of the many types of symbioticprokaryotes living within our bodies. How might thesetypes of mutualistic relationships have arisen? Considernatural selection in your answer.570 Unit 6: Classification and Diversity

For more test practice,go to ClassZone.com.1.Set-up for Antibiotic TestingAntibiotic A B CInfected mice tested 30 15 5% Effectiveness 83% 25% 100%Scientists are testing three antibiotics—A, B,and C—on 50 mice with bacterial infections.Their experimental design is shown above. Theyconclude that they need to do more testing onantibiotic C becauseA it unexpectedly worked better than A and B.B 100 percent of the mice were cured.C the sample size was too small.D it will likely have the worst side effects.2. Impetigo is a highly contagious skin infectioncaused by staph or strep bacteria that arenormally found on the skin, where they areharmless. This infection is most likely tooccur whenA scraped skin provides the bacteria with accessto tissues they do not normally contact.B the bacteria have been genetically altered byscientists doing research.C the infected person did not receive regularlyscheduled vaccinations.D the bacteria form endospores on the surface ofthe skin.3. Some scientists think that measures of anecosystem’s health—such as usable nitrogenlevels in the soil—may become more variable asthe diversity of organisms on Earth declines.This is because usable soil nitrogen dependson a variety ofA animals that return nitrogen to the soilthrough respiration.B animals that return nitrogen to the soil afterthey die.C bacteria and other decomposers that fixnitrogen into a usable form.D plants, which produce nitrogen as a byproductof photosynthesis.4. The main reason that viruses are not consideredto be living things is that they do notA die when exposed to antibiotics.B reproduce on their own.C contain a nucleus.D undergo meiosis before replication.THINK THROUGH THE QUESTIONAll of these answer choices correctly describeviruses, so do not be tricked! Look at each answerchoice and try to think of a living organism thatfits the characteristic described, making thatanswer choice wrong.5. Hepatitis B is a viral disease that attacks cellsin the liver. When should a person receive avaccination against hepatitis B?A before being exposed to the virusB as soon as viral symptoms begin to appear.C after being diagnosed with the diseaseD never; the overuse of vaccines leads toantibiotic resistance6.??reproduceshasdoesn’tasexuallyhavenucleusDNAmembrane-boundorganelleschlorophyllWhich of the following is described by thisconcept map?A animal cellB plant cellC bacteriumD virusChapter 18: Viruses and Prokaryotes 571