Antimicrobial Drugs

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M34_ADAM9811_03_SE_CH34.QXD 12/30/09 1:16 PM Page 481 Chapter 34 <strong>Drugs</strong> for Bacterial Infections 481 RNA synthesis inhibitors: • Rifampin DNA Cell wall synthesis inhibitors: • Carbapenums • Cephalosporins • Isoniazid • Penicillins • Vancomycin Protein synthesis inhibitors: • Aminoglycosides • Chloramphenicol • Clindamycin • Linezolid • Macrolides • Streptogramins • Tetracyclines mRNA Replication DNA synthesis inhibitors: • Fluoroquinolones Proteins Enzymes or essential metabolites Antimetabolites: • Sulfonamides ➤ Figure 34.1 Mechanisms of action of antimicrobial drugs 34.5 Acquired Resistance Microorganisms have the ability to replicate extremely rapidly. For example, under ideal conditions E. coli can produce a million cells every 20 minutes. During this rapid replication, bacteria make frequent errors while duplicating their genetic code. These mutations occur spontaneously and randomly throughout the bacterial chromosome. Although most mutations are harmful to the organism, mutations occasionally result in a bacterial cell that has reproductive advantages over its neighbors. The mutated bacterium may be able to survive in harsher conditions or perhaps grow faster than surrounding cells. Mutations that are of particular importance to medicine are those that confer drug resistance to a microorganism. Antibiotics help promote the development of drug-resistant bacterial strains. Killing populations of bacteria that are sensitive to the drug leaves behind those microbes that possess mutations that made them insensitive to the effects of the antibiotic. These drug-resistant bacteria are then free to grow, unrestrained by their neighbors that were killed by the antibiotic, and the patient develops an infection that is resistant to conventional drug therapy. This phenomenon, acquired resistance, is illustrated in ➤ Figure 34.2. Bacteria may pass the resistance gene to other bacteria through conjugation, the transfer of small pieces of circular DNA called plasmids. It is important to understand that the antibiotic did not create the mutation that caused bacteria to become resistant. The mutation occurred randomly. The role that the antibiotic plays in resistance is to kill the surrounding cells that were susceptible to the drug, leaving the mutated ones plenty of room to divide and infect the host. It is the bacteria that have become resistant, not the patient. An individual with an infection that is resistant to certain antibacterial agents can transmit the resistant bacteria to others. The widespread and sometimes unwarranted use of antibiotics has led to a large number of resistant bacterial strains. At least 60% of Staphylococcus aureus infections are now resistant to penicillin, and resistant strains of Enterococcus faecalis, Enterococcus faecium, and Pseudomonas aeruginosa have become major clinical problems. The longer an antibiotic is used in the population and the more often it is prescribed, the larger the percentage of resistant strains. Infections acquired in a hospital or other health care setting, called nosocomial infections, are often resistant to common antibiotics. Resistant nosocomial infections are especially troublesome in critical care units, where seriously ill patients are often treated with high amounts of antibiotics. Two particularly serious resistant infections are those caused by methicillin-resistant Staphylococcus aureus (MRSA) and vancomycin-resistant enterococci (VRE). MyNursingKit Veterinary Antibiotics and Human Health # 102887 Cust: PE/NJ/CHET Au: ADAMS Pg. No. 481 Title: Pharmacology for Nurses Server: Jobs2 C/M/Y/K Short / Normal DESIGN SERVICES OF S4CARLISLE Publishing Services
M34_ADAM9811_03_SE_CH34.QXD 12/30/09 1:16 PM Page 482 482 Unit 5 The Immune System Resistant organism Antibiotic Time ➤ Figure 34.2 Acquired resistance 1. Infection 2. Antibiotic kills all organisms except the resistant one. 3. Resistant organism that remained has rapidly divided to infect the client. Antibiotic is no longer effective. Health care providers play important roles in delaying the emergence of resistance. The following are five principles recommended by the CDC: ● Prevent infections whenever possible. It is always easier to prevent an infection, than to treat one. This includes teaching the patient the importance of getting immunizations. ● Use the right drug for the infection. Infections should be cultured so that the offending organism can be identified and the correct drug chosen (see Section 34.6). ● Restrict the use of antibiotics to those conditions deemed medically necessary. Antibiotics should only be prescribed when there is a clear rationale for their use. ● Advise the patient to take anti-infectives for the full length of therapy, even if symptoms disappear before the regimen is finished. Prematurely stopping antibiotic therapy allows some pathogens to survive, thus promoting the development of resistant strains. ● Prevent transmission of the pathogen by using proper infection control procedures. This includes the use of standard precautions and teaching patients methods of proper hygiene for preventing transmission in the home and community settings. In most cases, antibiotics are given when there is clear evidence of bacterial infection. Some patients, however, receive antibiotics to prevent an infection, a practice called prophylactic use, or chemoprophylaxis. Examples of patients who might receive prophylactic antibiotics include those who have a suppressed immune system, those who have experienced deep puncture wounds such as from dog bites, or those who have prosthetic heart valves and are about to have medical or dental procedures. 34.6 Selection of an Effective Antibiotic The selection of an antibiotic that will be effective against a specific pathogen is an important task of the health care provider. Selecting an incorrect drug will delay proper treatment, giving the microorganisms more time to invade. Prescribing ineffective antibiotics also promotes the development of resistance, and may cause unnecessary adverse effects in the patient. Ideally, laboratory tests should be conducted to identify the specific pathogen prior to beginning anti-infective therapy. Lab tests may include examination of urine, stool, spinal fluid, sputum, blood, or purulent drainage for microorganisms. Organisms isolated from the specimens are grown in the laboratory and identified. After identification, the laboratory tests several different antibiotics to determine which is most effective against the infecting microorganism. This process of growing the pathogen and identifying the most effective antibiotic is called culture and sensitivity (C&S) testing. Because antibiotic therapy alters the composition of infected fluids, samples should be collected prior to starting pharmacotherapy. However, laboratory testing and identification may take several days and, in the case of viruses, several weeks. If the infection is severe, therapy is often begun with a broad-spectrum antibiotic, one that is effective against a wide variety of different microbial species. After laboratory testing is completed, the drug may be changed to a narrowspectrum antibiotic, one that is effective against a smaller group of microbes or only the isolated species. In general, narrowspectrum antibiotics have less effect on normal host flora, thus causing fewer side effects. For mild infections, laboratory identification is not always necessary; skilled health care providers are often able to make an accurate diagnosis based on patient signs and symptoms. In most cases, antibacterial therapy is best conducted using a single drug. Combining two antibiotics may actually decrease each drug’s efficacy, a phenomenon known as antagonism. If incorrect combinations are prescribed, the use of multiple antibiotics also has the potential to promote resistance. Multidrug therapy is warranted, however, if several different organisms are causing the patient’s infection or if the infection is so severe that therapy must be started before laboratory tests have been completed. Multidrug therapy is clearly warranted in the treatment of tuberculosis or in patients infected with HIV. One common adverse effect of anti-infective therapy is the appearance of secondary infections, known as superinfections, # 102887 Cust: PE/NJ/CHET Au: ADAMS Pg. No. 482 Title: Pharmacology for Nurses Server: Jobs2 C/M/Y/K Short / Normal DESIGN SERVICES OF S4CARLISLE Publishing Services
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