Microbiology, 2021

398 10 • Biochemistry of the Genome
Clinical Focus
Resolution
Within 24 hours, the results of the diagnostic test analysis of Alex’s stool sample revealed that it was
positive for heat-labile enterotoxin (LT), heat-stabile enterotoxin (ST), and colonization factor (CF),
confirming the hospital physician’s suspicion of ETEC. During a follow-up with Alex’s family physician, this
physician noted that Alex’s symptoms were not resolving quickly and he was experiencing discomfort that
was preventing him from returning to classes. The family physician prescribed Alex a course of
ciprofloxacin to resolve his symptoms. Fortunately, the ciprofloxacin resolved Alex’s symptoms within a
few days.
Alex likely got his infection from ingesting contaminated food or water. Emerging industrialized countries
like Mexico are still developing sanitation practices that prevent the contamination of water with fecal
material. Travelers in such countries should avoid the ingestion of undercooked foods, especially meats,
seafood, vegetables, and unpasteurized dairy products. They should also avoid use of water that has not
been treated; this includes drinking water, ice cubes, and even water used for brushing teeth. Using bottled
water for these purposes is a good alternative. Good hygiene (handwashing) can also aid the prevention of
an ETEC infection. Alex had not been careful about his food or water consumption, which led to his illness.
Alex’s symptoms were very similar to those of cholera, caused by the gram-negative bacterium Vibrio
cholerae, which also produces a toxin similar to ST and LT. At some point in the evolutionary history of
ETEC, a nonpathogenic strain of E. coli similar to those typically found in the gut may have acquired the
genes encoding the ST and LT toxins from V. cholerae. The fact that the genes encoding those toxins are
encoded on extrachromosomal plasmids in ETEC supports the idea that these genes were acquired by E.
coli and are likely maintained in bacterial populations through horizontal gene transfer.
Go back to the previous Clinical Focus box.
Viral Genomes
Viral genomes exhibit significant diversity in structure. Some viruses have genomes that consist of DNA as
their genetic material. This DNA may be single stranded, as exemplified by human parvoviruses, or double
stranded, as seen in the herpesviruses and poxviruses. Additionally, although all cellular life uses DNA as its
genetic material, some viral genomes are made of either single-stranded or double-stranded RNA molecules,
as we have discussed. Viral genomes are typically smaller than most bacterial genomes, encoding only a few
genes, because they rely on their hosts to carry out many of the functions required for their replication. The
diversity of viral genome structures and their implications for viral replication life cycles are discussed in
more detail in The Viral Life Cycle.
CHECK YOUR UNDERSTANDING
• Why do viral genomes vary widely among viruses?
MICRO CONNECTIONS
Genome Size Matters
There is great variation in size of genomes among different organisms. Most eukaryotes maintain multiple
chromosomes; humans, for example have 23 pairs, giving them 46 chromosomes. Despite being large at 3
billion base pairs, the human genome is far from the largest genome. Plants often maintain very large
genomes, up to 150 billion base pairs, and commonly are polyploid, having multiple copies of each
chromosome.
The size of bacterial genomes also varies considerably, although they tend to be smaller than eukaryotic
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