The Power of Bacteria
- More and more research groups are seeking to show that changes in the microbiota of a particular site are involved in diseases such as periodontal disease, inflammatory bowel disease, and premature birth. Critics object that showing an association is not the same as demonstrating cause and effect. In the case of the obesity study, scientists tried to do this by inoculating germfree mice with different variations of the microbiota. Clearly, this would not be possible in humans. How might you prove cause and effect in humans?
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- Members of the microbiota cause some quite serious diseases. How could a bacterium that normally lives in a beneficial or neutral association with its human host cause serious disease?
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- The assertion is made in this chapter that scientists now believe that transfer of DNA by conjugation in the colon is occurring across species and genus lines. Suppose you found the same type of antibiotic-resistance gene in members of two different genera. What criteria might you use to show that the gene was transferred horizontally? How might you suspect that the gene was transferred by conjugation?
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- PCR combined with sequencing can provide a quick identification of bacteria. What are the limitations of this approach?
- Why are scientists so reluctant to let go of Koch’s postulates even though they often chafe – as Koch did – under their restrictions> Why are Koch’s postulates, with all of their problems, still as relevant today as they were when Koch first proposed them?
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- We mentioned the possibility that some diseases might be caused by shifts in the microbiota of a site on the human body. Along these lines, some oral microbiologists have pointed to an association between gingivitis (gum disease), caused by microbes found in dental plaques and cardiovascular disease. Could you formulate Koch’s postulates for such a disease? If so, how would you set up an experiment to satisfy these postulates?
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- Are there certain disease cases where it might be allowable to use human volunteers as experimental subjects in order to satisfy Koch’s postulates? What are those instances?
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Solving Problems in Bacterial Pathogenesis
- You have just received an urgent call from the Centers for Disease Control and Prevention (CDC) to consult on the following case of an unusual disease outbreak that they have been investigating. A new Gram-positive bacterium related to Listeria monocytogenes was isolated from an outbreak of food poisoning in Wisconsin due to contaminated cheese that appears to cause painful gastritis and, in about half of the exposed individuals, sudden onset of bleeding ulcers, followed by death from toxic shock within two to three days. Upon biopsy of the infected individuals, it was found that the bacteria were growing on the surface of epithelial cells lining the gastric pit of the stomach. Autopsy of individuals who died showed that bacteria were found only in the stomach and not in any of the other body organs. The researchers at the CDC have subsequently determined that, like Listeria monocytogenes, this new species of Listeria invades epithelial cells in tissue culture. Which, if any, of Koch’s postulates have been satisfied for the involvement of this new Listeria strain with gastritis, bleeding ulcers, and toxic shock? What additional measures could the researchers take to help satisfy Koch’s postulates?
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- In 1976, the Bellevue-Stratford Hotel in downtown Philadelphia hosted a convention of members of the American Foreign Legion. During the convention, 221 legionnaires developed a new form of pneumonia and 24 people died. Infectious disease specialists working for the CDC were sent to Philadelphia to identify the cause of this new, mysterious illness, which not surprisingly was named Legionnaires’ disease. Using Koch’s postulates, list the steps that those specialists had to take to complete their assignment. Briefly explain the significance of each step. These specialists understood that it is not always possible to follow Koch’s postulates to the letter. As they embarked on their quest to identify the causative agent of this new illness, list at least two problems that they might have faced in attempting to fulfill Koch’s postulates.
