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Evolutionary Lessons From Superbugs

Virulent drug-resistant "superbugs" are back in the news. We have a lot to learn from these small but smart creatures. To the dismay of many in the pubic health field, the FDA just dropped plans to enforce a 1977(!) decision to limit the use of antibiotics in animal feed, which facilitates the emergence of antibiotic-resistant pathogens. A December 23, 2011 article in Wired by Maryn McKenna ("FDA Won't Act Against Ag Antibiotic Use") and a December 27, 2011 New York Times blog by Mark Bittman ("Bacteria 1, F.D.A. 0") tell the story. I'll leave it to others to discuss the political ramifications of this disastrous (in)action. Here, we'll look at it as another reflection on public misunderstanding of modern evolutionary science.

How do bacteria acquire antibiotic resistance? How do they become pathogens? We currently know a great deal about the genetic basis of these critically important bacterial properties. We also know how resistance and virulence are acquired and spread to new species. The story of how we came to this knowledge is a fascinating and instructive chapter in the history of science -- it illuminates the insight that scientific "fact" consists of more than experimentally confirming hypothetical predictions.

In the early days of molecular biology, bacterial geneticists applied conventional evolutionary concepts from the pre-DNA period to explain the evolution of antibiotic resistance. The theory was that mutations could alter the structure of cell components and either block entry of the drugs into the bacteria or prevent their action on cellular targets, such as the enzymes essential to cell wall synthesis. Even if the initial mutation did not confer a high degree of resistance, accumulation of several sequential changes would result in resistance to the antibiotic levels used in clinical medicine. Indeed, a wide variety of laboratory experiments confirmed this theory, and bacterial geneticists isolated the predicted mutant strains. In virtually all cases, the resistant mutants grew less well than the parental sensitive bacteria, leading to the comforting conclusion that resistant bacteria would not significantly accumulate in nature. The degree of confidence was so great that the U.S. Surgeon General in 1967 declared that "the war against infectious diseases has been won" (Fauci 2001).

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