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roundtable: the evolving enemy Watch Show 4:
"The Evolutionary Arms Race"
on PBS
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bottle of prescription antibiotics
In the battle against infectious disease, humankind has inadvertently given rise to deadly enemies. Antibiotic resistance is a stunning example of evolution by natural selection. Bacteria with traits that allow them to survive the onslaught of drugs can thrive, re-ignite infections, and launch to new hosts on a cough. Evolution generates a medical arms race. The bad news is that bacteria -- with their fast doubling times and ability to swap genes like trading cards -- evolve quickly. The good news is that in the 150 years since Darwin, we have grown to understand the rules of the race. But can we win this war?
Tamar Barlam
  George Beran
  Stuar Levy
  Stephen Palumbi
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Stephen R. PalumbiStephen R. Palumbi is professor of biology at Harvard University, where he teaches evolution, marine biology, and molecular ecology, and conducts research on populations and molecular genetics of marine animals. He is the author of The Evolution Explosion (2001), about how evolution, including antibiotic resistance, is sped up by human technology.

Approaches to reducing antibiotic resistance include undoing current damage, reversing it, and slowing evolution down. Undoing damage will often entail new antibiotics that overcome current resistance. But the basic arms race still churns, costly in terms of new drugs and even lives.

It is possible in some cases to reverse evolution -- to encourage the re-emergence of drug-susceptible bacteria by removing the selective pressure of drugs. Resistance mechanisms sometimes generate an energetic cost, like extra proteins that the bacteria must make for drug detoxification. In the drug's absence, non-resistant bacteria replicate faster, and eventually take over. Unfortunately, few opportunities arise to take advantage of this tradeoff, because many bacteria have evolved low-cost antibiotic resistance.
To slow evolution, we can reduce selective pressure by avoiding antibiotics when they are not useful, (e.g., viral infections), or substituting other anti-bacterial strategies instead of chemical control. In agriculture, integrated pest management employs physical reduction of insect pests and thereby reduces reliance on insecticides. Hospitals also use this strategy, except they call it hand-washing.
Another method is to change selective pressures periodically by changing the antibiotic used. This alters the trajectory of evolution and can delay the day when full resistance to one antibiotic evolves.
A third way to slow evolution is pyramiding, the use of multiple drugs to deliver a strong killing dose. Evolution only occurs in a variable population -- when some bacteria are able to survive an antibiotic dose, but others are not. In the presence of a drug overkill, there is no variation -- all bacteria die -- and evolution slows dramatically. This is what makes triple-drug therapy effective against the HIV virus.
In the face of rapid bacterial evolution, all drug strategies are temporary. But by studiously engineering the evolutionary process, we can extend the life of powerful drugs, slow the arms race, and reduce the social and economic costs of disease.
(Boldface added.)
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