Saturday, November 12, 2005

Peptide Antibiotics: Boot Camp for Bacteria?

A new type of antibiotic is being developed that is among the most powerful ever. Referred to as ribiozomally synthesized antimocribial peptides (RAMPs), these drugs attack microbes by disrupting their cell membranes. The drug effective spills the guts of bacteria by breaking them open. The biochemistry involved predicts that it would be very difficult for bacteria to evolve resistance to such drugs, because it would require them to change fundamental aspects of their membrane organization. Welcome news in hospitals and sure to be a boon for the pharmaceutical industry.

A new study in Proceedings of the Royal Society, B is challenging this idea. Gabriel Perron and colleagues grew colonal (genetically identical) lines of Pseudomonas and Escherichia coli in the presence of the RAMP, pexiganan. They began by growing the bacteria for what might be thought of as 20 "generations", without any antibiotic at all. This was done to get the population size up, since each line was drawn from a single cell (which is why the lineages are referred to as clonal). Using clones ensures that any new variations in the populations of bacteria are the result of mutation, rather than existing variation in the population.

Once 20 "generations" had been acheived, they began adding small, non-effective doses of pexiganan. Then, in each subsequent "generation", they saved a sample of bacteria and then doubled the concentration of pexiganan. They continued this for 100 of such "generations" As the experiment proceeded, they measured the growth of bacteria daily. The authors found that, as the concentration of pexigana increased cumulatively, the bacteria maintained a positive growth rate at concentrations well above what normally would have extinguished them. The bacteria had evolved resistance.

Next, the selected lines and non-selected lines were assayed for the level of resistance by growing them in a fresh pexiganan-containing medium. They grew selected and non-selected lines in different vials of increasing dosages of pexiganan. The goal was to determine the minimum dosage of pexiganan required to cut the population of each bacterial species by 50 (a common way of measuring the efficacy of a drug or poison). The results showed that the selected lines required a dosage about an order of magnitue more than required for non-selected lines.

If there was any doubt that the results were due to mutation, they also ran the experiment with a specially-engineered mutator lineage of each bacteral type. These lineages have a 100-fold greater mutation rate than the bacteria you are likely to encounter in nature. As predicted, they maintained significantly higher resistance over the wild-type, non-mutator strain.

Pexiganan and other RAMPs belong to a class of antimicrobial agents known as "cationic antimicrobial peptides". Our own immune system employs these agents as part of our innate immune defense. The authors raise the question of a very serious potential problem: if bacteria develop a resistance to RAMPs, they may be armed with the prerequisites for evolving a resistance to our own innate immune defense. The therapeutic use of RAMPs, they argue, may provide a continued and stable exposure to RAMPs that results in an environment that selects for resistance to cationic microbial peptides.

Perron et al. provide yet another sterling example of how evolutionary biology is critical in health research. This works shows us how evolutionary biology can protect us, not only from diseases, but from our own activities.
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