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Tuesday, November 15, 2005 

Evolution of resistance--bacteria win again

Resistance to antibiotics has been a concern of scientists almost since their widespread use began. In a 1945 interview with the New York Times, Alexander Fleming himself warned that the misuse of penicillin could lead to selection of resistant forms of bacteria, and indeed, he’d already derived such strains in the lab by varying doses of penicillin the bacteria were subjected to. A short 5 years later, several hospitals had reported that a majority of their Staph isolates were, as predicted, resistant to penicillin. This decline in effectiveness has led to a search for new sources and kinds of antimicrobial agents. One strategy involves going back to a decades-old approach researched by Soviet scientists: phage therapy. Here, they pit one microbe directly against another, using viruses called bacteriophage to infect, and kill, pathogenic bacteria. Vincent Fischetti at the Rockefeller University has used this successfully to kill anthrax, Streptococcus pyogenes, and others. Another novel source of antibiotics has come from our own innate immune system, one of our initial defenses against microbial invaders.

An enormous variety of organisms produce compounds called cationic antimicrobial peptides. A component of our own innate immune system, these are fairly short strings of amino acids (less than 100 a.a.’s) that have a net positive charge. It is thought that these peptides work primarily by disrupting the integrity of the bacterial cell wall, essentially poking holes in the wall, causing death of the cell. Since the peptides are targeted at the bacterial cell wall structure, it was thought that resistance would require a fundamental change in membrane structure, making it an exceedingly rare event. Therefore, these antimicrobial peptides might make an excellent weapon in the fight against multiply drug-resistant bacteria. Additionally, the remarkable diversity of these peptides, combined with the presence of multiple types of peptides with different mechanisms of action present at the infection site, rendered unlikely the evolution of resistance to these molecules (or so the common thinking went). However, evolutionary biologists have pointed out that therapeutic use of these peptides would differ from natural exposure: concentration would be significantly higher, and a larger number of microbes would be exposed. Additionally, resistance to these peptides has been detailed in a few instances. For example, resistance to antimicrobial peptides has been shown to be essential for virulence in Staphylococcus aureus and Salmonella species, but we didn't *witness* that resistance develop--therefore, it might simply be that those species have physiological properties that render them naturally resistant to many of these peptides, and were never susceptible in the first place.

Antimicrobial resistance is always a problem—it can render antibiotics much less useful, and make deadly infections almost untreatable. But resistance to these peptides could make us all vulnerable. The peptides of our innate immune system are one of our first lines of defense against an immense variety of pathogens, and we don’t know what the outcome may be if we compromise this essential level of protection. But realistically, could such resistance evolve within the bacterial population?

Dr. Michael Zasloff of Georgetown University was originally a doubter. In this 2002 Nature article, he states in conclusion:
Studies both in the laboratory and in the clinic confirm that emergence of resistance against antimicrobial peptides is less probable than observed for conventional antibiotics, and provides the impetus to develop antimicrobial peptides, both natural and laboratory conceived, into therapeutically useful agents.
Certainly in the short term, resistance was unlikely to evolve for the reasons I mentioned above. However, if these peptides are used over an extended period of time, could the mutations necessary to confer resistance accumulate? This was the question asked in a new study by Dr. Zasloff along with colleagues Gabriel Perron and Graham Bell. Following publication of his 2002 paper where he called evolution of resistance to these peptides “improbable,” Bell challenged Zasloff to test this theory. Zasloff took him up on the offer, and they’ve published their results in Proceedings of the Royal Society.

They tested this using strains of E. coli and Pseudomonas fluorescens. They started out growing these bacteria with low concentrations of a peptide antibiotic called pexiganan, a derivative of a peptide originally isolated from a frog. (Carl Zimmer has an excellent post on this same topic here). The experimental design was quite simple. They grew the bacteria, took a portion of the growth, and added that to a new tube with fresh media. Gradually, they increased the concentration of pexiganan in the growth medium. In all, they did 100 serial transfers of the bacteria (correlating to ~500-600 generations of bacteria), and the end result were--voilà!--bacterial populations that were resistant to the peptides.

Creationists/ID advocates (such as chemist Phil Skell) often claim that “evolutionary theory contributes little to experimental biology,” or that “evolution has little to do with almost all research in biology and biotechnology”, etc. etc. And sure, the theory of evolution didn’t *directly* result in the discovery of peptide antibiotics. But advances in biotechnology do not exist in a vacuum, and we have seen what can occur from the misapplication of these types of technologies, unguided by an understanding of underlying evolutionary principles. Peptide antibiotics have not yet been used clinically to treat human infections, but imagine if they had gone into widespread use without a thought given to the evolution of resistance to these peptides. Imagine if they had gone into widespread use prior to an investigation of the relatedness of various peptides to those produced by humans. Imagine if, as a result of not considering these implications, we had lost an ancient protection against bacteria—-which *evolved* over millions of years of host-pathogen interaction--due to a mere advancement in biotechnology. While I enjoy proving the evolution-doubters wrong, I hope it never comes down to that kind of situation in order to do so, and I hope this example is instructive to those who claim that evolution isn't useful.


About me

  • I'm Tara C. Smith
  • From Iowa, United States
  • I'm a mom and a scientist, your basic stressed-out, wanna-have-it-all-and-do-it-all Gen Xer. Recently transplanted from Ohio to Iowa, I've spent most of my life in the midwest (with 4 years of college spent out east in "soda" territory). My main interest, and the subject of my research, is infectious disease: how does the microbe cause illness? What makes one strain nasty, and another "avirulent?" Are the latter really not causing any disease, or could some of those be possible for the development of chronic disease years down the road? Additionally, I've spent a lot of time discussing the value of teaching evolution, and educating others about "intelligent design" and other forms of creationism. My interest in history of science and medicine is also useful as a way to tie all of the above interests together. [Disclaimer: the views here are solely my own, and do not represent my employer, my spouse, that guy who's always sitting by the fountain when I come into work, or anyone else with whom I may be remotely affiliated.]
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