Evolution of resistance–bacteria win again

I’m swamped today, so alas, nothing new from me. However, since many of you are newer readers, I thought I’d totally cheat and dig up one from the archives on antimicrobial resistance. This one I cross-posted to Panda’s Thumb where it received some decent discussion; it was also mentioned in a write-up of Panda’s Thumb featured in Science magazine. I also find it very fitting since we have a number of commenters discussing a number of things microbes “can’t” do, as the post tells the story of one scientist who made a similar comment, was taken up on that, and proven wrong.

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.

9 Replies to “Evolution of resistance–bacteria win again”

  1. Of the two ideas only one seems to have been refuted.
    >>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.< < This was falsified. >>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<< Unless they used the full cocktail of peptides( i only saw the abstract and it referred to the singular peptide) or proved cross resistance

  2. Cayte,

    Zasloff didn’t think that resistance would develop (as he quotes, “both in the laboratory and in the clinic.”) You’re correct that it only evolved to the one peptide they used experimentally, and as such, may not occur in a more natural environment, but the fact that it evolved at all was a surprise to many.

  3. Is this similar to Coley’s Toxins ans cancer immunotherapy?

    Nope. Coley’s toxins were heat-killed strep and Serratia bacteria, whose actions were thought to be due largely to streptokinase (if I’m remembering the story correctly). The chemicals described above are short peptides–I don’t know if they’ve been looked into as a cancer therapy.

  4. “Oh, and Eddie–si, hablo espanol. He sends it to my email as well. Charming, eh?”

    Oh yeah. Very charming. Kind of like Rasputin or Yaponchik.


  5. So- the CT was dead, these are live. CT is more like a killed vaccine, but in a very non-scientific wording and understanding. The crazy thing is, there are instructions on the web on how to make and inject yourself with Coley’s Toxins- which seems a bit risky to me, seeing as it starts with hamburger. That has nothing to do with anything other than as a creepy aside. http://www.second-opinions.co.uk/coleys_instructions.html

    (and I only looked because after I posted, I figured I was probably asking something I could find out myself. I was reading Commotion in the Blood last week and it mentioned Coley stuff. I obviously (take note of this other non science based people—– did not understand EVERYTHING I WAS READING- even though I probably know a bit more than someone who is not even interested in science at all and therefore has no base. Valuable lesson here! Isn’t there a maxim that a little bit of knowledge can be a dangerous thing?).

    And you are a much nicer person than I am- the conversation with Hank and the other very angry disagreeable ones would make me want to block their butts. How condescending some of his remarks have been. You are obviously a professional- something I would not extend to the other side.

  6. Cayte,

    It is true that the experiment did not address the mixture effect of antimicrobial peptides and the evolution of resistance. However, it is important to note that a single peptide will most likely be used commercially. This will create conditions “similar” to the presented experiment, where the given peptide is disproportionately present in the environment of the bacteria.

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