co-evolution of virulence and anti-microbial peptides

[gusl]

I've been analyzing more evolution stuff, inspired by tdj. Maybe I'm going a bit overboard (I probably misinterpreted the original meaning, but such speculations are fascinating anyway)

In response to Study reveals a way disease bacteria sense antimicrobials and initiate a counter-defense:

Many living things, from fruit flies to people, naturally produce disease-fighting chemicals, called antimicrobial peptides, to kill harmful bacteria. In a counter move, some disease-causing bacteria have evolved microbial detectors. The bacteria sense the presence of antimicrobial peptides as a warning signal. The alarm sets off a reaction inside the bacteria to avoid destruction.

University of Washington (UW) and McGill researchers have revealed a molecular mechanism whereby bacteria can recognize tiny antimicrobial peptide molecules, then respond by becoming more virulent.

I wrote:

Whoa, such a mechanism could evolve even if it kills the host and stops the bacteria from multiplying. How? Co-evolution: the conditional virulence causes hosts to stop producing antimicrobials (since the ones who produce them die more).

Therefore, the bacteria populations that *do* respond by becoming more virulent have a stable strategy. This is game theory! The 2 players are: HOST'S GENES, and BACTERIA'S GENES, and each player has 2 strategies.

Let's assume virulent reactions to peptide kills the host (-10 for the host).

 \   HOST     peptide   no peptide
BACTERIA

virulent     (-10, -1)    (-1, +1)

non-virulent (0, 0)       (-1, +1)

(HOST_GENES, BACTERIA_GENES)

as long as there is a credible threat of virulence, hosts may "choose" to not produce peptide. I think the evolutionarily stable solution is "mixed strategies".

Individual bacteria do better by not killing the host, but whole bacterial populations that co-evolve with the host do better by having some individuals who become virulent (sacrificing themselves for the greater good of their family), thus "forcing" the host populations' genes to play "no peptide".

I find it plausible that group selection is a strong enough force in bacterial evolution.