This is only a guess on my part, but I think that mutations in response to a lethal assault such as pesticides, in general is not that likely to produce a stronger organism, since often the target enzymatic pathways are essential.  The 2 illustrative examples that I can think of off the top of my head are bacterial resistance to erythromycin via methylation of the ribosomal binding area to which erythromycin binds, thereby reducing it's ability to block protein production.  This resultant bacteria are significantly weaker than wildtype because methylation severely hampers protein synthesis, it's inducible (the cladinose on Ery A causes induction) and I think reversible.  I am pretty sure there are also ribosomal mutations that are constitutively resistant but also much weaker than wildtype for the same reason.  So in the absence of Ery A wildtype bacteria are going to predominate.

Penicillin resistance is interesting case because the bacteria produce a "extra" enzyme, beta-lactamase" which inactivates the penicillin by chemically altering it's structure, I don't think expression of beta-lactamase has any negative effects on the bacteria, except taking up a little of the available protein production capacity.  I would expect resistant organisms to linger significantly in the absence of penicillin, since a resistant bacterium isn't worse off, nor any better off than a non-resistant one.

I think this is an interesting discussion and so I am just throwing out some thoughts.  Perhaps the more figuratively relevant aspect of antibotic resistance in bacteria, to mite treatments and resistance to acaracides, is that virtually none of the prescription antibiotics available are cidal.  They are almost all bacteriastatic, they don't kill the bacteria, they just stop it from reproducing.  take away the antibiotic and the bacteria start right back up again.  The drugs just give your body's immune system time to work, it does the killing.  I am fascinated by the idea that there might be bees that purposely disrupt mite mating cycles to keep their numbers in check.  Mark

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