Taq writes:
So how did this antibiotic resistance come about? What are your hypotheses?
My hypothesis is that mutation(s) that occured during the growth of your one organism into trillions are the source of the antibiotic resistance. This mutation (assuming only one's involved) would probably have been near neutral on arrival.
The reasoning is this. If the resistance had existed in the original clone, then virtually all the descendents would have had it, and growth would have occured all over the antibiotic plate. Also, from what I know of bacterial mutation rates, billions of mutations must have taken place in the course of the growth from one into trillions, so there would be considerable variation on the original genome already, as many mutations would be near enough neutral. So, a "hit" (thinking retrospectively) in one or two strains of variants is plausible.
The end result is that a tiny fraction, perhaps one in a few billion, of the organisms have a variation that happens to be useful, which is what you see on your plate.
I don't know the specific paper you're talking about, but that seems to me to be the most plausible hypothesis.
So, If I'm right, the mutation was random in respect to fitness on arrival, at least, so far as its known advantage in relation to the antibiotic is concerned.
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