I read this:

"And they multiply incredibly quickly, leaving their equally resistant progeny in much greater numbers. The resistant bacteria will spread the way bacteria do, but now they will outnumber the vulnerable ones in the population. Then, when the same antibiotic is used again, it can’t gain a toehold because a far greater proportion of the newly-produced bacteria are unaffected by its use. The bacteria have evolved. Not taking a full course of antibiotics, or taking them when they can do no good, as with viral infections like colds or flu, hastens the selection of the resistant germs."

And then I started thinking...

This is a great example of natural selection at work in a bottlenecked population. The gene pool in your body (bacteria with a variety of different traits) responds to environmental factors (antibiotics) and the frequency of some of those traits changes. That's a genetic shift.

But is it, technically, evolution? (Serious question!) Mutation and natural selection are at work, and I understand how this could lead to the rise of a new species, but are bacterial strains "species"?

If the population was let loose, instead of confined to a single body, and under different environmental circumstances, it's possible that the population genetics would shift back, right? The bacteria haven't diverged to the point that they can no longer exchange genetic material, have they? (What's the difference between a bacterial strain and a bacterial species?) I've always though of antibiotic resistance as a trait like blue eyes: it can be more or less prevalent in a population, but it doesn't make that population an entirely different kind of bacteria. Or does it?