jenk | Why antibiotics drive drug resistance, and soap & bleach don't

Marlene Zuk, a professor of biology at the University of California at Riverside, writing in the NY Times blog Well (emphasis added):

Bacteria don’t “develop” resistance, as if it were a muscle nurtured by going to a microbial gym. Instead, they had it all along, or more accurately a small proportion of them did. [...] Antibiotics mainly kill bacteria by targeting components in the cell wall, a structure that surrounds bacteria but which our own cells lack. Antibiotics are highly selective — unlike soap and water, which get rid of bacteria indiscriminately, through mechanical means.

When you take an antibiotic, a few of the bacterial cells in your body already happen to have genes that enable them to be resistant to it, just by random chance. You have many millions of bacteria, so it’s not too surprising that they vary, the way a big city will tend to have at least a few people with unusual eye color, exceptionally small feet or any other characteristic. If you don’t take the whole course of antibiotics, say the 5 or 7 or 14 days your doctor recommends — or sometimes even if you do — enough of the resistant bacteria may remain to establish a new infection.

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.

In contrast, although soap and water don’t completely annihilate the bacteria either, they aren’t selective. The bacteria that remain are genetically similar to the ones that went swirling down the drain, and so their offspring are equally vulnerable to the next scrubbing. [...] So using soap or bleach-based cleansers is good, but inappropriate application of antibiotics will be worse than ineffective because it drives evolution.

Antibiotics can be lifesavers, but ... ow. There's this post on how antibiotics are often over-prescribed for ear infections.