Earlier quoted context omitted.
I’d guess that since they don’t die, they can go on to reproduce elsewhere.
But that's the thing, it's not very intuitive but selection pressure doesn't distinguish between dead and banished. Selection pressure means "instead of 100k bacteria, there are now only 100 bacteria that are the best at surviving this environment, and there is enough food for 100k, so they are going to grow back to 100k very quickly, all of them with the better survival skills"
Interesting point! I was thinking washed away germs remain in the gene pool so they aren’t selected out, but I guess the odds that they creep all the way back into the hospital to reproduce with the general population is quite low.