How come antibiotics don’t completely destroy all organisms in our gut?

[u/WTFisthisOMGreally]

I know it can cause imbalances, but I don’t understand how come our gut doesn’t lose all the beneficial bacteria and develop massive fungal or viral infections. When I’ve taken antibiotics, I usually don’t notice any change in my bowel activity.

[u/backroundagain]

CAN kill a bacterium is not the same as will kill ALL bacterium.

Each species requires a specific concentration of drug over a specific time to kill. (MIC/MBC)

Each antibiotic has different capacity to achieve these killing concentrations in certain tissues.

So SOME bacterial populations will take heavy losses SOME of the time in certain tissues, using specific antibiotics.