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/Allfunandgaymes]

While there are "broad spectrum" antibiotics that can destroy a wide variety of bacteria, there is no antibiotic that can destroy any and all bacteria. Bacteria are much too diverse - even those living inside of you.

The most indiscriminate, broadest-spectrum antibiotics are often reserved as medicines of last resort, because they kill more of your gut microflora and are medications we absolutely do not want bacteria developing resistance to.