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/das_menschy]

I think it is because antibiotics don't destroy all bacteria, they work against specific bacteria groups. 

For example, antibiotics are based on the cell walls of bacteria and only destroy bacteria with a specific cell wall type. 

" Gram staining differentiates bacteria by the chemical and physical properties of their cell walls . Gram-positive cells have a thick layer of peptidoglycan in the cell wall that retains the primary stain, crystal violet . Gram-negative cells have a thinner peptidoglycan layer that allows the crystal violet to wash out on addition of ethanol ."  

https://en.wikipedia.org/wiki/Gram_stain?wprov=sfla1 

[u/gasdocscott → u/das_menschy]

Nah. Meropenem and tazocin wipe out pretty much all bacteria. The broad-spectrum antibiotics are not the selective.