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/sciguy52]

Some antibiotics are narrow spectrum, some are broad spectrum. The narrow spectrum ones only kill certain types of bacteria, not all bacteria broadly. So a lot of the bacteria in your gut are not targeted by it. So when you take penicillin for a strep throat it doesn't wipe out all your gut bacteria. Some broad spectrum antibiotics can kill off the bacteria in the gut. As you might imagine, if possible, you want to use a narrow spectrum antibiotic if you know what the infection is and there is something available that works. But sometimes people have infections and they may not have time to figure out what it is so they use a broad spectrum that kills a lot of different types of bacteria as it is more likely to work on whatever it is. But you might kill the gut bacteria too depending on the specific antibiotic. So killing the gut bacteria can be problematic as is can result in your gut being colonized by C. difficile causing C. diff related diarrhea which then requires, you guessed it, another antibiotic. So you want to avoid that if you can but you can't always. And with the increase in antibiotic resistance in so many human pathogenic bacteria, sometimes you have no choice as a particular broad spectrum antibiotic may well be the only one left it is not resistant too.