Protected anal sex prevents STI’s

Patient: Is it possible that bacteria naturally found in the colon could cause an infection after unprotected anal sex? I was tested before I had sex with my boyfriend and I did not have any STIs, he was tested before me and he too was clean, but now he is showing symptoms that are typical of the bacterial STIs, and we don’t know what has happened.

Doctor: You need to figure out now what kind of bacteria causes this very symptoms and then get a proper treatment. And anyway, do not forget to protect yourself.