Part of the reason ATM is so popular in porn is for this very reason; the likely hood of fecal matter (even if so small you can't see it) getting into the girls mouths.

let's be honest, any girl who is doing gonzo style porn is not too concerned about germs, fecal matter, drops of urine, or stuff from the floor. It has tone down a lot these days but in the glory years of gonzo the girls were licking cum, that fell from asses onto dirty floors (and thus would have some fecal matter in it even with the best enemas). The big prolapsing craze now has girls licking these things over and over again; I remember sheena shaw saying they smelled like shit and blood, but they lick them with glee.

There is no care about germs and bacteria from any girls doing gonzo. They just don't think about it, do the job and get paid. The nastiness of it turns some of them on as they do get to a point where 'normal sex" can't do it for them so they have to be doing stuff like this just to get a rush.