Eh, the bacteria in vaginas, or more appropriately the vaginal flora since it isn't just bacteria isn't as bad as you'd think. That's because, barring sexually transmitted pathogens, everything that's in there is already on all of us, and in our mouths, already.

Now, I'm not downplaying the possibilities of STD transmission. You can catch a handful of them via oral sex.

But the stuff that's in every vagina, everywhere, aren't pathogens in the vagina, on the skin, or in the mouth. You've got your basic assortment of yeast, a handful of lactobacilli, and the trace of staph. You'll sometimes have e-coli in the mix, but it can't really compete in that environment, it gets killed off. Only the proximity of the anus keeps it there, and the amounts are unlikely to cause problems what with the need of a fairly decent "dose" to get past the oral flora and immune response.

Simply put, if it isn't an STD (and a fairly limited group of those), you don't run into contagion via oral sex.

STDs area not part of the discussion because they specifically transmit in that way, and aren't in/on everyone the way the others are.

However! You can run into issues where the exact balance of flora can be a problem. It isn't unusual to get minor issues when playing with a new partner because the exact strains and proportions aren't the same. That's when you run into things like bacterial vaginosis, yeast infections, thrush (which is just an oral yeast infection), or the occasional strep infection.

So, with a new partner, if they aren't going to be long term, you'd be a fool not you use a barrier of some kind. The likelihood of an imbalance is usually higher for the vagina being exposed to a mouth or sexual organs, but it can go both ways. So, if they aren't going to me a regular partner, there's no point I letting the imbalance occur in the first place. With someone you'll be with for years, you might as well go without a barrier, be prepared for the imbalance to occur, and let your bodies' flora mingle and find a new balance.

Now, oral to oral contact, aka kissing, is actually similar. Our immune systems adapt to a small degree to whatever mix we have, and it takes something throwing the immune system out of order for something like thrush to set up shop. That is, until we get a load of some strange spit in our mouths. While it isn't common, you can end up with issues after kissing someone new. Thrush is the usual one that will rise up until your own mix of yeast and lactobacilli kick foreign ass and/or the foreign strains find balance new.

I have seen, or read about unusual things though. Obviously, strep and staph strains can cause problems in the throat and mouth. But, there's this nasty little fucker called streptococcus mutans that is particularly good at wrecking enamel via acid production. It is aided in that by lactobacilli, of course.

Sometimes, a foreign strain of the S mutans can set off a burst of tooth decay in a previously healthy set of teeth, even when good oral hygiene is practiced. The foreign version can, in those rare cases, slide by hygiene the same as your usual strain does, but the immune system takes a few weeks to address those leftovers, so you get some weakening of the enamel via the acid activity on hydroxyapatite. Well, I think that's the right mineral, if any dentists or hygienists come along and I'm remembering wrong, please correct me.

So, even kissing isn't risk free. Unfortunately, unlike analingus, felatio, and cunnilingus, there aren't any good barriers for kissing. Mouths just aren't a good space to design a disposable barrier around. Also unfortunately, even immediate brushing and use of mouthwash isn't going to guarantee you get all of the foreign strains.

There's a reason I advocate for the use of oral barriers during any "lingus" activity. If it isn't a regular partner, you're simply risking hassles. Doesn't matter what body parts and genitalia are going to be in contact with each other, condoms and dental dams are cheap and easy to use.