premjan

isn't there some sort of ph difference within different parts such as the vagina, uterus, etc.?

Roland98

That's part of it. Additionally, recall that the cervix isn't only epithelial tissue, but it's also covered with a thick layer of mucus that's fairly difficult to penetrate. The sperm are hypermotile and self-propelled; most common vaginal bacteria are not. In some cases, sperm can actually carry bacteria into the uterus as "hitch hikers," and this can lead to infection (pelvic inflammatory disease), or simply death of the bacteria, depending on the dose of bacteria carried into the uterus, the immune and nutrition status of the woman, and a variety of other factors.

While GBS can indeed cause infection of neonates, it typically causes either bacteremia or meningitis, so it doesn't have much to do with having normal bowel flora at that point of the infant's life. And while vaginal carriage of GBS is often looked at in pregnant women, it also is a normal inhabitant of the bowel flora in many adults as well.

Prester John

The Cervix forms a narrow tube that is filled with mucous. This mucous fulfills the dual roles of preventing/reducing bacteria getting to the higher parts of the female reproductive tracts and allowing sperm through. There appear to be a number of differnt types of mucous with differing roles. Mucous production is sensitive to Hormones, so high eostrogen levels encourage production of mucous that make the cervix a nice place for sperm and high progesterone levels make it a bad place for sperm. Relativly speaking of course.

Its interesting that Chlamydia, probably the most common Sexually Transmitted Disease (STD) is an obligate intracellelular parasite, and does penetrate the cervix to cause (potentially)Pelvic Inflammatory Disease (PID) resulting (in some case) in fertility problems. Is it getting a ride up inside cells (spermatozoa?) and past the cervix's defences?

Amit Misra

So, fine: there are barriers b/w the vaginal lumen (plus epithelium) and the uterus. These can be classed into physiological (pH, composition of mucus secretion), morphological (the structure of the cervix) and immunological (the cytokine/chemokine profile and nature of cells patrolling the area).
All barriers, however, have a limit. They can be crossed, avoided or broken down. The spermatozoa are adapted to just that: overwhelm the barrier with sheer numbers, use the flagellum to wriggle past loopholes, use the prostaglandins in the semen to dialate the opening, etc., etc.
The interesting thing with vaginal flora, to my mind, is the standoff that they have established with the barrier(s). The barrier is vulnerable, but at the same time surprisingly strong: patients with vulvo-vaginitis do not necessarily have endometritis. It can be breached by ludicrously facile means: recall the polyfilament thread tail of Dalkon Shield. (I hasten to add that I'm not badmouthing the product: only pointing out that clumsiness in applying Dalkon Shield caused grief to thousands of women).
The fact remains, however, that for workable commensalism, the microbes have to know their limits. They can play around in the c***, but no infiltrating into the womb!

premjan

bad boy, you need to work on your bedside manner!

I suppose the shield was badly designed at that.

Amit Misra

Ah, but I never get to bedsides other than the matrimonial!
The shield story is quite intriguing. The thread that hangs out after insertion was made of several filaents, woven into a braid. It was demonstrated in the 1980s that fluid could rise (by capillary action AFAIR) all the way to the uterus, presumably carrying microbes into the no-no area. When they replaced the thread with monofilament yarn, things suddenly got better.