Here's a post I wrote last year that I am rather proud of for two reasons. First, I think it is well written and makes the relevant points effectively. Second, it pissed off a idiot and ended up spawning several follow up posts. Regardless, I noted that this post is receiving a bunch of traffic because of Powerball. So without further ado...
Discussions on the interface between Science and Society, Politics, Religion, Life, and whatever else I decide to write about.
Thinking about the vagina
I spent the better part of
several days last week discussing a fungus that lives in the vagina. I invited
a seminar speaker, who studies Candida
albicans colonization of mucosal surfaces, which includes the mouth,
intestine, and vagina, among other sites. C.
albicans is a normal part of the human flora that does not appear to be
detrimental or beneficial (a commensal).
During our discussions, which usually took place a local watering holes,
it became clear to me that different mammalian species can vary markedly when
it comes to the indigenous flora and ultimately the physiology of the vaginal
tract.
In women, the vagina is a
fairly acidic environment. This is due to the action of Lactobacillus, a
bacterium that grows well in the vaginal tract. Lactobacilli generates copious
amounts of lactic acid from the metabolism of complex sugars. Lactic acid, as
the name suggests, is acidic. The acidic pH in the vagina, due to lactic acid
accumulation, inhibits the growth of most other microbes with the notable exception
of C. albicans, a fungus. Fungi are
extremely tolerant of acidic environments, at least in comparison to many
bacteria.
Although C. albicans can grow reasonable well at
the pH found in the vagina, it’s growth is limited by the vast abundance of
Lactobacilli. Nutrients are not unlimited, so the faster growing Lactobacilli
outcompete the slower growing C. albicans.
Not surprisingly, if the Lactobacillus population is reduced, for example by
broad spectrum antibiotic use or douching, then C. albicans can flourish, which can result in a yeast infection.
(It can also lead to bacterial vaginosis, but this is less common.)
What is interesting, at
least to me, is that vaginal pH of a female mouse is neutral. Based on this bit
of information, you may not be surprised to learn that the murine vaginal tract
is not colonized by Lactobacillus (therefore no lactic acid made and no vaginal
acidic pH). Since the murine vaginal pH is not as acidic as a woman’s, you
might expect C. albicans to do well
there (as observed in women with a less acidic vaginal pH). However, C. albicans is not a normal colonizer of
the mouse vagina. That being said, we can infect the murine vaginal tract in
the lab with C. albicans. Indeed,
when we do this, it appears that mice respond similarly to C. albicans as women with a yeast infection. What this tells us is
that in the vaginal trac, C. albicans
does not appear to be concerned with the environmental pH in regards to disease
symptoms. This actually makes sense as recent studies have shown that women who
have recurrent vaginal yeast infections (3 or more incidents a year during
their child bearing years (yes hormones play a big role here)), the problem is
how their bodies respond to C. albicans
not due to anything specifically the fungus is doing.
To someone who works on
how C. albicans responds to
environmental pH, these results may seem disappointing. However, I for one find
them intriguing. One primary reason we study the C. albicans response to environmental pH is that environmental pH
has a dramatic effect on how this fungus grows. In acidic environments, C. albicans grows as a yeast
indistinguishable from the Bakers yeast (Saccharomyces
cerevisiae) you can buy in the grocery store. In neutral or alkaline
environments, C. albicans grows in
the hyphal form (analogous to the hairy mold growing on the bread you baked but
left sitting around too long). Besides being a striking phenotype, the
transition between the yeast and hyphal growth forms is critical for disease
and presumably for colonization as a commensal.
In summary, the ability to
switch between the yeast and hyphal growth forms is critical for disease and
environmental pH directly controls morphology. The problem or point of interest
is that the environmental pH in the vagina of two distinct mammals varies
markedly yet disease appears similar. This suggests that other environmental
factors contribute to morphogenesis (true) and/or that within the vaginal tract
C. albicans morphology is not
important (potentially true). Support for this latter idea comes from Candida glabrata, a yeast that commonly
colonizes the vaginal tract that is closely related to S. cerevisiae (the one you can buy in the supermarket). The key
here is that C. glabrata only grows
in the yeast form and is readily able to cause yeast infections.
A couple of accessory points. 1, A picture may be worth a thousand words, but some posts are not amenable to google searches; 2, You can get some interesting looks from patrons of the local watering hole discussing the microbial contingent of the vagina.