Normal vaginal flora has an acidic pH, Lactobacillus being responsible for the same. Candida thrives in acidic medium. Antibiotic administration causes decrease in acid forming Lactobacilli. How one explains presence of candidiasis and decrease in acid forming Lactobacilli, which seem contradictory statements.
Correct pH is only one factor that any organism needs to colonize an environment. In addition to lowering the pH of the vagina through their metabolism of glycogen, Lactobacilli also compete with Candida for binding sites, and for nutrients. Some Lactobacilli also make H2O2, which kills other organisms (both some Candida, and also other infectious agents which could damage the vaginal epithelia and potentially make Candida co-infections more likely).
In practice, where microscopy is performed and vaginal pH measured with a view to establishing a diagnosis of Bacterial vaginosis and candida is serendipitously identified, there appears to be no corelation between vaginal pH and the presence of Lactobacilli with the diagnosis of vaginal cadida. In other words, Candida could be present in the vagina even with normal lactobacillus population and a vaginal pH in the acidic range. I think what prtobably happens in cases of vaginal candidiasis induced by broad spectrum antibiotics is not so much the change in the vaginal lactobacillus microbial flora but an interference with the gut microbial flora by the broad spectrum antibiotics. This may result in the loss of the fine balance that exists between the bacterial flora and the candidal reservoir in the gut resulting in the overgrowth of candidal reserovior in the bowel. This may lead to the excessive spillage of candida onto the perianal area and its eventual migration into thye vagina. Additionally there may be an iterference by the broad spectrum amntibiotics on the immunological mediation of the parameters that keep the candidal population in the gut from overgrowing in the normal state, which is as yet uninvestigated.
Candida thrives in both an acidic medium and an alkaline medium, the difference being the form that it assumes. In an acidic pH, you're more likely to find the yeast form, and in an alkaline pH, the hyphal/fungal form. Vulvovaginal candidiasis is a diagnosis associated with the fungal form. Once antibiotics have eliminated or reduced Lactobacillus species, the pH can shift allowing for candida's morphogenesis. Candida can then inhibit the regrowth and recolonization of Lactobacilli.
The production of hydrogen peroxide by bacteria is thought to play a role in the formation of hypothiocyanate against candida, as opposed to H2O2 alone. Candida itself is able to produce H2O2.
Dear Jeff,
How does one explain presence of Vulvovaginal candidiasis in pregnancy, wherein vaginal pH is acidic? Is it the yeast form of Candidiasis? Is there any difference in the severity of infection between two forms of candidiasis? If presence of bacteria is adversely affecting growth of candida because of H2O2, what happens to the effect of H2O2 produced by candida itself?
Candida can exist as the yeast form. Studies are mixed on its presence during pregnancy -http://www.ncbi.nlm.nih.gov/pubmed/23689173. The yeast form is usually asymptomatic and commensal. The hyphal form is much more symptomatic.
Candida has shown an adaptive ability towards expression of H2O2 in its environment -http://www.ncbi.nlm.nih.gov/pubmed/14587100, http://www.ncbi.nlm.nih.gov/pmc/articles/PMC1265892/ I believe the production of H2O2 by candida is more minimal and designed to affect WBCs -http://www.ncbi.nlm.nih.gov/pubmed/6299966,http://www.ncbi.nlm.nih.gov/pmc/articles/PMC550832/.
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