Clinical opinion
General gynecology
Contemporary perspectives on vaginal pH and lactobacilli

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Ever since the pH of the vagina was found to be much more acidic than blood and interstitial fluids, a belief going back more than a century has persisted that the vagina is protected from pathogenic organisms by the high level of hydronium ions present. A corollary of this belief is that the pH of the vagina and antipathogen activity is due to colonizing Lactobacilli. Unfortunately, this dogma lacks empirical research support. The vaginal pH is determined by the interplay between vaginal physiological processes and microbiology. An acidic vaginal pH and Lactobacilli are components of multiple defense mechanisms active in protection against infection in the lower female genital tract.

Section snippets

Vaginal pH and the microbial inhabitants of the vagina

Although the pH of the vagina undoubtedly influences the composition of the microbial ecosystem at that site, the magnitude of an acidic vaginal pH in effecting protection from invasion by pathogenic microorganisms, especially those that are sexually transmitted, remains an open question. Clinical vaginal infections by Candida species readily occur at a pH of 4.5 or less. It is also not generally acknowledged that sexually transmitted pathogens are acquired by transfer of the male ejaculate and

Vaginal mucosa vs Lactobacillus as the source of acidic pH

The vaginal mucosa of reproductive-age women is known to be an important source of lactic acid in the vaginal lumen. With its limited blood supply, the human epithelium must depend on diffusion of glucose, oxygen, and various essential nutrients from the underlying submucosal tissues. This metabolically precarious situation results in a relatively anaerobic environment within the vaginal mucosa. Thus, lactic acid is a significant metabolic byproduct of obligate anaerobic glucose metabolism

Vaginal mucosal immune interaction with microbes

In recent years, research has disclosed significant immune capability of the female genital tract to regulate vaginal microbes. PubMed lists more than 3000 research papers that have been published in recent years associated with the topic of vaginal immunology. Although immunology of the female genital tract remains a complex subject, comprehensive reviews have been published.26, 27, 28 Recent research has led to the identification of innate immune system antimicrobial components such as

Influence of pH on the vaginal epithelium

As a consequence of a relatively anaerobic environment within the mucosa, a mildly acidic intraepithelial pH reflects normal skin metabolic activity, including the production of various substances that are important for vaginal immune defense. In keratinized epithelium, stratum corneum acidity within the epithelium is known to be essential for a normal inflammatory response and optimal skin barrier function.33

This observation most likely also applies to vaginal mucosal epithelium. It is not

Clinical value of vaginal pH testing

Whatever the mechanism for its occurrence, measurement of vaginal pH can be of clinical value as an initial screening tool in assessing altered vaginal physiology. A change to an elevated vaginal pH in an individual woman in the absence of recent exposure to semen, menstruation, or exogenous products indicates an alteration of her vaginal ecosystem. This might signify a transition to a bacterial vaginosis (BV)-like flora, the presence of aerobic vaginitis,39 or a sexually transmitted infection

Future directions

Whereas an alteration in vaginal pH to less acidic values is strongly associated with vaginal flora changes it remains unclear in some situations whether the pH change is a primary event or a secondary consequence of an altered microbial ecosystem. T vaginalis, C trachomatis, and N gonorrhoeae infections induce host cell changes (cervical bleeding or transmucosal water loss) and thereby secondarily raise the vaginal pH. Vaginal infections by Candida species do not affect the underlying vaginal

Summary

History presents Lactobacillus as the prominent determinant of vaginal pH as well as a major regulatory factor of microbial composition of the vaginal flora. This reductionist thinking negates the complexities of mucosal metabolism, immune defenses, and microbial colonization. Contemporary research suggests that classic ideas about pH and vaginal flora are not entirely correct. Acid pH may be influenced by estrogen-regulated mucosal metabolism to a greater extent than glycogen-mediated

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    Cite this article as: Linhares IM, Summers PR, Larsen B, et al. Contemporary perspectives on vaginal pH and lactobacilli. Am J Obstet Gynecol 2011;204:120.e1-5.

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