The Healthy Vaginal Microbiota
Multiple kinds of normal vaginal microbial communities are found in healthy women. The data provide strong evidence that more species than lactobacilli can dominate the vaginal microbial ecosystem of healthy women. The community function of maintaining low pH is highly conserved among women despite the difference in their vaginal microbial community composition and structure. These communities are postulated to provide different levels of protection against disease and infection, and their ability to offer protection may be lessened if the communities are disrupted. We propose that all vaginal microbial communities are not equally resilient and their stabilities differ in the face of disturbances. Moreover, differences in the resilience of various vaginal microbial communities may account for the differential susceptibility of races to HIV, BV and other urogenital infectious diseases.
Over the years, the definition of BV and the diagnostic criteria commonly used have been conflated, and they remain mired in controversy. The Amsel test, which is often used for the clinical diagnosis of BV, is based on four criteria: (a) a vaginal pH of >4.5, (b) the presence of clue cells, (c) a fishy odor upon addition of 10% KOH to vaginal discharge, and (d) a white, thin, homogenous vaginal discharge.139 The diagnosis of BV is made if at least three of these criteria are confirmed. The gold-standard for the diagnosis of BV in research and laboratory settings has been the Nugent score.140 This diagnostic test is a scored scale based on (1) the presence of Gram-positive rods (Lactobacillus morphotypes) (2) the presence of Gram-variable rods and cocci (Gardnerella vaginalis, Prevotella, Porphyromonas, and peptostreptococci morphotypes) and (3) the presence of curved Gram-variable rods (Mobiluncus spp. morphotypes). In a formal sense, an obvious potential problem is the logic of the Nugent score premise that high numbers of Lactobacillus spp. define “health,” and this imposes a bias against normal vaginal microbial communities that lack appreciable numbers of lactobacilli, yet maintain a low pH. The Amsel test, on the other hand, may lack sensitivity due to the subjectivity of the clinician’s interpretation. Reports comparing the two diagnostic measures arrive at opposing conclusions,141, 142 which has led many to suspect the accuracy of these tests. Hickey 2012
A second fallacy is directly tied to the notion that the vaginas of normal healthy women are populated by high numbers of Lactobacillus spp. This statement is accurate so far as it goes. However, the converse statement – that women whose vaginal communities have few or no Lactobacillus spp. – are abnormal is unsupported by data. We postulate that because of this logical fallacy, BV is often over-diagnosed. This could partly account for the reported high incidence of so-called ‘asymptomatic’ BV in reproductive age women,143, 144 and also explain a proportion of BV treatment failures and apparent recurrences of BV in women. Acknowledging that not all vaginal communities of healthy women are dominated by Lactobacillus spp. would also be in accordance with the observation that the vaginal communities of post-menopausal women (not taking hormone replacement therapy) often lack appreciable numbers of Lactobacillus spp., yet these individuals do not exhibit other untoward symptoms. We suspect that the causes and cures of BV will continue to be enigmatic until it is recognized that while ‘normal and healthy’ can be equated with high numbers of lactobacilli, the converse statement (“unhealthy” is equated with low numbers of or no lactobacilli) is not necessarily true. We must be vigilant and realize that for a significant proportion of women ‘normal and healthy’ can also occur in the absence of appreciable numbers of Lactobacillus spp.
Sexually Transmitted Infections