Women’s vulvovaginal microbiota: how can it help in clinical practice?

Historical perspective

The beginning of the long journey towards the understanding of the vaginal microbiome can be attributed to Albert Döderlein, at the end of the 19th century. In his book Das Scheidensekret und seine Bedeutung für das Puerperalfieberhe pointed out that “normal”, healthy women have the vagina dominated by Grampositive bacilli, which he named Lactobacillus acidophilus.

This concept still shapes contemporary interpretations of the vaginal microbiome, but the reality is probably far more complex.

Understanding of vaginitis is still incomplete, and its management mostly empirical, despite being one of the most common causes for  women to seek a medical appointment¹.

In 2011, Ravel et al. published one of the most important and mind-changing papers in terms of understanding of the human vaginal  microbiome. In that paper they demonstrated that asymptomatic is not synonymous with “normal” (leading also to the question of what 
is a “normal” – or, probably more accurate, “optimal” – vaginal microbiome) and that there are striking differences according to ethnicity².

Diversity is the rule in nature, but the human vagina seems to be an exception: the accepted “optimal” vaginal microbiome is dominated by one or two species of lactobacilli (low alpha diversity). If we think about other organs or anatomical regions, dominance by one species 
is usually synonymous with disease (infection). If we perform the same exercise considering any ecological system, it represents the last 
step before collapse (e.g. monocultures of plants never occur in nature, and when performed artificially they must be limited in time). We can look for further explanations for this apparent “abnormality” (or “exception”) in nature, but it does seem to lead to a dead end. 

Should we, instead, change the focus of our lens and investigate gene pools rather than species or genera to overcome this apparent 
biological abnormality

The ultimate goal of living beings seems to be passing genes to the next generations and evolution seems to be much driven by this 
primordial “instinct”. Therefore, we can easily assume that the human vaginal microbiome should be a fulcral part of the end-product 
of evolution to optimise the reproductive process. If this premise is correct, we can expect

1. evolutionary congruence (as has been shown, for instance, in the gut);
2. any differences should be more or less easily explained (mating process, diet, geographical location, etc.) and, naturally
3. higher similarity in closely related species.

Surprisingly, none of the three premises are satisfied. In nature, phylogeny cannot be related with the vaginal pH (a very indirect marker of the vaginal microbiome composition), and dominance by lactobacilli is unique to the human species. Even when comparing humans with other primates, the differences are huge and, currently, not easily explainable^3,4. What made the human vagina so unique? Was it the fruit of random events or the evolutionary corollary of the continuous ovarian cycle, high risk of lacerations and infection at birth, or agriculture and the consequent high consumption of starch³?

The microbiome and pregnancy

One issue seems to be beyond doubt: lactobacilli are fundamental for the success of pregnancy – but it is not so clear if the same 
applies to achieving a pregnancy⁵.

The available data clearly show that a vagina not dominated by lactobacilli during pregnancy is associated with negative obstetrical and puerperal outcomes, including preterm labour, premature rupture of membranes, and puerperal infections (Figure 1). Of note, one million babies die every year from prematurity-related complications⁶.

We are used to repeating that lactobacilli have a protective role and that their presence is desirable, but to assume that means we must ignore some obvious facts, such as that this dominance does not occur in children, during breastfeeding, nor in postmenopausal women. Therefore, we can theorise that our symbiotic relation with lactobacilli serves us a purpose during the reproductive years. We can consider that this purpose includes a reduction in the risk of sexually transmitted infection (STIs) (which pose a risk to the success of reproduction and to the offspring), of ascending genital infections (and consequent abortion, stillbirth, and preterm birth), as well as of puerperal complications. The role of the microbiome in achieving pregnancy seems to be more limited. For instance, populations with high rates of vaginal dysbiosis do not seem to be less fertile⁷

Figure 1. Vaginal lactobacilli dominance is associated with beneficial obstetrical and puerperal outcomes

In the same way, the impact of the cervicovaginal microbiome on the outcome of fertility treatments is also unclear⁵.

One of the biggest evolutionary differences between humans and other mammals has to do with delivery – the difficult balance between being born with a large cephalic perimeter and negotiating it with a pelvis that had to adapt to bipedalism. Humans have the most difficult deliveries – perhaps surpassed only by hyenas. Can this hold the key to understanding the uniqueness of the human vaginal microbiome? Whatever the evolutionary purpose was, for most women of reproductive age, even out of pregnancy, the dominance of lactobacilli in the vagina is the desirable state. But lacking lactobacilli, despite representing a dysbiotic state, is not synonymous with disease.

Our understanding of the role of the vaginal microbiota is still very limited. Even apparently simple questions, such as how lactobacilli
colonise the vagina still does not have a clear answer.

The vulvovaginal microbiome in heath and disease

The most noticeable effect of an altered microbiome is vaginitis. Most women will suffer at least one episode of candidiasis and, in some populations, more than half of reproductive-aged women have bacterial vaginosis (BV), asymptomatic most of the time (figure 2A and 2B). We have a limited understanding of what drives these shifts (“normal” – colonisation/asymptomatic state – symptomatic)⁸.

Figure 2. Microphotographs of wet mount microscopy preparations (phase contrast 400x)

(A) Normal lactobacilli, presence of Candida spp. blastospores (circle); (B) Bacterial vaginosis
(absence of lactobacilli and overgrowth of anaerobic and facultative bacteria).

However, the bacterial profile of the vagina,regardless of symptoms, may confer differentprofiles of risk or protection. In general, it is considered that Lactobacillus spp. tendto confer health benefits. However, not all species are equal and only a limited number of the existing species are usually found in dominating states in the vagina. L. iners, with a significantly smaller genome and different metabolic profile, is usually associated with dysbiotic or transition states⁹.

Table 1. Gynaecological and obstetrical conditions and their associations with the vulvovaginal microbiome.

While there are no current recommendations to treat asymptomatic dysbiosis (e.g. BV), it has been associated with obstetrical and non-obstetrical complications (including risk of acquiring STIs [HPV, HIV])⁸ (Table 1).

Once efficacious (preferably non-antibiotic) strategies are available, it may be advisable to screen and treat dysbiosis in women at increased risk of STIs or even in those infected with HPV. This, however, may prove more complex than it appears. The STI-like behaviour of BV has long been acknowledged, but recent data have confirmed it, as well as suggesting that reduction of recurrences may require treatment of partners, which may represent a great obstacle to prevention strategies¹⁰.

BV is a common syndrome, in which there is depletion of lactobacilli and an overgrowth of several strictly and facultative anaerobic bacteria, associated with formation of a biofilm that seems to contribute to the frequent relapses after treatment. The composition of BV is variable from woman to woman – and probably even in the same woman over time¹¹. Currently it is possible to diagnose BV using molecular tests, but it is expected that with the increasing knowledge of the vaginal microbiome, these tests will allow for the “profiling” of BV, the evaluation of the resistome, and a more rationale choice of treatments⁸.

The relationship between Candida spp. and the vaginal microbiome is very complex and far from fully understood. While candidiasis can exist with any vaginal microbiome, it tends to be more common with lactobacilli dominance (and the consequent low pH)¹².

Several gynaecological conditions have been associated with specific microbiome characteristics and, almost systematically, a reduction in lactobacilli confers an increased risk for STIs and gynaecological cancers (even of the upper genital tract). However, a causal relationship between the microbiome deviations and the specific conditions is not always straightforward. We can, however, expect to one day assess or modulate the risk of cancer through the evaluation of the vaginal microbiome – especially for cervical cancer¹³.

The interest and knowledge in the vulvar microbiome is more recent and the amount of data is scarce, but its role in entities such as vulvodynia, vulvar dermatoses, vulvar intraepithelial neoplasia and cancer is being studied^14-16.

What’s next?

Giant strides are being made towards the understanding of the vulvovaginal microbiome. Until we fully understand the microbiome, we can start by respecting it and its functional role, recognising that each woman is unique (and that this uniqueness is mutable), avoiding
unnecessary use of antibiotics and antiseptics, and properly diagnosing STIs and vaginitis, rather than relying on empiricism. Accurate diagnosis will minimise wrong treatments, with a potential long-term impact.

At this stage, it is essential to distinguish what is investigational and what is clinically relevant. We are in a process of learning and attempting to use investigational techniques and concepts in clinical practice that often leads to unnecessary testing, expenses, and
treatments – for instance, metagenomics is a very useful research tool, but it currently has no place in the clinical evaluation of vaginitis.

In the last two decades we have amassed a huge quantity of information, which will soon translate into better health care for women, including specific dietary recommendations, and pre- and probiotics. We can expect that this knowledge will substantially reduce preterm labour, gynaecological cancers, as well as the recurrence of vaginitis and cystitis.

The next chapters will undoubtedly be the most exciting ones!