The vaginal microbiome Lactobacilli clusters Pregnancy Menopause Other links
There has been increasing interest in the normal human microbiological environment over recent years. Previous studies have been based around culture of the organisms which showed micrococci, haemolytic streptococci, diphtheroids, lactobacilli, Staphylococcus aureus, and coagulase negative staphylococci on the vulva. With advances in molecular techniques, the ability to demonstrate many new and different species has shown distinct microbiomes that vary with site. The microbiological environment is a balance between resident and transient organisms. Resident organisms are ones that live at a site and multiply there, whereas transient ones come from another source but do not stay. The vaginal microbiota has been studied intensively, but far less is known about the normal microbiology of the vulva. However, the vaginal microbiome is important in the management of vulval disease as the vulva can be affected by changes in the vaginal environment.
The vulval microbiome
The vulva is a unique site as it is a junction between keratinised skin and non‐keratinised mucosa. It is also close to areas that have their own distinct microbiome such as the urethra, anus, and inguinal fold. Contamination with transient organisms from these sites can therefore complicate the assessment of what is normal for the vulval micro‐environment. The normal microbiome is important as disruption will lead to an increase in organisms that may cause bacterial vaginosis, candidiasis, and urinary tract infections. The vulval microbiome is complex and diverse. In a series of four women studied with targeted 16S rRNA gene studies, the microbiota was different in each case, and no single organism was common to all individuals [1]. In a previous study of 39 healthy women, microbial counts were significantly higher on the vulva than on the forearm, with a greater incidence of S. aureus (67% versus 11%) [2].
The microbiome is unique to the individual but can be influenced by the local environment and host factors such as menstruation, pregnancy, and menopause. Hygiene practices, the oral contraceptive, and spermicides will all have an effect. The vulval micro‐environment can be influenced by the use of sanitary products, but vapour‐permeable panty liners did not seem to have a significant effect as opposed to those that are non‐permeable [3].
Variations in site
The human skin microbiome has distinct patterns at different sites [4]. The inguinal fold is an occluded site with increased temperature and humidity. The commonest organisms at these sites are Corynebacterium species and S. aureus as they thrive in the moist conditions found here.
Moving to the inner vulva, differences between the labia majora and labia minora have been demonstrated [1]. The microbiological environment on the labia majora was similar to that of the skin, with S. epidermis and Corynebacterium species predominating, but Enterococcus faecalis was also present. Lactobacilli were common on the labia minora, as in the vagina, but Finegoldia was an additional finding. In terms of bacterial adherence, the epithelium of the labia majora is generally more amenable for adherence than that of the labia minora [5]. Enteric bacteria can pass to the vulva, and numbers of these fall gradually from perineum to the vestibule and then to the vagina [6].
Variations with physiological changes
The microbiota can vary with the menstrual cycle. An effect on the diversity of organisms is seen in 60% of women at different times in the cycle, but there is little effect on overall bacterial counts [7]. In an earlier study using culture techniques, there was no significant difference in the microbiota on the labia majora measured on days 2, 4, and 21 of the menstrual cycle [8].
Body mass index (BMI) may also have an effect. When comparing the influence of obesity on the microbiome of the vulva and abdominal folds, there were changes on the vulva with increasing BMI but not on the abdomen [9]. Those with a raised BMI had a higher pH with a predominance of Finegoldia and Corynebacterium, but lactobacilli made up the majority of organisms found in those with a normal BMI.
Vulval and vaginal atrophic changes seen after the menopause are associated with a relative reduction in lactobacilli species and an increase in Streptococci and Prevotella [10].
Variations with age
Contrary to previous theories about the changes that occur at puberty in the vaginal microbiome, a study of pre‐pubertal girls showed that the vaginal microbiome was similar to that of healthy adults [11]. It was possible to classify lactobacilli into specific dominant clusters at least 12 months prior to menarche. Gardnerella vaginalis was commonly seen as part of the normal vaginal flora in those who had no symptoms of bacterial vaginosis or prior sexual contact. This study also looked at vulval organisms, and several of the vaginal flora were present on the vulva but the species Segniliparus, Murdochiella, and Fusobacteria were more commonly found on the vulva.
The vaginal microbiome
The predominant organisms in the healthy vagina are lactobacilli, and it is recognised that this is the best environment for vaginal health. The vagina has many defence mechanisms against pathogenic organisms. The rapid turnover of the lining squamous epithelium does not allow any organisms adherent to this layer to take hold as they are shed and hence prevented from ascending to the higher genital tract. These cells are also able to release antimicrobial peptides and cytokines that stimulate phagocytosis [12]. Lactobacilli protect the vagina by reducing the pH via lactic acid production, but they can also produce other substances such as hydrogen peroxide which inhibit growth of pathogenic bacteria [13]. If there is any change to the equilibrium, then this allows growth of G. vaginalis, C. albicans, and E.coli, which can cause symptoms in both the vulva and vagina. Environmental factors, the use of antibiotics, practices such as douching, and the use of sanitary products can all affect this balance [14].
Lactobacilli clusters
There are four types of lactobacilli which are present in the vagina: L. crispatus, L. iners, L. gasseri, and L. jensenii. Another cluster is recognised where there are lower numbers of lactobacilli and an increase in anaerobes [15]. There is variation in the clusters and also vaginal pH with ethnicity [15, 16]. L. crispatus and L iners are more common in pre‐menopausal women.
Pregnancy
During pregnancy, there are generally higher levels of lactobacilli [17] but the microbiome is relatively stable throughout [18]. However, there is a link of preterm birth with lower levels of lactobacilli [19]. If there is a raised pH of greater than 4.7, this is a fairly good indicator of reduced lactobacilli. This can be measured during pregnancy, and it has been shown that treatment with antibiotics or probiotics can reduce the incidence of preterm birth in this group [20, 21].
Menopause
After menopause, oestrogen levels fall, leading to reduced glycogen levels and therefore decreased lactic acid production. The pH will increase, but hormone replacement therapy can reverse this and increase the lactobacilli numbers [22].