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EDITORIAL REVIEW

The vaginal microbiota and susceptibility to HIV

Buvé, Annea,*; Jespers, Vickya; Crucitti, Taniab; Fichorova, Raina N.c,*

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doi: 10.1097/QAD.0000000000000432
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Abstract

Introduction

Differences in the epidemic spread of HIV between populations where the predominant mode of transmission is sexual intercourse can be ascribed to diversity in sexual behaviour patterns and to variations in biological and behavioural co-factors that enhance the HIV transmission probability per sexual act [1,2]. Co-factors that have been implicated in HIV transmission include the following: the stage of disease in the HIV-infected partner and his/her treatment with antiretroviral drugs; the presence of another sexually transmitted infection (STI) and bacterial vaginosis; condom use and anal intercourse; male circumcision and hormonal contraception. A commonly used estimate of the HIV transmission probability per heterosexual act, in the absence of co-factors, is 0.001. This estimate was derived from studies on HIV-discordant heterosexual couples in high-income countries. Recent meta-analyses found that the risk of HIV infection per heterosexual act is higher in low-income countries than in high-income countries, and it is not clear whether these discrepancies can be explained by variations in the prevalence of known co-factors [3,4]. The authors of the meta-analyses concluded that more research is needed to explain these divergences.

We hypothesize that variations in the composition of the vaginal microbiota may be at least partially responsible for population differences in HIV transmission probability. Here we discuss data that are in support of this hypothesis.

The vaginal microbiota: what is healthy and what is not healthy?

Until about a decade ago, studies of the vaginal microbiota relied on culture techniques. Lactobacillus species were found to be the dominant bacterial species in the healthy vaginal microbiota, ‘healthy’ being defined as the absence of vaginal discharge or other symptoms of vaginitis and a low pH. The development of molecular techniques and especially pyrosequencing has increased our understanding of the complexity of the vaginal microbiota. It appears that there is no single ‘healthy’ vaginal microbiota formula, but that a variety of compositions of the vaginal microbiota can be found in healthy, asymptomatic women of reproductive age. According to two studies, a sizeable proportion of healthy women (20–27%) have a vaginal microbiota that is not dominated by lactobacilli [5,6]. Furthermore, those studies found that vaginal microbial communities that are not dominated by lactobacilli were more frequently found in African-American and Hispanic women than in Caucasian and Asian women [5,6]. Important variations also exist in the vaginal microbiota that are dominated by Lactobacillus spp. Ravel et al.[5] distinguished five community groups, four of which were dominated by Lactobacillus spp., including L. crispatus (group I), L. gasseri (group II), L. iners (group III) and L. jensenii (group V).

Prospective studies of women who frequently took a vaginal swab found more or less large variations in the composition of the vaginal microbiota over the course of the menstrual cycle [7–9]. During menses, the concentration of Gardnerella vaginalis appears to increase, whereas the concentration of L. crispatus decreases, suggesting that during menses, women are especially vulnerable to disturbances of the vaginal microbiota. A possible explanation would be that oestrogen levels and glycogen production are at their lowest during menses, which would hamper the growth of the lactobacilli, whereas the iron in the menstrual blood stimulates the growth of G. vaginalis[7]. Likewise, changes in the oestrogen levels and related vaginal mucosal changes may be responsible for variations in the vaginal microbiota over the lifetime of women. Pre-pubertal girls and post-menopausal women have a vaginal microbiota that is characterized by less lactobacilli as compared with women of reproductive age [10].

Clinical criteria are used to distinguish between healthy vaginal microbiota and disturbed vaginal microbiota. Bacterial vaginosis, worldwide the most common cause of vaginal symptoms, is usually diagnosed in clinical settings using Amsel's clinical criteria [11]. In research settings, the scoring system developed by Nugent et al.[12] is considered the gold standard for the diagnosis of bacterial vaginosis. It consists of semi-quantifying the lactobacilli and bacterial vaginosis-related bacterial morphotypes in a Gram-stained smear of the vaginal fluid. Each morphotype is scored according to the number observed per field. The sum of the different scores is interpreted according to a scale ranging from normal (score = 0–3) through intermediate (score = 4–6) to bacterial vaginosis (score = 7–10). The ‘intermediate vaginal microbiota’ is characterized by a decreasing number of lactobacilli and an increasing number of anaerobes. This condition is transient and can revert back to a normal microbiota dominated by lactobacilli, or evolve to bacterial vaginosis with disappearance of the lactobacilli and replacement by anaerobic bacteria [13]. Using molecular techniques, a wide range of microorganisms have been detected that are associated with disturbances of the vaginal microbiota, in particular bacterial vaginosis. In fact, bacterial vaginosis is also known as polybacterial dysbiosis because of the higher bacterial diversity associated with a higher Nugent score [5,14,15]. The microorganisms found to be associated with bacterial vaginosis include G. vaginalis, Atopobium vaginae, Prevotella spp., Leptotrichia spp., Megasphaera spp., Eggerthella spp., Dialister spp., Bifidobacterium spp., Slackia spp. and bacteria of the Clostridium phylum (BVAB1, BVAB2, BVAB3) [16–19]. Of these microorganisms, G. vaginalis has been proposed as sexually transmitted, causative agent of bacterial vaginosis [20]. This seems to contradict studies that have found G. vaginalis to be present in women without bacterial vaginosis as assessed by the Nugent score [5,9,21] unless one accepts that there are differences in virulence between different strains of G. vaginalis[22].

In conclusion, we do not yet have clear criteria to determine what is a healthy vaginal microbiota. Ma et al.[23] propose to further characterize the function of the vaginal ecosystem and to define disturbances or ‘unhealthy’ vaginal microbiota on the basis of functional criteria.

The protective role of the normal vaginal microbiota

The healthy vaginal microbiota plays an essential role in the natural defence system against HIV and other STIs. Several mechanisms have been identified or postulated, including antagonisms between microbiota constituents, direct microbicidal action of bacteria-derived substances, and bacterial-host cross-talk leading to changes in the host immune function.

Cumulative clinical and experimental evidence suggests that a group of vaginotropic Lactobacillus species hamper bacterial colonization and survival of urogenital pathogens, for example, Escherichia coli, Candida albicans, Staphylococcus aureus, group B streptococci and Neisseria gonorrhoeae, by producing synergistic microbicide factors such as H2O2, bacteriocins and lactic acid, and likely more mediators yet to be identified [24–28]. The lack of H2O2-producing lactobacilli associated with bacterial vaginosis has been regarded as a particular risk of HIV acquisition and transmission given the fact that H2O2, especially in combination with host-derived peroxidases and halides, is virucidal to HIV in vitro[29]. However, the powerful microbicidal properties of H2O2 and its efficiency in suppressing the growth of bacterial vaginosis bacteria are reduced in the context of human vaginal fluid and semen, possibly due to naturally occurring antioxidants [30,31]. In contrast, the lactic acid has powerful microbicidal properties against HIV and bacterial vaginosis bacteria that are maintained in the context of the vaginal fluid as long as the pH is acidic [30,32,33].

In addition to their direct microbicidal properties, some of these Lactobacillus-produced factors stimulate signalling and immune responses by the cervical and vaginal epithelial cells. For example, the L-isomer of lactic acid, produced by a number of vaginal bacteria and the human vaginal epithelial cells, correlates with metalloproteinase levels in the vaginal secretions in vivo[34]. In vitro, it stimulates the IL23/IL17 lymphocyte pathway in the presence of lipopolysaccharide and acidic pH [35], and also enhances immune responses to viral RNA by the vaginal epithelial cells [36]. Lactobacilli also have been shown to suppress various checkpoints of pro-inflammatory signalling by intestinal epithelial cells by cross-talk mediators yet to be defined [37–39].

The microbiota that is classified as bacterial vaginosis and intermediate flora by Nugent scoring has been associated with a myriad of disturbances of the vaginal innate immunity [40–42]. In experimental models, in contrast to dominant vaginal lactobacilli, bacterial vaginosis-associated bacteria G. vaginalis, A. vaginae and Prevotella bivia trigger pro-inflammatory cascades including nuclear factor–kappa B (NF-kB) and differential downstream up-regulation of cytokines and chemokines by human vaginal and cervical epithelial cells [43–47]. Importantly, P. bivia has immunosuppressive effects, reducing levels of immunoregulatory chemokines, whereas A. vaginae and G. vaginalis amplify inflammatory chemokine responses to other STI pathogens, for example, Trichomonas vaginalis and its endosymbiont viruses [45]. These pro-inflammatory and immunosuppressive effects of the bacterial vaginosis-associated bacteria could enhance the risk of HIV transmission per sexual act by recruitment and activation of CD4+-positive HIV host cells at the mucosal interface and by direct up-regulation of the HIV replication cycle [48–50].

The experimental evidence of immune modulation by vaginal bacteria is supported by growing clinical evidence. In addition to the increased levels of cytokines that can promote HIV replication, for example, IL-1β, bacterial vaginosis has been associated with increased vaginal levels of heat shock protein (hsp)70 and correlating increased levels of nitric oxide, known to be stimulated by hsp70 [51]. Nitric oxide is a double-sword immune factor with protective wide-range microbicidal activity, and also with unwanted tissue damaging, pro-inflammatory and immunosuppressive effects [52].

Among the most concerning effects of bacterial vaginosis is the suppression of protective innate immunity effectors and, in particular, the secretory leukocyte protease inhibitor (SLPI) [40,53]. SLPI has direct HIV inhibitory properties in vitro[54]. Vaginal SLPI levels are reduced in women with STIs [40,55,56] and reduced levels of SLPI are associated with higher probability of HIV acquisition [57]. Thus, apart from directly suppressing SLPI, by increasing the risk of STIs, bacterial vaginosis has an additional indirect negative effect on SLPI and the associated risk of HIV acquisition.

Disturbances of the vaginal microbiome and transmission of HIV and other sexually transmitted infections

Since the 1990s, numerous epidemiological studies have explored the association between bacterial vaginosis and HIV infection. In 2008, a meta-analysis was published which included 23 papers up to the end of 2005 [58]. Three of these papers reported on longitudinal studies that could establish the temporal relationship between bacterial vaginosis and HIV acquisition. The summary relative risk of incident HIV infection associated with bacterial vaginosis was estimated at 1.6 [95% confidence interval (CI) 1.2–2.1]. Using the same key words as Atashili et al. we searched the PubMed and retrieved another five papers that were published after 2005 and that reported on longitudinal studies (Table 1). One of these publications was a meta-analysis of data from 13 prospective studies [59]. All studies found a statistically significant association between incident HIV infection and bacterial vaginosis at baseline or a visit close to the HIV-seroconversion visit. The adjusted hazard ratios ranged from 1.5 to 2.1 [59–62]. Three of the studies also considered intermediate flora, defined as a Nugent score of 4–6 on a Gram-stained vaginal smear, and in two of these studies acquisition of HIV infection was significantly associated with intermediate flora with adjusted hazard ratios of 1.4 and 2.0 [59,61,62].

T1-1
Table 1:
Effect of bacterial vaginosis/intermediate flora on the risk of HIV acquisition.

There is also growing evidence from epidemiological studies for an association between bacterial vaginosis and acquisition of other STIs, in particular, herpes simplex virus 2 (HSV-2) infection, T. vaginalis and, more recently, human papilloma virus (HPV) infection. The hazard ratios for incident HSV-2 infection associated with bacterial vaginosis ranged from 1.6 to 2.4 [63–65]. Rathod et al. found bacterial vaginosis to be associated with a nine-fold increased risk of acquiring T. vaginalis infection, whereas other studies found a hazard ratio around 2 [66–68]. Brotman et al.[67] in their longitudinal study of more than 3000 women in Alabama also found an increased risk of gonorrhoea and of chlamydial infection associated with bacterial vaginosis. Two longitudinal studies that explored the association between bacterial vaginosis and incident HPV infection found odds ratios (ORs) of 1.2 and 1.4 [69,70]. Furthermore, King et al.[70] found bacterial vaginosis to be associated with delayed clearance of HPV.

It is well established that HSV-2 infection, gonorrhoea, chlamydial infection and trichomoniasis increase the risk of HIV acquisition. As bacterial vaginosis increases the risk of these infections, the association between bacterial vaginosis and incident HIV infection might be confounded (and overestimated) by the association between bacterial vaginosis and other STIs, especially HSV-2, which is a strong co-factor in the transmission of HIV [71]. More recent studies, however, adjusted for potential confounding by HSV-2 and found adjusted hazard ratios for incident HIV infection associated with bacterial vaginosis, in the range of 1.5–2.1 (Table 1). The trial by Reid et al.[62] was conducted among female sex workers who were all HSV-2-infected, and also this prospective study found an increased risk of HIV infection associated with bacterial vaginosis, independent of HSV-2 infection. One can thus conclude that bacterial vaginosis has a direct effect on HIV acquisition and an indirect effect through its association with other STIs.

Lastly, there is growing evidence that bacterial vaginosis enhances onward transmission of HIV from women to their male partners. A longitudinal study among HIV-discordant couples found an increased risk of female-to-male transmission if the female partner had bacterial vaginosis [72]. Indirect evidence of increased female-to-male transmission associated with bacterial vaginosis is provided by a study of HIV genital shedding among women in Burkina Faso who were on antiretroviral treatment [73].

Prevalence of bacterial vaginosis in different populations across the world

In 2013, a review was published of data on the prevalence of bacterial vaginosis in different countries across the world [74]. We used this review as a basis to try and compare the prevalence of bacterial vaginosis in different parts of the world and discern a pattern. We completed the review by Kenyon et al. by searching the PubMed for additional papers. In order to minimize bias in the selection of the study participants, we only considered population-based studies that included women who were not at particular risk of STIs, and studies among pregnant women. We excluded studies among women who were symptomatic and/or were HIV-infected. Inequalities in the performance of techniques to assess bacterial vaginosis can make comparison of prevalence data difficult and therefore we only considered studies that used the Nugent score. However, we made an exception for China as large-scale population-based studies have been reported from this country.

Table 2 gives an overview of the prevalence of bacterial vaginosis in different parts of the world. The lowest prevalence rates were found in Western Europe with the exception of the population of Greenland, which also has a high prevalence of other STIs. Most studies from Asia found prevalence rates that were somewhat higher than in Europe – between 10 and 20%. Several large-scale studies have been reported from China, but they used Amsel's criteria to diagnose bacterial vaginosis. A study among more than 53 000 women in rural China found a prevalence of bacterial vaginosis of 12% [113], which is probably an underestimate as the sensitivity of the Amsel's criteria to detect bacterial vaginosis is lower than the Nugent score. The prevalence rates in the Americas varied widely from 5.8% in pregnant women in Philadelphia in the United States to 40.8% among women from communities in the jungle of Peru and 49.1% in Jamaica. Overall, a high prevalence of bacterial vaginosis was found in sub-Saharan Africa, especially South and Eastern Africa. Among pregnant women in Jamaica, the prevalence of bacterial vaginosis was similar to the prevalence among pregnant women in South Africa and in Uganda. Likewise, a high prevalence was found in African-American women in the general population in the United States – 51.4 versus 31.9% in Hispanic women and 23.2% in non-Hispanic white women [75]. Among pregnant women in London, the overall prevalence of bacterial vaginosis was 14.5%, but it was 33.6% among Afro-Caribbean and African women [101].

T2-1
Table 2:
Prevalence of bacterial vaginosis among women from the general population and among pregnant women in different parts of the world.

In conclusion, there seems to be an association between bacterial vaginosis and ethnic background, with the highest prevalence rates of bacterial vaginosis found in black populations that are in general heavily affected by the HIV epidemic. In addition, isolated communities of low socio-economic status, such as the communities in the studies from Peru and from Greenland, also appear to be more affected by bacterial vaginosis.

What are the risk factors for bacterial vaginosis and can they explain the variations in prevalence between different populations?

Whether bacterial vaginosis is an STI on its own is a matter of debate [114,115]. Several risk factors related to sexual behaviour have been identified that are consistent with bacterial vaginosis being an STI. Bacterial vaginosis has been found to be associated with acquisition of a new sex partner and number of sex partners, male or female, whereas condom use has been found to be protective [114]. Other arguments in favour of bacterial vaginosis being an STI are the concordance of the vaginal microbiota in couples of women who have sex with women [116] and the protective effect of male circumcision against bacterial vaginosis in their female partners [117]. Furthermore, bacteria that have recently been found to be associated with bacterial vaginosis in women including BVAB2, BVAB3, Megasphaera and Leptotrichia/Sneathia spp., have been detected in male urine and were associated with urethritis [118].

There is some evidence that suggests that other STIs, in particular, HSV-2 infection enhance the risk of bacterial vaginosis [119,120]. The association between high-risk sexual behaviour and bacterial vaginosis may thus, in part, be confounded by other STIs as was illustrated in the study by Cherpes et al.[119]. They found that incident bacterial vaginosis was associated with number of sex partners in univariate analysis, but after adjusting for HSV-2 infection, this association was not statistically significant anymore, whereas there was a significant association between positive HSV-2 serology and incident bacterial vaginosis [119]. Furthermore, data from experimental and clinical research suggest that T. vaginalis alters the composition of the vaginal microbiota toward a bacterial vaginosis state and that endosymbionts carried by T. vaginalis promote survival of bacterial vaginosis-associated bacteria [45,121,122].

Clinical data also support an argument that bacterial vaginosis is an opportunistic reproductive tract infection rather than a classic STI. So far, no causative agent for bacterial vaginosis has been identified that is transmitted between sex partners and treatment of male partners of women with bacterial vaginosis fails to decrease recurrences [123]. Furthermore sexual practices that carry a minimal risk of transmission of an STI are associated with bacterial vaginosis, including receptive oral sex and non-penetrative digital–genital contact [124]. Another argument against bacterial vaginosis being an STI is the observations of bacterial vaginosis in sexually inexperienced women. Table 3 gives an overview of studies that have compared the prevalence of bacterial vaginosis in virgins and sexually active women. These data, however, have to be interpreted with caution as the validity of self-reported sexual behaviour is low. Possible explanations for the occurrence of bacterial vaginosis in these women include oral sex, and douching or other intra-vaginal practices. Regarding the latter risk factors, the evidence is not conclusive, as some studies have found an association with bacterial vaginosis that is not confirmed by the other studies [59,129–131]. In conclusion, data do suggest that high-risk sexual behaviour, that is having multiple sex partners, does not per se increase the risk of bacterial vaginosis, but that the risk is rather increased by sexual activity and interference with the vaginal mucosa. This led Verstraelen et al.[115] to coin the term ‘sexually enhanced disease’. The complex relationship between disturbances of the vaginal microbiota, sexual behaviour, STIs, and HIV risk is presented in Fig. 1.

T3-1
Table 3:
Prevalence of bacterial vaginosis in women who reported that they never had sexual intercourse vs. women who had passed their sexual debut.
F1-1
Fig. 1:
Schematic representation of risk factors for disturbances of the vaginal microbiota and risk of HIV.

There is now compelling evidence that oral and injection/implant hormonal contraceptives is protective against bacterial vaginosis [132]. This is all the more interesting as the use of injectable hormonal contraceptives has been found to be associated with an increased risk of HIV acquisition in some but not all observational studies [133], and both injectable and oral contraceptives have been associated with untoward pro-inflammatory changes in the cervical immune mediator levels [57].

Studies from the United States have consistently found bacterial vaginosis by classic Nugent score to occur more frequently in African-American women. These racial characteristics in occurrence of bacterial vaginosis apparently cannot be fully explained by known risk factors, including sexual behaviour, presence of another STI, smoking, contraceptive use, douching and socio-economic status [75,134]. But studies that used culture-independent techniques to characterize the vaginal microbiota have found racial differences in the composition of the ‘healthy’ vaginal microbiota [5,6], healthy African-American women having more often a vaginal microbiota that is not dominated by lactobacilli as compared to Caucasian women. Zhou et al. hypothesized that healthy African-American women have more often a vaginal microbiota that is less resilient to stress than Caucasian women. Ravel et al.[5] found that a vaginal microbiota that is dominated by L. crispatus is less frequent in African-American women than in Caucasian women, whereas this type of microbiota has been found to be more stable and less prone to shifts towards a disturbed microbiota [135]. These observations need to be confirmed in larger population studies in different countries and continents. The reasons why women of different race or ethnicity may have a different composition of their vaginal microbiota and are thus more prone to disturbances of the vaginal microbiota that enhance their risk of acquiring HIV infection, are unclear. It has been hypothesized that host genetic factors may play a role among other factors, including nutrition, stress, depression and environmental exposures [5].

Interventions to reduce disturbances of the vaginal microbiota and maintain a healthy vaginal environment

In populations where the prevalence of bacterial vaginosis and intermediate flora is high, for example, in Southern Africa, a sizeable proportion of HIV infections are attributable to disturbances of the vaginal microbiota. Van de Wijgert et al.[61] estimated that in their cohort of women in Uganda and Zimbabwe, 17% of new HIV infections were attributable to bacterial vaginosis and 12% to intermediate flora. Interventions that can reduce the incidence and prevalence of disturbances of the vaginal microbiota could thus have an important impact on the spread of HIV.

Treatment of bacterial vaginosis with metronidazole was part of the STI mass treatment intervention that was tested in the randomized controlled trial in Rakai, Uganda [89]. No effect was found on the prevalence of bacterial vaginosis or on the incidence of HIV infection. The explanation has to be sought in the high recurrence rates of bacterial vaginosis after treatment, the reasons for which are not clear. Suppressive antibacterial therapy has been shown to reduce the rate of recurrences in two trials in the United States and in Kenya [136,137]. An alternative approach that may be promising is the use of probiotics, defined as ‘live microorganisms which when administered in adequate amounts confer a health benefit to the host’ [138]. In their 2009 review, Senok et al.[139] identified four randomized controlled trials of probiotics in combination with antibiotics or an estriol preparation. Results suggest a beneficial effect on the short-term cure rate of bacterial vaginosis, but the evidence was not yet strong enough to recommend probiotics for the treatment of bacterial vaginosis. A trial comparing oral metronidazole with a Lactobacillus acidophilus vaginal-probiotic containing also oestriol and oral metronidazole with 2% vaginal clindamycin cream did not find any significant effect of the interventions on the 6-month recurrence of bacterial vaginosis [140]. The challenge is in finding a strain or combination of strains that colonizes the vagina and persists after treatment [141]. Experiments with primates suggest that the way forward may be transfer of the whole vaginal content of healthy women [142]. In a primate model restoring the resistance to uropathogenic E. coli could be partially achieved by vaginal instillation of a mixture of selected lactobacilli, but fully achieved only by swabbing and transferring the whole vaginal content from healthy animals [142]. These findings underscore the importance of the integrals of bacterial communities rather than individual microbiome players.

Prevention of disturbances of the vaginal microbiota could in theory be achieved by acting on risk factors for bacterial vaginosis and intermediate flora. So far, two randomized trials have assessed the effects of a behavioural intervention on the incidence or persistence of bacterial vaginosis with disappointing results. An intervention of douching cessation found an effect on the occurrence of bacterial vaginosis only among women who reported as reason for douching cleansing after menstruation [143]. A behavioural intervention targeting women who have sex with women did not find an effect on the persistence of bacterial vaginosis, although there was an effect on their behaviour [144].

Conclusion

There is strong evidence that bacterial vaginosis increases the risk of acquiring HIV infection in women. Recent studies also found less severe disturbances of the vaginal microbiota, that is, intermediate flora, as assessed by the Nugent scoring system, to be associated with an increased incidence of HIV. Considering the high prevalence of disturbances of the vaginal microbiota in some populations, especially in African populations or populations from African descent, disturbances of the vaginal microbiota may play an important role in the spread of HIV. However, efforts to reduce the occurrence of bacterial vaginosis and intermediate flora are hampered by the fact that the causes and mechanisms of disturbances of the vaginal microbiota are poorly understood.

In the United States, racial differences have been found in the occurrence of bacterial vaginosis and intermediate flora that cannot fully be explained by variations in known risk factors. Studies among healthy women of different ethnic background in the United States suggest that there are racial differences in the composition of the vaginal microbiota which begs the question what is to be considered ‘healthy’ [23]. Larger population-based studies comparing functional and genetic characteristics of the vaginal microbiota in different geographic areas of the world are needed to confirm or reject differences based on host genetic background, to define ‘health’ and to establish the microbiological correlates of HIV transmission risk that may vary from one population to another. If differences are confirmed and the risk factors as well as population-specific norms are established, we would be one step closer to understanding variations in the spread of HIV.

Acknowledgements

A.B. wrote the first outline and draft. V.J. and A.B. wrote the parts on the epidemiology. T.C. and R.N.F. wrote the parts dealing with the microbiological and immunological aspects. All authors reviewed the manuscript and contributed to the conclusions.

Conflicts of interest

There are no conflicts of interest.

References

1. Buvé A, Caraël M, Hayes RJ, Auvert B, Ferry B, Robinson NJ, et al. Multicentre study on factors determining differences in rate of spread of HIV in sub-Saharan Africa: summary and conclusions. AIDS 2001; (Suppl 4):S127–S131.
2. Pettifor AE, Levandowski BA, Macphail C, Miller WC, Tabor J, Ford C, et al. A tale of two countries: rethinking sexual risk for HIV among young people in South Africa and the United States. J Adolesc Health 2011; 49:237–243.
3. Powers KA, Poole C, Pettifor AE, Cohen MS. Rethinking the heterosexual infectivity of HIV-1: a systematic review and meta-analysis. Lancet Infect Dis 2008; 8:553–563.
4. Boily M-C, Baggaley RF, Wang L, Masse B, White RG, Hayes RJ, et al. Heterosexual risk of HIV-1 infection per sexual act: systematic review and meta-analysis of observational studies. Lancet Infect Dis 2009; 9:118–129.
5. Ravel J, Gajer P, Abdo Z, Schneider GM, Koenig SS, McCulle SL, et al. Vaginal microbiome of reproductive-age women. Proc Natl Acad Sci U S A 2011; 108 (Suppl 1):4680–4687.
6. Zhou X, Brown CJ, Abdo Z, Davis CC, Hansmann MA, Joyce P, et al. Differences in the composition of vaginal microbial communities found in healthy Caucasian and black women. ISME J 2007; 1:121–133.
7. Srinivasan S, Liu C, Mitchell CM, Fiedler TL, Thomas KK, Agnew KJ, et al. Temporal variability of human vaginal bacteria and relationship with bacterial vaginosis. PLoS One 2010; 5:e10197.
8. Santiago GL, Cools P, Verstraelen H, Trog M, Missine G, El Aila N, et al. Longitudinal study of the dynamics of vaginal microflora during two consecutive menstrual cycles. PLoS One 2011; 6:e28180.
9. Jespers V, Menten J, Smet H, Poradosú S, Abdellati S, Verhelst R, et al. Quantification of bacterial species of the vaginal microbiome in different groups of women, using nucleic acid amplification tests. BMC Microbiol 2012; 12:83.
10. Hillier SL, Lau RJ. Vaginal microflora in postmenopausal women who have not received estrogen replacement therapy. Clin Infect Dis 1997; 25 (Suppl 2):S123–S126.
11. Amsel R, Totten PA, Spiegel CA, Chen KC, Eschenbach D, Holmes KK. Nonspecific vaginitis. Diagnostic criteria and microbial and epidemiologic associations. Am J Med 1983; 74:14–22.
12. Nugent RP, Krohn MA, Hillier SL. Reliability of diagnosing bacterial vaginosis is improved by a standardized method of gram stain interpretation. J Clin Microbiol 1991; 29:297–301.
13. Hillier SL, Krohn MA, Nugent RP, Gibbs RS. Vaginal Infections and Prematurity Study Group. Characteristics of three vaginal flora patterns assessed by gram stain among pregnant women. Am J Obstet Gynecol 1992; 166:938–944.
14. Ling Z, Liu X, Chen X, Zhu H, Nelson KE, Xia Y, et al. Diversity of cervicovaginal microbiota associated with female lower genital tract infections. Microb Ecol 2011; 61:704–714.
15. Pépin J, Deslandes S, Giroux G, Sobéla F, Khonde N, Diakité S, et al. The complex vaginal flora of West African women with bacterial vaginosis. PLoS One 2011; 6:e25082.
16. Verhelst R, Verstraelen H, Claeys G, Verschraegen G, Delanghe J, Van Simaey L, et al. Cloning of 16S rRNA genes amplified from normal and disturbed vaginal microflora suggests a strong association between Atopobium vaginae, Gardnerella vaginalis and bacterial vaginosis. BMC Microbiol 2004; 4:16.
17. Menard JP, Fenollar F, Henry M, Bretelle F, Raoult D. Molecular quantification of Gardnerella vaginalis and Atopobium vaginae loads to predict bacterial vaginosis. Clin Infect Dis 2008; 47:33–43.
18. Haggerty CL, Totten PA, Ferris M, Martin DH, Hoferka S, Astete SG, et al. Clinical characteristics of bacterial vaginosis among women testing positive for fastidious bacteria. Sex Transm Infect 2009; 85:242–248.
19. Marrazzo JM. Interpreting the epidemiology and natural history of bacterial vaginosis: are we still confused?. Anaerobe 2011; 17:186–190.
20. Schwebke JR, Muzny CA, Josey WE. Role of Gardnerella vaginalis in the pathogenesis of bacterial vaginosis: a conceptual model. J Infect Dis 2014; [Epub ahead of print].
21. Schwebke JR, Flynn MS, Rivers CA. Prevalence of Gardnerella vaginalis among women with lactobacillus-predominant vaginal flora. Sex Transm Infect 2014; 90:61–63.
22. Hickey RJ, Forney LJ. Role of Gardnerella vaginalis in the pathogenesis of bacterial vaginosis (letter). J Infect Dis 2014; [Epub ahead of print].
23. Ma B, Forney LJ, Ravel J. Vaginal microbiome: rethinking health and disease. Annu Rev Microbiol 2012; 66:371–389.
24. Witkin SS, Linhares IM, Giraldo P. Bacterial flora of the female genital tract: function and immune regulation. Best Pract Res Clin Obstet Gynaecol 2007; 21:347–354.
25. Cadieux PA, Burton J, Devillard E, Reid G. Lactobacillus by-products inhibit the growth and virulence of uropathogenic Escherichia coli. J Physiol Pharmacol 2009; 60 (Suppl 6):13–18.
26. Kalyoussef S, Nieves E, Dinerman E, Carpenter C, Shankar V, Oh J, et al. Lactobacillus proteins are associated with the bactericidal activity against E. coli of female genital tract secretions. PLoS One 2012; 7:e49506.
27. Zarate G, Nader-Macias ME. Influence of probiotic vaginal lactobacilli on in vitro adhesion of urogenital pathogens to vaginal epithelial cells. Lett Appl Microbiol 2006; 43:174–180.
28. Schwebke JR. Role of vaginal flora as a barrier to HIV acquisition. Curr Infect Dis Rep 2001; 3:152–155.
29. Klebanoff SJ, Coombs RW. Viricidal effect of Lactobacillus acidophilus on human immunodeficiency virus type 1: possible role in heterosexual transmission. J Exp Med 1991; 174:289–292.
30. O’Hanlon DE, Moench TR, Cone RA. In vaginal fluid, bacteria associated with bacterial vaginosis can be suppressed with lactic acid but not hydrogen peroxide. BMC Infect Dis 2011; 11:200.
31. O’Hanlon DE, Lanier BR, Moench TR, Cone RA. Cervicovaginal fluid and semen block the microbicidal activity of hydrogen peroxide produced by vaginal lactobacilli. BMC Infect Dis 2010; 10:120.
32. Aldunate M, Tyssen D, Johnson A, Zakir T, Sonza S, Moench T, et al. Vaginal concentrations of lactic acid potently inactivate HIV. J Antimicrob Chemother 2013; 68:2015–2025.
33. O’Hanlon DE, Moench TR, Cone RA. Vaginal pH and microbicidal lactic acid when lactobacilli dominate the microbiota. PLoS One 2013; 8:e80074.
34. Witkin SS, Mendes-Soares H, Linhares IM, Jayaram A, Ledger WJ, Forney LJ. Influence of vaginal bacteria and D- and L-lactic acid isomers on vaginal extracellular matrix metalloproteinase inducer: implications for protection against upper genital tract infections. MBio 4: e00460–13.
35. Witkin SS, Alvi S, Bongiovanni AM, Linhares IM, Ledger WJ. Lactic acid stimulates interleukin-23 production by peripheral blood mononuclear cells exposed to bacterial lipopolysaccharide. FEMS Immunol Med Microbiol 2011; 61:153–158.
36. Mossop H, Linhares IM, Bongiovanni AM, Ledger WJ, Witkin SS. Influence of lactic acid on endogenous and viral RNA-induced immune mediator production by vaginal epithelial cells. Obstet Gynecol 2011; 118:840–846.
37. Zeuthen LH, Fink LN, Frokiaer H. Epithelial cells prime the immune response to an array of gut-derived commensals towards a tolerogenic phenotype through distinct actions of thymic stromal lymphopoietin and transforming growth factor-beta. Immunology 2008; 123:197–208.
38. Donato KA, Gareau MG, Wang YJJ, Sherman PM. Lactobacillus rhamnosus GG attenuates interferon-{gamma} and tumor necrosis factor-{alpha}-induced barrier dysfunction and pro-inflammatory signalling. Microbiology 2010; 156 (Pt 11):3288–3297.
39. Neish AS, Gewirtz AT, Zeng H, Young AN, Hobert ME, Karmali V, et al. Prokaryotic regulation of epithelial responses by inhibition of IkappaB-alpha ubiquitination. Science 2000; 289:1560–1563.
40. Novak RM, Donoval BA, Graham PJ, Boksa LA, Spear G, Hershow RC, et al. Cervicovaginal levels of lactoferrin, secretory leukocyte protease inhibitor, and RANTES and the effects of coexisting vaginoses in human immunodeficiency virus (HIV)-seronegative women with a high risk of heterosexual acquisition of HIV infection. Clin Vaccine Immunol 2007; 14:1102–1107.
41. Hedges SR, Barrientes F, Desmond RA, Schwebke JR. Local and systemic cytokine levels in relation to changes in vaginal flora. J Infect Dis 2006; 193:556–562.
42. Fichorova RN, Lai JJ, Schwartz JL, Weiner DH, Mauck CK, Callahan MM. Baseline variation and associations between subject characteristics and five cytokine biomarkers of vaginal safety among healthy nonpregnant women in microbicide trials. Cytokine 2011; 55:134–140.
43. Libby EK, Pascal KE, Mordechai E, Adelson ME, Trama JP. Atopobium vaginae triggers an innate immune response in an in vitro model of bacterial vaginosis. Microbes Infect 2008; 10:439–446.
44. Yamamoto HS, Xu Q, Fichorova RN. Homeostatic properties of Lactobacillus jensenii engineered as a live vaginal anti-HIV microbicide. BMC Microbiol 2013; 13:4.
45. Fichorova RN, Buck OR, Yamamoto HS, Fashemi T, Dawood HY, Fashemi B, et al. The villain team-up or how Trichomonas vaginalis and bacterial vaginosis alter innate immunity in concert. Sex Transm Infect 2013; 89:460–466.
46. Fashemi B, Delaney ML, Onderdonk AB, Fichorova RN. Effects of feminine hygiene products on the vaginal mucosal biome. Microb Ecol Health Dis 2013; 24: 19703 - http://dx.doi.org/10.3402/mehd.v24i0.19703.
47. Fichorova RN, Yamamoto HS, Delaney ML, Onderdonk AB, Doncel GF. Novel vaginal microflora colonization model providing new insight into microbicide mechanism of action. MBio 2011; 2: e00168-00111.
48. Hladik F, McElrath MJ. Setting the stage: host invasion by HIV. Nat Rev Immunol 2008; 8:447–457.
49. Alfano M, Poli G. The cytokine network in HIV infection. Curr Mol Med 2002; 2:677–689.
50. Shattock RJ, Haynes BF, Pulendran B, Flores J, Esparza J. Improving defences at the portal of HIV entry: mucosal and innate Immunity. PLoS Med 2008; 5:e81.
51. Genç MR, Delaney ML, Onderdonk AB, Witkin SS. Microbiology and Prematurity (MAP) Study Group. Vaginal nitric oxide in pregnant women with bacterial vaginosis. Am J Reprod Immunol 2006; 56:86–90.
52. Bogdan C. Nitric oxide and the immune response. Nat Immunol 2001; 2:907–916.
53. Valore EV, Wiley DJ, Ganz T. Reversible deficiency of antimicrobial polypeptides in bacterial vaginosis. Infect Immun 2006; 74:5693–5702.
54. McNeely TB, Shugars DC, Rosendahl M, Tucker C, Eisenberg SP, Wahl SM. Inhibition of human immunodeficiency virus type 1 infectivity by secretory leukocyte protease inhibitor occurs prior to viral reverse transcription. Blood 1997; 90:1141–1149.
55. Draper DL, Landers DV, Krohn MA, Hillier SL, Wiesenfeld HC, Heine RP. Levels of vaginal secretory leukocyte protease inhibitor are decreased in women with lower reproductive tract infections. Am J Obstet Gynecol 2000; 183:1243–1248.
56. Huppert JS, Huang B, Chen C, Dawood HY, Fichorova RN. Clinical evidence for the role of Trichomonas vaginalis in regulation of secretory leukocyte protease inhibitor in the female genital tract. J Infect Dis 2013; 207:1462–1470.
57. Morrison C, Fichorova R, Mauck C, Chen P-L, Kwok C, Chipato T, et al. Cervical inflammation and immunity associated with hormonal contraception, pregnancy and HIV-1 seroconversion. J AIDS 2014; 66:109–117.
58. Atashili J, Poole C, Ndumbe PM, Adimora AA, Smith JS. Bacterial vaginosis and HIV acquisition: a meta-analysis of published studies. AIDS 2008; 22:1493–1501.
59. Low N, Chersich MF, Schmidlin K, Egger M, Francis SC, van de Wijgert JH, et al. Intravaginal practices, bacterial vaginosis, and HIV infection in women: individual participant data meta-analysis. PLoS Med 2011; 8:e1000416.
60. Kapiga SH, Sam NE, Bang H, Ni Q, Ao TT, Kiwelu I, et al. The role of herpes simplex virus type 2 and other genital infections in the acquisition of HIV-1 among high-risk women in northern Tanzania. J Infect Dis 2007; 195:1260–1269.
61. Van de Wijgert JHHM, Morrison CS, Brown J, Kwok C, Van Der Pol B, Chipato T, et al. Disentangling contributions of reproductive tract infections to HIV acquisition in African women. Sex Transm Dis 2009; 36:357–364.
62. Reid SE, Dai JY, Wang J, Sichalwe BN, Akpomiemie G, Cowan FM, et al. Pregnancy, contraceptive use, and HIV acquisition in HPTN 039: relevance for HIV prevention trials among African women. J Acquir Immune Defic Syndr 2010; 53:606–613.
63. Cherpes TL, Meyn LA, Krohn MA, Lurie JG, Hillier SL. Association between acquisition of herpes simplex virus type 2 in women and bacterial vaginosis. Clin Infect Dis 2003; 37:319–325.
64. Gallo MF, Warner L, Macaluso M, Stone KM, Brill I, Fleenor ME, et al. Risk factors for incident herpes simplex type 2 virus infection among women attending a sexually transmitted disease clinic. Sex Transm Dis 2008; 35:679–685.
65. Chohan V, Baeten JM, Benki S, Graham SM, Lavreys L, Mandaliya K, et al. A prospective study of risk factors for herpes simplex virus type 2 acquisition among high-risk HIV-1 seronegative women in Kenya. Sex Transm Infect 2009; 85:489–492.
66. Rathod SD, Krupp K, Klausner JD, Arun A, Reingold AL, Madhivanan P. Bacterial vaginosis and risk for Trichomonas vaginalis infection: a longitudinal analysis. Sex Transm Dis 2011; 38:882–886.
67. Brotman RM, Klebanoff MA, Nansel TR, Yu KF, Andrews WW, Zhang J, Schwebke JR. Bacterial vaginosis assessed by gram stain and diminished colonization resistance to incident gonococcal, chlamydial, and trichomonal genital infection. J Infect Dis 2010; 202:1907–1915.
68. Balkus JE, Richardson BA, Rabe LK, Taha TE, Mgodi N, Kasaro MP, et al. Bacterial vaginosis and the risk of Trichomonas vaginalis acquisition among HIV-1-negative women. Sex Transm Dis 2014; 41:123–128.
69. Watts DH, Fazzari M, Minkoff H, Hillier SL, Sha B, Glesby M, et al. Effects of bacterial vaginosis and other genital infections on the natural history of human papillomavirus infection in HIV-1-infected and high-risk HIV-1-uninfected women. J Infect Dis 2005; 191:1129–1139.
70. King CC, Jamieson DJ, Wiener J, Cu-Uvin S, Klein RS, Rompalo AM, et al. Bacterial vaginosis and the natural history of human papillomavirus. Infect Dis Obstet Gynecol 2011; 2011: doi:10.1155/2011/319460.
71. Glynn JR, Biraro S, Weiss HA. Herpes simplex virus type 2: a key role in HIV incidence. AIDS 2009; 23:1595–1598.
72. Cohen CR, Lingappa JR, Baeten JM, Ngayo MO, Spiegel CA, Hong T, et al. Bacterial vaginosis associated with Increased risk of female-to-male HIV-1 transmission: a prospective cohort analysis among African couples. PLoS Med 2012; 9:e1001251.
73. Low AJ, Konate I, Nagot N, Weiss HA, Kania D, Vickerman P, et al. Cervicovaginal HIV-1 shedding in women taking antiretroviral therapy in Burkina Faso: a longitudinal study. J Acquir Immune Defic Syndr 2014; 65:237–245.
74. Kenyon C, Colebunders R, Crucitti T. The global epidemiology of bacterial vaginosis: a systematic review. Am J Obstet Gynecol 2013; 209:505–523.
75. Koumans EH, Sternberg M, Bruce C, McQuillan G, Kendrick J, Sutton M, et al. The prevalence of bacterial vaginosis in the United States, 2001-2004; associations with symptoms, sexual behaviors, and reproductive health. Sex Transm Dis 2007; 34:864–869.
76. de Lima Soares V, de Mesquita AM, Cavalcante FG, Silva ZP, Hora V, Diedrich T, et al. Sexually transmitted infections in a female population in rural north-east Brazil: prevalence, morbidity and risk factors. Trop Med Int Health 2003; 8:595–603.
77. Miranda AE, Merçon-de-Vargas PR, Corbett CE, Corbett JF, Dietze R. Perspectives on sexual and reproductive health among women in an ancient mining area in Brazil. Rev Panam Salud Publica 2009; 25:157–161.
78. Oliveira FA, Pfleger V, Lang K, Heukelbach J, Miralles I, Fraga F, et al. Sexually transmitted infections, bacterial vaginosis, and candidiasis in women of reproductive age in rural Northeast Brazil: a population-based study. Mem Inst Oswaldo Cruz 2007; 102:751–756.
79. García PJ, Chavez S, Feringa B, Chiappe M, Li W, Jansen KU, et al. Reproductive tract infections in rural women from the highlands, jungle, and coastal regions of Peru. Bull World Health Organ 2004; 82:483–492.
80. Ramia S, Kobeissi L, El Kak F, Shamra S, Kreidieh K, Zurayk H. Reproductive tract infections (RTIs) among married nonpregnant women living in a low-income suburb of Beirut, Lebanon. J Infect Dev Ctries 2012; 6:680–683.
    81. Eriksson K, Adolfsson A, Forsum U, Larsson PG. The prevalence of BV in the population on the Åland Islands during a 15-year period. APMIS 2010; 118:903–908.
    82. Datcu R, Gesink D, Mulvad G, Montgomery-Andersen R, Rink E, Koch A, et al. Vaginal microbiome in women from Greenland assessed by microscopy and quantitative PCR. BMC Infect Dis 2013; 13:480.
    83. Patel V, Weiss HA, Mabey D, West B, D'Souza S, Patil V, et al. The burden and determinants of reproductive tract infections in India: a population based study of women in Goa, India. Sex Transm Infect 2006; 82:243–249.
    84. Nayab D. Reproductive tract infections among women in Pakistan: an urban case study. Pak Dev Rev 2005; 44:131–158.
    85. Lan PT, Lundborg CS, Phuc HD, Sihavong A, Unemo M, Chuc NT, et al. Reproductive tract infections including sexually transmitted infections: a population-based study of women of reproductive age in a rural district of Vietnam. Sex Transm Infect 2008; 84:126–132.
    86. Go VF, Quan VM, Celentano DD, Moulton LH, Zenilman JM. Prevalence and risk factors for reproductive tract infections among women in rural Vietnam. Southeast Asian J Trop Med Public Health 2006; 37:185–189.
    87. Kirakoya-Samadoulougou F, Nagot N, Defer MC, Yaro S, Fao P, Ilboudo F, et al. Epidemiology of herpes simplex virus type 2 infection in rural and urban Burkina Faso. Sex Transm Dis 2011; 38:117–123.
    88. Walraven G, Scherf C, West B, Ekpo G, Paine K, Coleman R, et al. The burden of reproductive-organ disease in rural women in The Gambia, West Africa. Lancet 2001; 357:1161–1167.
    89. Wawer MJ, Sewankambo NK, Serwadda D, Quinn TC, Paxton LA, Kiwanuka N, et al. Control of sexually transmitted diseases for AIDS prevention in Uganda: a randomised community trial. Rakai Project Study Group. Lancet 1999; 353:525–535.
    90. Goldenberg RL, Klebanoff MA, Nugent R, Krohn MA, Hillier S, Andrews WW. Bacterial colonization of the vagina during pregnancy in four ethnic groups. Vaginal Infections and Prematurity Study Group. Am J Obstet Gynecol 1996; 174:1618–1621.
    91. Tolosa JE, Chaithongwongwatthana S, Daly S, Maw WW, Gaitán H, Lumbiganon P, et al. The International Infections in Pregnancy (IIP) study: variations in the prevalence of bacterial vaginosis and distribution of morphotypes in vaginal smears among pregnant women. Am J Obstet Gynecol 2006; 195:1198–1204.
    92. Wenman WM, Tataryn IV, Joffres MR, Pearson R, Grace MG, Albritton WL, et al. Demographic, clinical and microbiological characteristics of maternity patients: a Canadian clinical cohort study. Can J Infect Dis 2002; 13:311–318.
    93. Gondo F, da Silva MG, Polettini J, Tristao Ada R, Peracoli JC, Witkin SS, et al. Vaginal flora alterations and clinical symptoms in low-risk pregnant women. Gynecol Obstet Invest 2011; 71:158–162.
    94. Krauss-Silva L, Almada-Horta A, Alves MB, Camacho KG, Moreira ME, Braga A. Basic vaginal pH, bacterial vaginosis and aerobic vaginitis: prevalence in early pregnancy and risk of spontaneous preterm delivery, a prospective study in a low socioeconomic and multiethnic South American population. BMC Pregnancy Childbirth 2014; 14:107.
    95. Kamara P, Hylton-Kong T, Brathwaite A, Del Rosario GR, Kristensen S, Patrick N, et al. Vaginal infections in pregnant women in Jamaica: prevalence and risk factors. Int J STD AIDS 2000; 11:516–520.
    96. Desseauve D, Chantrel J, Fruchart A, Khoshnood B, Brabant G, Ancel PY, et al. Prevalence and risk factors of bacterial vaginosis during the first trimester of pregnancy in a large French population-based study. Eur J Obstet Gynecol Reprod Biol 2012; 163:30–34.
    97. Daskalakis G, Papapanagiotou A, Mesogitis S, Papantoniou N, Mavromatis K, Antsaklis A. Bacterial vaginosis and group B streptococcal colonization and preterm delivery in a low-risk population. Fetal Diagn Ther 2006; 21:172–176.
    98. Cristiano L, Rampello S, Noris C, Valota V. Bacterial vaginosis: prevalence in an Italian population of asymptomatic pregnant women and diagnostic aspects. Eur J Epidemiol 1996; 12:383–390.
    99. Gratacós E, Figueras F, Barranco M, Ros R, Andreu A, Alonso PL, et al. Prevalence of bacterial vaginosis and correlation of clinical to Gram stain diagnostic criteria in low risk pregnant women. Eur J Epidemiol 1999; 15:913–916.
    100. Martínez de Tejada B, Coll O, de Flores M, Hillier SL, Landers DV. [Prevalence of bacterial vaginosis in an obstetric population of Barcelona.]. Med Clin (Barc) 1998; 110:201–204.[Article in Spanish].
      101. Oakeshott P, Hay P, Hay S, Steinke F, Rink E, Kerry S. Association between bacterial vaginosis or chlamydial infection and miscarriage before 16 weeks’ gestation: prospective community based cohort study. Br Med J 2002; 325:1334.1.
      102. Riduan JM, Hillier SL, Utomo B, Wiknjosastro G, Linnan M, Kandun N. Bacterial vaginosis and prematurity in Indonesia: association in early and late pregnancy. Am J Obstet Gynecol 1993; 169:175–178.
      103. Thammalangsy S, Sihavong A, Phouthavane T, Sayabounthavong K, Puapermpoonsiri S, Kitayaporn D, et al. The prevalence of lower genital tract infections among ante-natal care (ANC) clinic patients in two central hospitals, Vientiane, Lao People's Democratic Republic. Southeast Asian J Trop Med Public Health 2006; 37:190–199.
      104. Puapermpoonsiri S, Kato N, Watanabe K, Ueno K, Chongsomchai C, Lumbiganon P. Vaginal microflora associated with bacterial vaginosis in Japanese and Thai pregnant women. Clin Infect Dis 1996; 23:748–752.
      105. Goto A, Nguyen QV, Pham NM, Kato K, Cao TP, Le TH, et al. Prevalence of and factors associated with reproductive tract infections among pregnant women in ten communes in Nghe An Province, Vietnam. J Epidemiol 2005; 15:163–172.
      106. Shimano S, Nishikawa A, Sonoda T, Kudo R. Analysis of the prevalence of bacterial vaginosis and Chlamydia trachomatis infection in 6083 pregnant women at a hospital in Otaru, Japan. J Obstet Gynaecol Res 2004; 30:230–236.
      107. Kirakoya-Samadoulougou F, Nagot N, Defer MC, Yaro S, Meda N, Robert A. Bacterial vaginosis among pregnant women in Burkina Faso. Sex Transm Dis 2008; 35:985–989.
      108. Blankhart D, Müller O, Gresenguet G, Weis P. Sexually transmitted infections in young pregnant women in Bangui, Central African Republic. Int J STD AIDS 1999; 10:609–614.
      109. Romoren M, Velauthapillai M, Rahman M, Sundby J, Klouman E, Hjortdahl P. Trichomoniasis and bacterial vaginosis in pregnancy: inadequately managed with the syndromic approach. Bull World Health Organ 2007; 85:297–304.
      110. Govender L, Hoosen AA, Moodley J, Moodley P, Sturm AW. Bacterial vaginosis and associated infections in pregnancy. Int J Gynaecol Obstet 1996; 55:23–28.
      111. Frohlich JA, Abdool Karim Q, Mashego MM, Sturm AW, Abdool Karim SS. Opportunities for treating sexually transmitted infections and reducing HIV risk in rural South Africa. J Adv Nurs 2007; 60:377–383.
      112. Tann CJ, Mpairwe H, Morison L, Nassimu K, Hughes P, Omara M, et al. Lack of effectiveness of syndromic management in targeting vaginal infections in pregnancy in Entebbe, Uganda. Sex Transm Infect 2006; 82:285–289.
      113. Li XD, Wang CC, Zhang XJ, Gao GP, Tong F, Li X, et al. Risk factors for bacterial vaginosis: results from a cross-sectional study having a sample of 53,652 women. Eur J Clin Microbiol Infect Dis 2014; 33:1525–1532.doi: 10.1007/s10096-014-2103-1.
      114. Fethers KA, Fairley CK, Hocking JS, Gurrin LC, Bradshaw CS. Sexual risk factors and bacterial vaginosis: a systematic review and meta-analysis. Clin Infect Dis 2008; 47:1426–1435.
      115. Verstraelen H, Verhelst R, Vaneechoutte M, Temmerman M. The epidemiology of bacterial vaginosis in relation to sexual behaviour. BMC Infect Dis 2010; 10:81.
      116. Marrazzo JM, Koutsky LA, Eschenbach DA, Agnew K, Stine K, Hillier SL. Characterization of vaginal flora and bacterial vaginosis in women who have sex with women. J Infect Dis 2002; 185:1307–1313.
      117. Gray RH, Kigozi G, Serwadda D, Makumbi F, Nalugoda F, Watya S, et al. The effects of male circumcision on female partners’ genital tract symptoms and vaginal infections in a randomized trial in Rakai, Uganda. Am J Obstet Gynecol 2009; 200:42.e1–7.
      118. Manhart LE, Khosropour CM, Liu C, Gillespie CW, Depner K, Fiedler T, et al. Bacterial vaginosis-associated bacteria in men: association of Leptotrichia/Sneathia spp. with nongonococcal urethritis. Sex Transm Dis 2013; 40:944–949.
      119. Cherpes TL, Hillier SL, Meyn LA, Busch JL, Krohn MA. A delicate balance: risk factors for acquisition of bacterial vaginosis include sexual activity, absence of hydrogen peroxide-producing lactobacilli, black race, and positive herpes simplex virus type 2 serology. Sex Transm Dis 2008; 35:78–83.
      120. Masese L, Baeten JM, Richardson BA, Bukusi E, John-Stewart G, Jaoko W, et al. Incident herpes simplex virus type 2 infection increases the risk of subsequent episodes of bacterial vaginosis. J Infect Dis 2014; 209:1023–1027.
      121. Martin DH, Zozaya M, Lillis RA, Myers L, Nsuami MJ, Ferris MJ. Unique vaginal microbiota that includes an unknown Mycoplasma-like organism is associated with Trichomonas vaginalis infection. J Infect Dis 2013; 207:1922–1931.
      122. Fichorova RN, Lee Y, Yamamoto HS, Takagi Y, Hayes GR, Goodman RP, et al. Endobiont viruses sensed by the human host: beyond conventional antiparasitic therapy. PLoS One 2012; 7:e48418.
      123. Colli E, Landoni M, Parazzini F. Treatment of male partners and recurrence of bacterial vaginosis: a randomised trial. Genitourin Med 1997; 73:267–270.
      124. Fethers KA, Fairley CK, Morton A, Hocking JS, Hopkins C, Kennedy LF, et al. Early sexual experiences and risk factors for bacterial vaginosis. J Infect Dis 2009; 200:1662–1670.
      125. Bump RC, Buesching WJ. Bacterial vaginosis in virginal and sexually active adolescent females: evidence against exclusive sexual transmission. Am J Obstet Gynecol 1988; 158:935–939.
      126. Yen S, Shafer M-A, Moncada J, Campbell CJ, Flinn SD, Boyer CB. Bacterial vaginosis in sexually experienced and nonsexually experienced young women entering the military. Obstet Gynecol 2003; 102:927–933.
      127. Jones FR, Miller G, Gadea N, Meza R, Leon S, Perez J, et al. Prevalence of bacterial vaginosis among young women in low-income populations of coastal Peru. Int J STD AIDS 2007; 18:188–192.
      128. Vaca M, Guadalupe I, Erazo S, Tinizaray K, Chico ME, Cooper PJ, Hay P. High prevalence of bacterial vaginosis in adolescent girls in a tropical area of Ecuador. BJOG 2009; 117:225–228.
      129. Schwebke JR. New concepts in the etiology of bacterial vaginosis. Curr Infect Dis Rep 2009; 11:143–147.
      130. Baeten JM, Hassan WM, Chohan V, Richardson BA, Mandaliya K, Ndinya-Achola JO, et al. Prospective study of correlates of vaginal Lactobacillus colonisation among high-risk HIV-1 seronegative women. Sex Transm Infect 2009; 85:348–353.
      131. Hilber AM, Francis SC, Chersich M, Scott P, Redmond S, Bender N, et al. Intravaginal practices, vaginal infections and HIV acquisition: systematic review and meta-analysis. PLoS One 2010; 5:e9119.
      132. Vodstrcil LA, Hocking JS, Law M, Walker S, Tabrizi SN, Fairley CK, et al. Hormonal contraception is associated with a reduced risk of bacterial vaginosis: a systematic review and meta-analysis. PLoS One 2013; 8:e73055.
      133. Polis CB, Curtis KM. Use of hormonal contraceptives and HIV acquisition in women: a systematic review of the epidemiological evidence. Lancet Infect Dis 2013; 13:797–808.
      134. Ness RB, Hillier S, Richter HE, Soper DE, Stamm C, Bass DC, et al. Can known risk factors explain racial differences in the occurrence of bacterial vaginosis?. J Natl Med Assoc 2003; 95:201–212.
      135. Verstraelen H, Verhelst R, Claeys G, De Backer E, Temmerman M, Vaneechoutte M. Longitudinal analysis of the vaginal microflora in pregnancy suggests that L. crispatus promotes the stability of the normal vaginal microflora and that L. gasseri and/or L. iners are more conducive to the occurrence of abnormal vaginal microflora. BMC Microbiol 2009; 9:116.
      136. Sobel JD, Ferris D, Schwebke J, Nyirjesy P, Wiesenfeld HC, Peipert J. Suppressive antibacterial therapy with 0.75% metronidazole vaginal gel to prevent recurrent bacterial vaginosis. Am J Obstet Gynecol 2006; 194:1283–1289.
      137. McClelland RS, Richardson BA, Hassan WM, Chohan V, Lavreys L, Mandaliya K, et al. Improvement of vaginal health for Kenyan women at risk for acquisition of human immunodeficiency virus type 1: results of a randomized trial. J Infect Dis 2008; 197:1361–1368.
      138. FAO/WHO. Probiotics in food: Health and nutritional properties and guidelines for evaluation. Rome, 2006. ftp://ftp.fao.org/es/esn/food/wgreport2.pdf.
      139. Senok AC, Verstraelen H, Temmerman M, Botta GA. Probiotics for the treatment of bacterial vaginosis. Cochrane Database Syst Rev 2009; CD006289.
      140. Bradshaw CS, Pirotta M, De Guingand D, Hocking JS, Morton AN, Garland SM, et al. Efficacy of oral metronidazole with vaginal clindamycin or vaginal probiotic for bacterial vaginosis: randomised placebo-controlled double-blind trial. PLoS One 2012; 7:e34540.
      141. Dover SE, Aroutcheva AA, Faro S, Chikindas ML. Natural antimicrobials and their role in vaginal health: a short review. Int J Probiotics Prebiotics 2008; 3:219–230.
      142. Winberg J, Herthelius-Elman M, Möllby R, Nord C. Pathogenesis of urinary tract infection: experimental studies of vaginal resistance to colonization. Pediatric Nephrol 1993; 7:509–514.
      143. Brotman RM, Ghanem KG, Klebanoff MA, Taha TE, Scharfstein DO, Zenilman JM. The effect of vaginal douching cessation on bacterial vaginosis: a pilot study. Am J Obstet Gynecol 2008; 198:628e1-7.
      144. Marrazzo JM, Thomas KK, Ringwood K. A behavioural intervention to reduce persistence of bacterial vaginosis among women who report sex with women: results of a randomised trial. Sex Transm Infect 2011; 87:399–405.
      Keywords:

      bacterial vaginosis; HIV transmission; intermediate flora; vaginal microbiome

      © 2014 Lippincott Williams & Wilkins, Inc.