Our study offers a unique perspective on the effects of sexual intercourse on the vaginal microbiota by comparing samples from before and after initiation of sexual intercourse. In this cohort of sexually inexperienced women, we found that both BV and vaginal colonization with BV-associated bacterial species was rare, and that the vaginal microbiota of women who were virgins appeared to be stable over the follow-up interval. Women who became sexually active during the study were more likely to gain colonization with G. vaginalisbut there was no loss of colonization with Lactobacillus species after initiation of sexual intercourse, nor increase in colonization with the fastidious BV-associated species.
While BV is associated with sexual activity,(2
) the mechanism by which women lose vaginal Lactobacillus
colonization and gain the polymicrobial community associated with BV is unclear. Eschenbach et al studied the vaginal microbiota in sexually experienced women within 8–12 hours after one episode of intercourse with or without a condom, and saw an increase in culture-based detection of E. coli
) but did not evaluate the fastidious BV-associated organisms described here. Fethers et al used similar qPCR techniques to evaluate colonization in a cross-sectional study that combined sexually naïve and sexually experienced women.(12
) They found that G. vaginalis, Sneathia
and BVAB3 were associated with more frequent penile-vaginal sex, as well as with unprotected sex. Women with no sexual experience, or who always used condoms were much less likely to be colonized with the BV-associated species that were tested for in their study. Of note, their analysis did not assess changes in the microbiota over time. Our study confirms that vaginal colonization with BV-associated bacterial species is rare in sexually inexperienced women, and does not significantly increase with initiation of penile-vaginal sexual activity. Although we did see an increase in the proportion of women colonized with G. vaginalis
among those who became sexually active, this microbe is widely prevalent in women with and without BV(21
), and for the purposes of this analysis is considered a commensal organism.
Our group has previously shown that the vaginal microbiota in sexually experienced women can be quite dynamic when sampled daily, due in part to the influence of menses.(18
) Other groups have shown significant variation in vaginal microbiota over time as well.(22
) Interestingly, in our data, women who remained virginal had very stable microbiota, while those who became sexually active began to show some variability in bacterial composition – though not to the extent reported in older women. This suggests that sexual activity, as well as menses, may play a role in disrupting or changing the vaginal microbial environment, though via what mechanism is not clear.
In a previous analysis from a similar cohort that excluded girls who remained virginal, presence of HPV was associated with diagnosis of BV, and more often preceded diagnosis of BV than the opposite (BV preceding HPV).(24
) In that analysis the median time to first detection of HPV was 4 months, while median time to first BV diagnosis was 12 months. Some have hypothesized that HPV infection, or the immune response, may predispose women to develop BV. In this analysis we had too few women with HPV at baseline to draw any firm conclusions, but did not find any association between HPV and detection of BV-associated bacteria.
Our study is limited in that we did not collect information on non-penetrative sexual contact; therefore, we were unable to evaluate any associations with these species and non-penetrative types of sexual activity, which have clearly been associated with an increased risk of BV. As our participants who became sexually active during the study had higher prevalence of BV-associated bacteria at the first visit, we suspect that these women were already participating in non-penetrative behaviors. This is consistent with a previous analysis of this cohort showing that women who initiated vaginal intercourse during the study had higher incidence of HPV infection prior to sexual debut, suggesting a non-penile exposure.(25
) Self-report of sexual activity could be incorrect, as some women may be uncomfortable disclosing sexual information to researchers. In addition, our sample size is small, due to the unique nature of the cohort, and thus affords limited power to detect any differences. Our participants are also a low-risk population for development of BV: primarily white, well-educated, non-smokers with few sexual partners. However, this allowed us to come closer to isolating the effect of sex alone on the vaginal microbiota. We tested for 11 individual bacterial species, both healthy commensals and well-described BV-associated species, but this is not a comprehensive assessment of all potential species that may colonize the vagina.
Sexual activity is clearly linked to the development of BV, but likely via a more complex mechanism than some other sexually transmitted infections. Some have hypothesized that the change in vaginal pH due to semen is what drives the shift in microbiota that results in BV. We did not have adequate numbers of women with reported information on condom use to draw any conclusions about the effect of semen, independent of sexual activity. Interestingly, the women who did gain colonization with BV-associated bacteria reported fewer penile-vaginal sex acts, and higher percentage of condom use with those acts.
Young, sexually inexperienced women are unlikely to have vaginal colonization with BV-associated bacterial species, and the composition of the vaginal microbiota does not change rapidly after initiation of sexual activity. This suggests that the association between sex and the development of BV is complex.