Objective L crispatus, L. jensenii, L. gasseri and L. iners are the predominant lactobacilli in the vaginal flora of reproductive aged women. Colonization of the vagina and rectum by lactobacilli has been associated with decreased risk of BV. We evaluated the species-specific role of Lactobacillus on the acquisition of BV.

Methods Two hundred forty four healthy asymptomatic women aged 18–40 were followed at 2 month intervals for up to 18 months. At each visit, vaginal and rectal swabs for culture detection of lactobacilli and a vaginal smear for diagnosis of BV using Nugent criteria were collected. Lactobacilli were identified to the species level using repetitive sequence PCR and/or 16S rDNA sequencing. The risk of BV acquisition using Lactobacillus colonisation vaginally and/or rectally as a time-varying covariate was evaluated using Cox proportional hazards models.

Results This analysis included 1481 follow-up visits at which 235 women were colonised by L. crispatus, L. jensenii, L. gasseri, or L. iners. Of 2734 vaginal and1861 rectal lactobacilli recovered, 1968 were L. crispatus, 1024 L. jensenii, 909 L. gasseri, 410 L. iners, and 284 other species. Eighty nine women acquired BV over 220.4 woman-years (WY) for an incidence of 40 per 100 WY. The rate of BV was lowest among women colonised by L. crispatus at the prior visit (25 per 100 WY, unadjusted hazards ratio 0.31, 95% confidence interval: 0.16–0.62), compared to a rate of 100 per 100 WY among women having only L. iners. Vaginal and/or rectal colonisation by L. jensenii or L. gasseri was not associated withlower rates of BV acquisition (60 and 76 per 100 WY, respectively (p > 0.05)) than the rate observed among women having only L. iners.

Conclusions Although there is Lactobacillus species diversity in the vaginal microbiome, L. crispatus has the greatest protective benefit against acquisition of BV.