Epidemiologic Evidence on the Role of Lactobacillus iners in Sexually Transmitted Infections and Bacterial Vaginosis: A Series of Systematic Reviews and Meta-Analyses

Although Lactobacillus crispatus–dominated vaginal microbiotas are thought to protect against bacterial vaginosis (BV) and sexually transmitted infections, the role of Lactobacillus iners–dominated microbiotas is less clear. To better understand the impact of L. iners on common cervicovaginal infections, we conducted systematic reviews of the associations between L. iners compared with L. crispatus and 8 outcomes: Chlamydia trachomatis (Ct), BV, human papillomavirus, cervical dysplasia, human immunodeficiency virus, genital herpes, Trichomonas vaginalis, and Neisseria gonorrhoeae. On April 30, 2021, we searched PubMed, Embase, Cochrane Library, and Web of Science for epidemiologic studies of reproductive-age, nonpregnant, cisgender women that used marker gene sequencing to characterize vaginal microbiota composition and presented an effect estimate for the association between L. iners, compared with L. crispatus, and outcomes of interest. For outcomes with ≥3 eligible results presenting the same form of effect estimate, we conducted random-effects meta-analysis. The review protocol was registered prospectively (PROSPERO CRD42020214775). Six Ct studies were included in meta-analysis, which showed L. iners–dominated microbiotas were associated with 3.4-fold higher odds of Ct compared with L. crispatus–dominated microbiotas (95% confidence interval, 2.1–5.4). Three BV studies were included in meta-analysis, which indicated L. iners–dominated microbiotas were associated with 2.1-fold higher prevalence of BV compared with L. crispatus–dominated microbiotas (95% confidence interval, 0.9–4.9). Evidence was too sparse to perform meta-analysis for the remaining outcomes. L. iners–dominated vaginal microbiotas may be suboptimal compared with L. crispatus–dominated microbiotas for BV and Ct. These reviews highlight evidence gaps regarding the remaining outcomes and opportunities to improve epidemiologic rigor in vaginal microbiome science.