Lactobacillus iners at the nexus of microbiota, immunity, and pregnancy.
Pregnancy induces a dynamic reconfiguration of the vaginal microbiome, typically marked by increased dominance of Lactobacillus species and reduced microbial diversity. Among these bacteria, Lactobacillus iners stands out for its unique genomic traits, controversial associations with vaginal health, and frequent presence across all stages of gestation. This review synthesizes current literature on the maternal microbiome with a focus on L. iners, exploring its strain-level diversity, metabolic idiosyncrasies, and inflammatory potential. We discuss how host factors such as ethnicity, sexual activity, maternal age, and especially obesity, influence microbial composition, and evaluate conflicting data surrounding L. iners in contexts like in vitro fertilization, preterm birth, and postpartum recovery. Emerging evidence suggests that L. iners may act as a transitional species, whose effect on pregnancy outcomes depends on its abundance, genetic features, and interactions with the host immune system. We also assess limitations of current animal models and propose future directions for understanding this enigmatic bacterium. Unraveling the role of L. iners will be essential to predicting, preventing, and managing adverse pregnancy outcomes in diverse populations.