Transmission of lactic acid bacteria: a case study of the human vaginal microbiome.

Lactic acid bacteria (LAB) are among the microorganisms to which humans are most frequently exposed, largely via consumption of fermented foods and probiotics and through interpersonal contact. Such continued exposure to LAB does not necessarily result in stable engraftment, raising fundamental questions about which factors mediate successful LAB transmission and niche specialization across human-associated microbiomes. Here, we rethink transmission of LAB across the human life course, spanning food-associated, within-host, and interindividual routes, highlighting that LAB transfer can occur across multiple contexts, but long-term engraftment depends less on exposure and more on host-mediated selection. As a paradigmatic case study, we present the human adult vaginal microbiome, a low-diversity environment generally dominated by strains of specific Lactobacillus species. These strains display clear signatures of niche specialization in the vaginal environment, making them taxonomically and functionally distinct from other LAB, and illustrating how host-mediated selection rather than exposure alone determines which LAB persist. This proposed conceptual framework, combined with the experimental and computational tractability of LAB, enables a better understanding of LAB engraftment in the human host, contributing to improved design and strain-level efficacy assessment of microbiome-based interventions, including probiotics, live biotherapeutic products, and microbiota transplants.