Plasmids in Lactic Acid Bacteria: From Functional Diversity to Ecological Adaptation and Fermentation Performance

The fermentation performance of lactic acid bacteria (LAB) is closely associated with their adaptation to distinct food ecosystems. As dynamic components of the accessory genome, plasmids provide an important genetic basis for the functional diversity of LAB strains. This review examines the structural features, individual plasmid size, per-strain plasmid number and composition, and functional roles of LAB plasmids, synthesizing recent advances in their contributions to ecological adaptation and fermentation performance. LAB plasmids harbor functional modules involved in lactose utilization, proteolysis, citrate transport, bacteriocin production, phage defense, and stress tolerance. These modules can broaden the metabolic repertoire of LAB, enhance ecological competitiveness, and contribute to acidification, flavor formation, and fermentation quality. Horizontal gene transfer and insertion sequence-mediated recombination can facilitate the acquisition, rearrangement, and dissemination of plasmid-borne functional modules, with their adaptive effects depending on the host genetic background and environmental conditions. Collectively, LAB plasmids connect genomic plasticity, metabolic capacity, and strain-specific fermentation phenotypes through interactions with host metabolism and environmental requirements. For starter culture selection, these functional contributions should therefore be considered alongside plasmid stability and the safety implications of acquired antimicrobial resistance.