Lifestyles in transition: evolution and natural history of the genus Lactobacillus

Lactobacillus species are found in nutrient-rich habitats associated with food, feed, plants, animals and humans. Due to their economic importance, the metabolism, genetics and phylogeny of lactobacilli have been extensively studied. However, past research primarily examined lactobacilli in experimental settings abstracted from any natural history, and the ecological context in which these bacteria exist and evolve has received less attention. In this review, we synthesize phylogenetic, genomic and metabolic metadata of the Lactobacillus genus with findings from fine-scale phylogenetic and functional analyses of representative species to elucidate the evolution and natural history of its members. The available evidence indicates a high level of niche conservatism within the well-supported phylogenetic groups within the genus, with lifestyles ranging from free-living to strictly symbiotic. The findings are consistent with a model in which host-adapted Lactobacillus lineages evolved from free-living ancestors, with present-day species displaying substantial variations in terms of the reliance on environmental niches and the degree of host specificity. This model can provide a framework for the elucidation of the natural and evolutionary history of Lactobacillus species and valuable information to improve the use of this important genus in industrial and therapeutic applications.