Emended descriptionof Lactobacillus

Lactobacillus species are Gram-positive, homofermentative, thermophilic and non-spore-forming rods. Most Lactobacillus species do not ferment pentoses and none of the organisms encode genes for the pentose–phosphate pathway or pyruvate formate lyase. The emended description of the genus includes all organisms that were previously assigned to the L. delbrueckii group [17]. Lactobacillus species are host-adapted; the Lactobacillus melliventris clade (previously termed the Firm-5 clade) is adapted to social bees [52] while all other Lactobacillus speciesare adapted tovertebrate hosts. Lactobacillus species ferment a relatively broad spectrum of carbohydrates and have the strain-specific ability to ferment extracellular fructans, starch, or glycogen [53, 54]. The L. melliventris cladespecies also ferment awider range of carbohydrates when compared to insect-adapted species in the genera Apilactobacillus and Bombilactobacillus. In intestinal habitats, Lactobacillus species are generally associated with heterofermentative lactobacilli. For specific examples, it was shown that co-habitation of Lactobacillus species with heterofermentative lactobacilli is based on long-term evolutionary relationships in biofilms [55] and a complementary preference for carbon sources [18, 56]. Many Lactobacillus species are able to ferment mannitol, which also reflects co-habitation with heterofermenters. The metabolic focus of L. delbrueckii on lactose [57] explains its dominance in yoghurt and cheese fermentations but also relates to its presence in the intestine of suckling piglets [54]. The genus Lactobacillus remains a relatively heterogenous genus with L. iners as the mostdistant member. L. iners has the smallest genome size among all Lactobacillaceae, which reflects its strict adaptation to the human vagina.

In addition to their relevance inintestinal andvaginal ecosystems, Lactobacillus species frequently occur in dairy and cereal fermentations and are widely used as starter cultures for production of fermented dairy products [58, 59].

Aphylogenetic treeof all species in the genus Lactobacillus is provided in Fig. S6A.

The type species of the genus Lactobacillus is L. delbrueckii. Although the nomenclature of species in the emended genus Lactobacillus remains unchanged, a listof species and a listof their properties is provided below.