dLactococcus lactis is predominantly associated with dairy fermentations, but evidence suggests that the domesticated organism originated from a plant niche. L. lactis possesses an unusual taxonomic structure whereby strain phenotypes and genotypes often do not correlate, which in turn has led to confusion in L. lactis classification. A bank of L. lactis strains was isolated from various nondairy niches (grass, vegetables, and bovine rumen) and was further characterized on the basis of key technological traits, including growth in milk and key enzyme activities. Phenotypic analysis revealed all strains from nondairy sources to possess an L. lactis subsp. lactis phenotype (lactis phenotype); however, seven of these strains possessed an L. lactis subsp. cremoris genotype (cremoris genotype), determined by two separate PCR assays. Multilocus sequence typing (MLST) showed that strains with lactis and cremoris genotypes clustered together regardless of habitat, but it highlighted the increased diversity that exists among "wild" strains. Calculation of average nucleotide identity (ANI) and tetranucleotide frequency correlation coefficients (TETRA), using the JSpecies software tool, revealed that L. lactis subsp. cremoris and L. lactis subsp. lactis differ in ANI values by ϳ14%, below the threshold set for species circumscription. Further analysis of strain TIFN3 and strains from nonindustrial backgrounds revealed TETRA values of <0.99 in addition to ANI values of <95%, implicating that these two groups are separate species. These findings suggest the requirement for a revision of L. lactis taxonomy.
Lactococcus lactis is a member of the lactic acid bacteria (LAB), a group of organisms used worldwide in the production of fermented dairy products. Three Lactococcus lactis subspecies exist: Lactococcus lactis subsp. cremoris, L. lactis subsp. lactis, and L. lactis subsp. hordniae. Many strains of L. lactis subsp. lactis and L. lactis subsp. cremoris are typically associated with dairy fermentations, but evidence suggests that these organisms originated from a plant niche, and they are now considered to be "domesticated" compared to their so-called "wild" counterparts. Conversely, L. lactis subsp. hordniae is unable to utilize lactose and has not been isolated previously from the dairy environment (1). A citrate-metabolizing biovariety also exists, L. lactis biovar diacetylactis, which is capable of imparting buttery aromas in dairy fermentations (2). Information regarding subspecies classification is important for the application of cultures in dairy fermentations, particularly cheese production, as L. lactis subsp. cremoris strains are often considered more suitable due to their association with cheeses free from off-flavors (3).Before the advent of molecular methods, subspecies classification of L. lactis was based on the possession of a number of phenotypic traits. The ability to grow in the presence of 4% NaCl, at 40°C, and at pH 9.2 and the ability to degrade arginine were assigned as traits for L. lactis subspecies...
