Elevation of Lactococcus lactis subsp. cremoris to the species level as Lactococcus cremoris sp. nov. and transfer of Lactococcus lactis subsp. tructae to Lactococcus cremoris as Lactococcus cremoris subsp. tructae comb. nov.

Currently, Lactococcus lactis contains four subspecies: L. lactis subsp. lactis , L. lactis subsp. hordniae , L. lactis subsp. cremoris and L. lactis subsp. tructae . In the study of Pérez et al. , these four subspecies could be clearly divided into two groups based on recA sequence analysis: L. lactis subsp. lactis and L. lactis subsp. hordniae ; L. lactis subsp. cremoris and L. lactis subsp. tructae . The two groups had a relatively low DNA–DNA hybridization value (about 60 %). In the present study, the taxonomic position of L. lactis subsp. cremoris and L. lactis subsp. tructae was re-examined based on sequence analyses of 16S rRNA, rpoB , recA and pheS genes, average nucleotide identity (ANI) values and digital DNA–DNA hybridization (dDDH) values. The result of 16S rRNA gene sequence analysis indicated that L. lactis subsp. cremoris NCDO 607 T and L. lactis subsp. tructae L105 T were phylogenetically related to the type strains of L. lactis subsp. hordniae , L. lactis subsp. lactis , Lactococcus taiwanensis , Lactococcus kimchii , Lactococcus allomyrinae , Lactococcus protaetiae , Lactococcus hircilactis , Lactococcus fujiensis and Lactococcus nasutitermitis . The 16S rRNA gene, rpoB , recA , pheS and concatenated rpoB , recA and pheS sequence similarities, ANI values, and dDDH values between the type strains of L. lactis subsp. cremoris , L. lactis subsp. tructae and phylogenetically related species were 93.5–99.4 %, 83.3–97.6 %, 80.6–92.4 %, 79.7–92.7 %, 83.5–94.3 %, 72.4–86.9 % and 21.4–32.5 %, respectively. Lower than 95–96 % ANI values and lower than 70 % dDDH values confirmed that the type strains of L. lactis subsp. cremoris and L. lactis subsp. tructae represent a novel species in the genus Lactococcus . Because L. lactis subsp. cremoris was proposed and validated before L. lactis subsp. tructae , L. lactis subsp. cremoris is elevated to the species level as Lactococcus cremoris sp. nov. and L. lactis subsp. tructae is transferred to L. cremoris as L. cremoris subsp. tructae comb. nov. The type strain of L. cremoris sp. nov. is NCDO 607 T (=ATCC 19257 T =DSM 20069 T =JCM 16167 T =LMG 6897 T =NBRC 100676 T ). The type strain of L. cremoris subsp. tructae comb. nov. is L105 T (=NBRC 110453 T =DSM 21502 T =JCM 31125 T =LMG 24662 T ).