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.

Abstract

Currently, <i>Lactococcus lactis</i> contains four subspecies: <i>L. lactis</i> subsp. <i>lactis</i>, <i>L. lactis</i> subsp. <i>hordniae</i>, <i>L. lactis</i> subsp. <i>cremoris</i> and <i>L. lactis</i> subsp. <i>tructae</i>. In the study of Pérez <i>et al.</i>, these four subspecies could be clearly divided into two groups based on <i>recA</i> sequence analysis: <i>L. lactis</i> subsp. <i>lactis</i> and <i>L. lactis</i> subsp. <i>hordniae</i>; <i>L. lactis</i> subsp. <i>cremoris</i> and <i>L. lactis</i> subsp. <i>tructae</i>. The two groups had a relatively low DNA-DNA hybridization value (about 60 %). In the present study, the taxonomic position of <i>L. lactis</i> subsp. <i>cremoris</i> and <i>L. lactis</i> subsp. <i>tructae</i> was re-examined based on sequence analyses of 16S rRNA, <i>rpoB</i>, <i>recA</i> and <i>pheS</i> genes, average nucleotide identity (ANI) values and digital DNA-DNA hybridization (dDDH) values. The result of 16S rRNA gene sequence analysis indicated that <i>L. lactis</i> subsp. <i>cremoris</i> NCDO 607^T and <i>L. lactis</i> subsp. <i>tructae</i> L105^T were phylogenetically related to the type strains of <i>L. lactis</i> subsp. <i>hordniae</i>, <i>L. lactis</i> subsp. <i>lactis</i>, <i>Lactococcus taiwanensis</i>, <i>Lactococcus kimchii</i>, <i>Lactococcus allomyrinae</i>, <i>Lactococcus protaetiae</i>, <i>Lactococcus hircilactis</i>, <i>Lactococcus fujiensis</i> and <i>Lactococcus nasutitermitis</i>. The 16S rRNA gene, <i>rpoB</i>, <i>recA</i>, <i>pheS</i> and concatenated <i>rpoB</i>, <i>recA</i> and <i>pheS</i> sequence similarities, ANI values, and dDDH values between the type strains of <i>L. lactis</i> subsp. <i>cremoris</i>, <i>L. lactis</i> subsp. <i>tructae</i> 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 <i>L. lactis</i> subsp. <i>cremoris</i> and <i>L. lactis</i> subsp. <i>tructae</i> represent a novel species in the genus <i>Lactococcus</i>. Because <i>L. lactis</i> subsp. <i>cremoris</i> was proposed and validated before <i>L. lactis</i> subsp. <i>tructae</i>, <i>L. lactis</i> subsp. <i>cremoris</i> is elevated to the species level as <i>Lactococcus cremoris</i> sp. nov. and <i>L. lactis</i> subsp. <i>tructae</i> is transferred to <i>L. cremoris</i> as <i>L. cremoris</i> subsp. <i>tructae</i> comb. nov. The type strain of <i>L. cremoris</i> 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 <i>L. cremoris</i> subsp. <i>tructae</i> comb. nov. is L105^T (=NBRC 110453^T=DSM 21502^T=JCM 31125^T=LMG 24662^T).