Streptococcus humanilactis sp.nov., isolated from healthy nursing mother's breast milk

Abstract

Two bacterial strains were isolated from the breast milk of two healthy nursing mothers. The isolates were Gram-positive, catalase-negative, coccus-shaped, chain-forming organisms. Analysis of the 16S rRNA gene sequences of strain IMAU99125^T shared 99.7 and 99.6% similarity with Streptococcus mitis ATCC 49456 ^T and Streptococcus pseudopneumoniae ATCC BAA-960 ^T, respectively. The nearly complete 16S rRNA gene sequences of IMAU99125^T and IMAU99674 strains were very closely related (with only 0.06% difference between them). Sequence analysis of the gyrB and rpoB genes also indicated that IMAU99125^T was closely related to S. mitis ATCC 49456 ^T (94.7% and 97.1%, respectively) and S. pseudopneumoniae ATCC BAA-960 ^T (94.4% and 97.1%, respectively). Average nucleotide identity (ANI) values between strain IMAU99125^T and S. mitis ATCC 49456 ^T and S. pseudopneumoniae ATCC BAA-960 ^T, were 93.3% and 92.7%, respectively. Genome-to-genome distance (GGD) values between strain IMAU99125^T and S. mitis ATCC 99125 ^T and S. pseudopneumoniae ATCC BAA-960 ^T were 53.4% (50.7-56.0) and 50.4% (47.7-53.0), respectively. The major fatty acids of the strain were C_16:0 (51.4%). On the basis of the results of phenotypic and phylogenetic analyses, we propose that the two strains be classified as representing a novel species of the genus Streptococcus, namely Streptococcus humanilactis sp.nov. The type strain is IMAU99125^T (= GDMCC 1.1876 ^T = KCTC 21157 ^T). The genome of Streptococcus humanilactis sp. nov. is comprised of 2,027,143 bp. The DNA G + C content of the strain is 40.0 mol%.

Keywords: Breast milk; Novel species; Polyphasic taxonomy; Streptococcus.