Streptococcus wuxiensis sp. nov., Streptococcus jiangnanensis sp. nov. and Streptococcus fermentans sp. nov.: three novel species of genus Streptococcus isolated from human breast milk.

Three novel coccoid-shaped strains, designated 21WXBC0057M1^T, 21WXBC0044M1^T and BJSWXB5TM5^T, were isolated from human breast milk in Wuxi, Jiangsu Province, China. These strains were facultative anaerobes, catalase-negative and Gram-positive. Through a comprehensive analysis of rRNA genes, protein-coding housekeeping genes and genomic phylogeny, we identified these strains as belonging to the genus Streptococcus. Specifically, strain 21WXBC0057M1^T was phylogenetically most closely related to Streptococcus infantis, strain 21WXBC0044M1^T was most closely related to Streptococcus oralis and strain BJSWXB5TM5^T displayed similarities to Streptococcus australis, Streptococcus peroris and S. infantis. The pairwise average nucleotide identity and digital DNA-DNA hybridization values for these three strains were below 95 and 70%, respectively, indicating that they occupied evolutionary branches distinct from all previously validly published Streptococcus species. Distinctive phenotypic characteristics discriminated these novel species from the type strains of their most closely related species. The major cellular fatty acids in the three strains were C_16 : 0 and C_18 : 0. Genome annotation and a thorough examination of carbohydrate-active enzyme distribution highlighted the observation that all strains possessed extensive capabilities for carbohydrate metabolism, particularly human milk oligosaccharides utilization. Thus, based on these findings, we proposed the classification of the strains as representing three novel species within the genus Streptococcus: Streptococcus wuxiensis sp. nov. (type strain 21WXBC0057M1^T=GDMCC 1.4126^T=KCTC 25760^T), Streptococcus jiangnanensis sp. nov. (type strain 21WXBC0044M1^T= GDMCC 1.4127^T=KCTC 25762^T) and Streptococcus fermentans sp. nov. (type strain BJSWXB5TM5^T=GDMCC 1.4130^T=KCTC 25759^T).