streptococcus lactarius streptococcus lactarius streptococcus lactarius is a species of streptococcus. first reported in 2010, it was found growing in the breast milk of three non-related healthy women. based on 16s rrna gene sequences, it is most similar to s. peroris. streptococcus lactarius scientific classification domain: bacteria phylum: bacillota class: bacilli order: lactobacillales family: streptococcaceae genus: streptococcus species: s. lactarius binomial name streptococcus lactariusmartín et al. 2011