streptococcus downei streptococcus downei streptococcus downei is a gram-positive bacterium, with type strain mfe28 (nctc 11391t). it is thought to be cariogenic. streptococcus downei scientific classification domain: bacteria phylum: bacillota class: bacilli order: lactobacillales family: streptococcaceae genus: streptococcus species: s. downei binomial name streptococcus downeiwhiley et al. 1988