High genetic similarity of Streptococcus agalactiae and Streptococcus difficilis: S. difficilis Eldar et al. 1995 is a later synonym of S. agalactiae Lehmann and Neumann 1896 (Approved Lists 1980)

Abstract

The genetic relationship betweenStreptococcus agalactiaeandStreptococcus difficiliswas studied.S. difficiliswas originally described as serologically non-typable but was later reported to be a group B, type Ib streptococcus. Upon comparative analysis of five gene sequences, it was found thatS. agalactiaeandS. difficilisare closely related. Sequence similarity values between these two species were 100·0 % for 16S rRNA, 99·6 % forgyrB, 98·6 % forsodA, 99·5 % forgyrAand 99·8 % forparCgenes. These data strongly suggest thatS. agalactiaeandS. difficilisare synonyms. The biochemical characteristics ofS. difficilis, which differ slightly from those of typicalS. agalactiae, are similar to those of other group B, type Ib streptococci isolated from fish and frogs. Whole genome DNA–DNA hybridization values between the type strains of both species were greater than 78·6 %. On the basis of these data, it is proposed thatS. difficilisis a later synonym ofS. agalactiae.