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)

The genetic relationship between Streptococcus agalactiae and Streptococcus difficilis was studied. S. difficilis was 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 that S. agalactiae and S. difficilis are closely related. Sequence similarity values between these two species were 100.0 % for 16S rRNA, 99.6 % for gyrB, 98.6 % for sodA, 99.5 % for gyrA and 99.8 % for parC genes. These data strongly suggest that S. agalactiae and S. difficilis are synonyms. The biochemical characteristics of S. difficilis, which differ slightly from those of typical S. 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 that S. difficilis is a later synonym of S. agalactiae.