Immunologic and genetic comparison of Streptococcus equi isolates from the United States and Europe.

A series of isolates of Streptococcus equi from the United States and Europe were compared by the bactericidal test, immunoblotting, DNA restrictions, and Southern hybridization analysis. All isolates tested were sensitive to the same bactericidal serum. In addition, immunoblotting revealed no differences in M proteins prepared by acid or mutanolysin extraction. Immunoblotting of acid extracts of the isolates with mucosal nasopharyngeal mucus from a convalescent horse revealed the presence of the 41,000- and 46,000-Mr polypeptide fragments of the M protein of S. equi known to be important in stimulating mucosal nasopharyngeal immune responses. DNA restriction analysis of total cell DNA digests, as well as Southern hybridizations using an S. equi M protein gene probe, did not detect any differences among these isolates. Our results, therefore, confirm the antigenic homogeneity of the M proteins of S. equi isolates and suggest that variation in this antigen is not a reason for the failure of commercial vaccines in the field. Interestingly, the protoplast M proteins of all isolates showed remarkable size homogeneity, in contrast to the size variation reported in M proteins of group A streptococci.