Disclosed is a Streptococcus vaccine strain comprising the following modifications in its genome: (i) attenuation of the aroB and pyrC genes or homologues thereof as specified in Table 7A, (ii) attenuation of one or more genes which encode a haemolytic toxin, or the sagA gene or a homologue thereof as specified in Table 7A, plus any one, two, or three of the following modifications: (iii) attenuation of one or more genes which encode a protein responsible for formation of a hyaluronate capsule, (iv) modification of the seM gene or a homologue thereof as specified in Table 7A to permit serological discrimination of the vaccine strain based on analysis of a protein encoded by said gene, and (v) attenuation of one or more genes which encode an enzyme responsible for recombination repair of the genome, wherein the Streptococcus vaccine strain is selected from any one of Streptococcus equi, Streptococcus zooepidemicus and Streptococcus pyogenes.
