Evaluation of Brucella abortus S19 vaccine strains by bacteriological tests, molecular analysis of ery loci and virulence in BALB/c mice

Two Brucella abortus S19 commercial vaccine strains used for vaccination against brucellosis in India and three S19 strains available as international reference were examined by microbiological assays and molecular analysis of the ery loci involved in erythritol metabolism, and tested for residual virulence in BALB/c mice. According to the sensitivity to penicillin and i-erythritol, the five strains tested had the phenotypic characteristics of strain S19. However, on culture medium containing i-erythritol, all strains developed spontaneous i-erythritol resistant colonies at mutation rates ranging from 1.42 × 10−2 to 1.33 × 10−6. The S19 characteristic 702 bp deletion in the erythrulose 1-phosphate dehydrogenase gene of the ery locus was present only in the three reference strains but not in the two commercial vaccines. Both commercial strains and one of the reference strains showed reduced virulence in BALB/c mice. The presence or absence in S19 strains of the 702 bp deletion in the ery locus had no correlation with either the rates of spontaneous mutation to erythritol resistance or the residual virulence in mice.