Evaluation of Brucella melitensis B115 as rough-phenotype vaccine against B. melitensis and B. ovis infections.

Brucella melitensis strain Rev1 is used as vaccine for the prophylaxis of brucellosis in sheep and goats. Because of its smooth phenotype, however, it induces antibodies directed to the O-polysaccharide (O-PS) of the lipopolysaccharide (LPS), thus unabling to distinguish between vaccinated and infected animals. It has been speculated that alternative vaccines could be live, attenuated Brucella rough strains, which are devoid of the O-PS. B. melitensis B115 is a natural, attenuated, rough strain. The O-PS is not exposed at the surface but is present in the cytoplasm. We tested the protective activity of B115 against B. melitensis and B. ovis infections in mice, in comparison with that of Rev1. The residual virulence and the humoral response following B115 vaccination were also evaluated. Vaccination with B115 conferred significant protective immunity against B. melitensis 16M and B. ovis challenge strains, equivalent to that provided by Rev1. No interfering antibodies to O-PS were detected, while the B115 vaccination was monitored by a specific B115-based complement fixation test. These promising features suggest further evaluation of B. melitensis B115 as vaccine for target animal hosts.