The Brucella melitensis M5-90 phosphoglucomutase (PGM) mutant is attenuated and confers protection against wild-type challenge in BALB/c mice

Brucellae are Gram-negative intracellular bacterial pathogens that infect humans and animals, bringing great economic burdens to developing countries. Live attenuated Brucella vaccines (strain M5-90 or others) are the most efficient means for prevention and control of animal brucellosis. However, these vaccines have several drawbacks, including residual virulence in animals, and difficulties in differentiating natural infection from vaccine immunization, which limit their application. A vaccine that can differentiate infection from immunization will have extensive applications. A Brucella melitensis (B. melitensis) strain M5-90 pgm mutant (M5-90Δpgm) was constructed to overcome these drawbacks. M5-90Δpgm showed significantly reduced survival in embryonic trophoblast cells and in mice, and induced high protective immunity in BALB/c mice. Moreover, M5-90Δpgm elicited an anti-Brucella-specific immunoglobulin G response and induced the secretion of gamma interferon (IFN-γ) and interleukin-2 (IL-2). In addition, M5-90Δpgm induced the secretion of IFN-γ in immunized sheep. Serum samples from sheep inoculated with M5-90Δpgm were negative by the Rose Bengal Plate Test (RBPT) and Standard Tube Agglutination Test (STAT). Furthermore, the PGM antigen allowed serological differentiation between infected and vaccinated animals. These results suggest that M5-90Δpgm is an ideal live attenuated vaccine candidate against B. melitensis 16 M and deserves further evaluation for vaccine development.