Evaluation of residual virulence and protective efficacy for Brucella melitensis vaccine strain M5-90 in mice and guinea pigs.

Brucellosis, a globally significant zoonosis caused by Brucella species, leads to severe reproductive complications including abortion and infertility in animals and humans. Vaccination remains a cornerstone strategy for the control of brucellosis transmission. The Brucella melitensis vaccine strain M5‑90 was developed in China during the 1970s-1980s by attenuation from virulent M28 strain. This study systematically evaluated the safety and protective efficacy of M5‑90 in mouse and guinea pigs models. Results indicated that M5‑90 was completely cleared from mice within 15 weeks after subcutaneous inoculation of 10⁸ CFU, exhibiting a 50 % recovery time (RT50) of 9.27 ± 1.26 weeks. Bacterial replication was not dose-dependent. The bacterial burden in M5‑90-infected mice reduced by 1-3 log10 compared with the virulent M28 group at a challenge dose of 10⁶ CFU (P < 0.05). Immunization with 10⁵ CFU conferred significant protection against virulent B. melitensis M28 and B. abortus 544 challenges at 45 and 150 days post-vaccination, respectively (P < 0.05 or P < 0.01). In guinea pigs, the splenic bacterial load of M5‑90 was markedly lower than that in the M28 group (2440 ± 240 vs. 223,000 ± 3800 CFU/g; P < 0.001). A dose of 3 × 10⁸ CFU of M5‑90 provided complete (100 %) and strong (90 %) protection against M28 and 544 challenges, respectively. In summary, Brucella melitensis vaccine strain M5‑90 demonstrates favorable safety and protective efficacy in mice and guinea pigs, supporting its potential as a promising vaccine for brucellosis control.