Development of attenuated live vaccine candidates against swine brucellosis in a non-zoonotic B. suis biovar 2 background
Brucella is a genus of gram-negative bacteria that cause brucellosis. B. abortus and B. melitensis infect domestic ruminants while B. suis (biovars 1-3) infect swine, and all these bacteria but B. suis biovar 2 are zoonotic. Live attenuated B. abortus S19 and B. melitensis Rev1 are effective vaccines in domestic ruminants, though both can infect humans. However, there is no swine brucellosis vaccine. Here, we investigated the potential use as vaccines of B. suis biovar 2 rough (R) lipopolysaccharide (LPS) mutants totally lacking O-chain (Bs2¿wbkF) or only producing internal O-chain precursors (Bs2¿wzm) and mutants with a smooth (S) LPS defective in the core lateral branch (Bs2¿wadB and Bs2¿wadD). We also investigated mutants in the pyruvate phosphate dikinase (Bs2¿ppdK) and phosphoenolpyruvate carboxykinase (Bs2¿pckA) genes encoding enzymes bridging phosphoenolpyruvate and the tricarboxylic acid cycle. When tested in the OIE mouse model at the recommended R or S vaccine doses (108 and 105 CFU, respectively), CFU/spleen of all LPS mutants were reduced with respect to the wild type and decreased faster for the R than for the S mutants. At those doses, protection against B. suis was similar for Bs2¿wbkF, Bs2¿wzm, Bs2¿wadB and the Rev1 control (105 CFU). As described before for B. abortus, B. suis biovar 2 carried a disabled pckA so that a double mutant Bs2¿ppdK¿pckA had the same metabolic phenotype as Bs2¿ppdK and ppdK mutation was enough to generate attenuation. At 105 CFU, Bs2¿ppdK also conferred the same protection as Rev1. As compared to other B. suis vaccine candidates described before, the mutants described here simultaneously carry irreversible deletions easy to identify as vaccine markers, lack antibiotic-resistance markers and were obtained in a non-zoonotic background. Since R vaccines should not elicit antibodies to the S-LPS and wzm mutants carry immunogenic O-chain precursors and did not improve Bs2¿wbkF, the latter seems a better R vaccine candidate than Bs2¿wzm. However, taking into account that all R vaccines interfere in ELISA and other widely used assays, whether Bs2¿wbkF is advantageous over Bs2¿wadB or Bs2¿ppdK requires experiments in the natural host.