Effect of polymyxin B and environmental conditions on isolation of Brucella species and the vaccine strain RB51

Brucella are resistant to polymyxin B (PB), but their relative susceptibility to PB and its derivative, colistin (COL) has not been rigorously or systematically studied. Comparative susceptibility of Brucella reference strains, vaccine strain RB51, and Brucella isolates from marine mammals to these two cationic peptides were determined by Etest. Vast differences among Brucella species were found in susceptibility to both PB and COL. Brucella demonstrated similar pattern of relative susceptibility to PB as that of COL, but they were less susceptible to COL. Both B. melitensis and B. suis were the least susceptible to polymyxins and rough strains were more susceptible to both PB and COL than the smooth except for the BvrR mutant. Strains were generally less susceptible to PB when cultured in CO(2) rather than ambient air; some became more susceptible in acidified medium. Results show that environment cultural conditions must be considered when selecting for CO(2)-independent strains of Brucella especially the vaccine strain RB51 on selective media containing PB. Our observations extend basic knowledge of the differential resistance of Brucella to polymyxins.