THE OUTER MEMBRANES OF BRUCELLA SPP ARE RESISTANT TO BACTERICIDAL CATIONIC PEPTIDES

The actions of polymyxin B, rabbit polymorphonuclear lysosome extracts , 14 polycationic peptides (including defensin NP-2, cecropin P1, lact oferricin B, and active peptides from cationic protein 18 and bactenec in), EDTA, and Tris on Brucella spp. were studied, with other gram-neg ative bacteria as controls. Brucella spp. were comparatively resistant to all of the agents listed above and bound less polymyxin B, and the ir outer membranes (OMs) were neither morphologically altered nor perm eabilized to lysozyme by polymyxin B concentrations, although both eff ects were observed for controls. EDTA and peptides increased or accele rated the partition of the hydrophobic probe N-phenyl-naphthylamine in to Escherichia coli and Haemophilius influenzae OMs but had no effect on Brucella OMs. Since Brucella and H. influenzae OMs are permeable to hydrophobic compounds (G. Martinet de Tejada and I. Moriyon, J. Bacte riol. 175:5273-5275, 1993), the results show that such unusual permeab ility is not necessarily related to resistance to polycations. Althoug h rough (R) B. abortus and B. ovis were more resistant than the contro ls were, there were qualitative and quantitative differences with smoo th (S) brucellae; this may explain known host range and virulence diff erences. Brucella S-lipopolysaccharides (LPSs) had reduced affinities for polycations, and insertion of Brucella and Salmonella montevideo S -LPSs into the OM of a Brucella R-LPS mutant increased and decreased, respectively, its resistance to cationic peptides. The results show th at the core lipid A of Brucella LPS plays a major role in polycation r esistance and that O-chain density also contributes significantly. It is proposed that the features described above contribute to Brucella r esistance to the oxygen-independent systems of phagocytes.