Brucella abortus and its closest phylogenetic relative, Ochrobactrum spp.,differ in outer membrane permeability and cationic peptide resistance

The outer membrane (OM) of the intracellular parasite Brucella abortus is p ermeable to hydrophobic probes and resistant to destabilization by polycati onic peptides and EDTA. The significance of these unusual properties was in vestigated in a comparative study with the opportunistic pathogens of the g enus Ochrobactrum, the closest known Brucella relative, Ochrobactrum spp, O Ms were impermeable to hydrophobic probes and sensitive to polymyxin B but resistant to EDTA. These properties were traced to lipopolysaccharide (LPS) because (i) insertion of B. abortus LPS, but not of Escherichia coli LPS, into Ochrobactrum OM increased its permeability; (ii) permeability and poly myxin B binding measured, with LPS aggregates paralleled the results with l ive bacteria: and (iii) the predicted intermediate results were obtained wi th B. abortus-Ochrobactrum anthropi and E. coli-O. anthropi LPS hybrid aggr egates. Although Ochrobactrum was sensitive to polymyxin, self-promoted upt ake and bacterial lysis occurred without OM morphological changes, suggesti ng an unusual OM structural rigidity. Ochrobactrum and B. abortus LPSs show ed no differences in phosphate, qualitative fatty acid composition, or acyl chain fluidity. However, Ochrobactrum LPS, but not B. abortus LPS, contain ed galacturonic acid. B, abortus and Ochrobactrum smooth LPS aggregates had similar size and zeta potential (-12 to -15 mV). Upon saturation with poly myxin, zeta potential became positive (1 mV) for Ochrobactrum smooth LPS wh ile remaining negative (-5 mV) for B. abortus smooth LPS, suggesting hinder ed access to inner targets. These results show that although Ochrobactrum a nd Brucella share a basic OM pattern, subtle modifications in LPS core caus e markedly different OM properties. possibly reflecting the adaptive evolut ion of B. abortus to pathogenicity.