Brucella abortusand Its Closest Phylogenetic Relative,Ochrobactrumspp., Differ in Outer Membrane Permeability and Cationic Peptide Resistance

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Abstract
The outer membrane (OM) of the intracellular parasiteBrucella abortusis permeable to hydrophobic probes and resistant to destabilization by polycationic peptides and EDTA. The significance of these unusual properties was investigated in a comparative study with the opportunistic pathogens of the genusOchrobactrum, the closest knownBrucellarelative.Ochrobactrumspp. OMs were impermeable to hydrophobic probes and sensitive to polymyxin B but resistant to EDTA. These properties were traced to lipopolysaccharide (LPS) because (i) insertion ofB. abortusLPS, but not ofEscherichia coliLPS, intoOchrobactrumOM increased its permeability; (ii) permeability and polymyxin B binding measured with LPS aggregates paralleled the results with live bacteria; and (iii) the predicted intermediate results were obtained withB. abortus-Ochrobactrum anthropiandE. coli-O. anthropiLPS hybrid aggregates. AlthoughOchrobactrumwas sensitive to polymyxin, self-promoted uptake and bacterial lysis occurred without OM morphological changes, suggesting an unusual OM structural rigidity.OchrobactrumandB. abortusLPSs showed no differences in phosphate, qualitative fatty acid composition, or acyl chain fluidity. However,OchrobactrumLPS, but notB. abortusLPS, contained galacturonic acid.B. abortusandOchrobactrumsmooth LPS aggregates had similar size and zeta potential (−12 to −15 mV). Upon saturation with polymyxin, zeta potential became positive (1 mV) forOchrobactrumsmooth LPS while remaining negative (−5 mV) forB. abortussmooth LPS, suggesting hindered access to inner targets. These results show that althoughOchrobactrumandBrucellashare a basic OM pattern, subtle modifications in LPS core cause markedly different OM properties, possibly reflecting the adaptive evolution ofB. abortusto pathogenicity.