EFFECT OF POLYMYXIN-B NONAPEPTIDE ON DAPTOMYCIN PERMEABILITY AND CELL-SURFACE PROPERTIES IN PSEUDOMONAS-AERUGINOSA, ESCHERICHIA-COLI, AND PASTEURELLA-MULTOCIDA

The present study was carried out to determine if sensitization of Gra m-negative bacteria to the polyanionic antibiotic daptomycin by cation ic molecules can be explained on the basis of decreased cell surface c harge in order to better understand intrinsic resistance. Turbidimetri c assessments of batch cultural growth kinetics revealed the outer mem brane permeabilizer polymyxin B nonapeptide sensitized Pseudomonas aer uginosa and Escherichia coli to the hydrophobic probe novobiocin, wher eas little or no sensitization was observed for two surface hydrophobi city variants of Pasteurella multocida. Polymyxin B nonapeptide and da ptomycin synergistically inhibited growth of P. aeruginosa only. A hyd rocarbon adherence assay revealed permeabilizing concentrations of pol ymyxin B nonapeptide increased cell surface hydrophobicity of P. aerug inosa and the hydrophobic P. multocida variant, while E. coli and the hydrophilic P. multocida variant remained unaffected. Measurement of c ellular electrophoretic mobility showed polymyxin B nonapeptide permea bilization of P. aeruginosa to daptomycin occurred concomitantly with a significant decrease in cell surface charge, while no such sensitiza tion occurred in organisms which failed to undergo polymyxin B nonapep tide-induced surface charge decreases. These data suggest that sensiti zation of Gram-negative bacteria to polyanionic lipopeptides by growth in the presence of polycationic outer membrane permeabilizers such as polymyxin B nonapeptide is dependent on diminution of overall cell su rface charge and polarity, thereby allowing outer cell envelope permea tion.