Adaptive resistance to polymyxin in Pseudomonas aeruginosa due to an outer membrane impermeability mechanism

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Abstract
The isolated outer membrane from cells of a Pseudomonas aeruginosa strain exhibiting adaptive resistance to polymyxin was not affected by polymyxin treatment, as monitored by electron microscopy of negatively stained preparations. This was in sharp contrast with extensive disruption by polymyxin of the outer membranes of the parent polymyxin-sensitive strain and the resistant strain following reversion to greater polymyxin sensitivity. The isolated cytoplasmic membrane of the polymyxin-resistant strain, on the other hand, remained sensitive to the disruptive effects of polymyxin treatment. The permeability characteristics of the resistant strains appeared to be altered, as indicated by differences in minimal inhibitory concentrations for a variety of antibiotics between the polymyxin-sensitive and polymyxin-resistant strains. No evidence was found for a polymyxin-inactivating enzyme in osmotic shock fluid from the polymyxin-resistant strain. No evidence for a cytoplasmic membrane repair mechanism was found in the polymyxin-resistant strain. These observations suggest that the mechanism of adaptive polymyxin resistance in this model system is the alteration of the outer membrane so that it excludes polymyxin from reaching the still sensitive cytoplasmic membrane.