Action of polymyxin B on bacterial membranes. Binding capacities for polymyxin B of inner and outer membranes isolated from Salmonella typhimurium G30

Radioactive mono-N-acetyl-14C-polymyxin B or natural polymyxin B are within 60 s absorbed by isolated inner (cytoplasmic) and outer membranes from Salmonella typhimuriumG30. The sigmoidal binding isotherms indicate saturation of inner and outer membranes with approximately 30 and 60 nmoles polymyxin B bound per mg membrane, respectively. Based on the known content of these membranes in lipopolysaccharide, phosphatidylglycerol, cardiolipin and phosphatidylethanolamine, a calculation of the theoretical binding capacities yields almost identical values if lipopolysaccharide, phosphatidylglycerol and cardiolipin are assumed to function as the actual binding sites for the antibiotic in the isolated membranes. The excellent agreement between theoretical evaluation and experimental determination of polymyxin B-binding capacities leaves little doubt that the named anionic compounds are the chemoreceptors for the cationic antibiotic. This is further substantiated by very similar binding and killing kinetics of polymyxin B.