Effect of polymyxin B on antibiotic-resistant Proteus mirabilis.

The surface properties of polymyxin B-resistant Proteus mirabilis are markedly altered by the antibiotic. The effects include the development of susceptibility to surface-active agents such as deoxycholate or tris(hydroxymethyl)aminomethane and a marked increased osmotic fragility. However, cell wall impermeability to various agents such as erythromycin, actinomycin D, bacitracin, lysozyme, and triphenyltetrazolium chloride apparently remains intact. These results support the concept that the actions of the polymyxins on gram-negative bacteria are multiple; the action(s) of the antibiotic responsible for lethality in vitro, presumably at the level of the cytoplasmic membrane, may represent only a portion of the potential therapeutic effects of the antibiotic in vivo.