Search

causingmorphological changes inthecell surface (4, 9)andinactivation ofthebiological activity of endotoxin (1, 5). Proteus mirabilis isresistant tothelethal action ofhighconcentrations ofpolymyxin B butisusually susceptible tothepenicillins. Several studies suggest thatpolymyxin Bresistance inthese organisms results fromacell wallwhich blocks access oftheantibiotic tothecytoplasmic membrane (3, 7,8). Teuber, using aP.mirabilis strain unusually susceptible toL-form generationbypenicillin, showed thattheL-forms are 400times moresusceptible topolymyxin Bthan intact organisms (8). Thelikely interpretation of this experiment isthatdamaged, orabsent, cell wallallows polymyxin Baccess tothepolymyxin B-susceptible cytoplasmic membrane. Inthe studies presented herein, wehavedemonstrated another typeofinteraction between polymyxin B andpenicillin onP.mirabilis. Atlow, but lethal, concentrations ofpenicillin there isantagonism between theantibiotics. Polymyxin B protects theorganisms fromlethal penicillin damage. FourP.mirabilis strains usedwereisolates

The presence of free fatty acids in liposome model membranes sensitizes these membranes to the action of the imidazole antifungals, clotrimazole, micronazole, and sulconazole. Unsaturation of the fatty acids is an important variable; the effect of linoleic and oleic acids is much greater than that of stearic acid. The imidazoles differ somewhat in action, with clotrimazole potency greatest both on membranes with and without fatty acids. Sulconazole has very little activity on membranes without fatty acids even at the highest concentrations tested. The data are discussed with reference to the susceptibility of various cells to the imidazoles and the specificity of imidazole action. A modification of the enzymatic method generally used for assay of marker glucose with liposome systems is also presented.

Proteus mirabilis is not killed by polymyxin B, normal serum, or sodium deoxycholate. Exposure to polymyxin B renders the cells susceptible to killing by the latter two agents. The data suggest that this synergism is due to polymyxin B-induced surface changes. The results point out an inadequacy of existing methods of screening for antibiotics; they fail to detect agents which, while showing no in vitro effect on growth, may alter a resistant organism so that it becomes more susceptible to other antimicrobials or host defense mechanisms. A method is described which can be used to detect such cell surface-modifying agents.

The lipids from three types of organisms—a Proteus mirabilis wild type highly resistant to polymyxin B, a polymyxin B-sensitive mutant derived from the wild type, and the wild type grown in the presence of sulfadiazine resulting in phenotypic conversion to polymyxin B sensitivity—were examined to determine the nature of polymyxin B resistance. The phospholipid compositions were nearly identical; each organism contained similar small amounts of N -methyl phosphatidylethanolamine in addition to comparable quantities of phosphatidylethanolamine, phosphatidylglycerol, and cardiolipin. the fatty acid compositions were similar in the exponential phase of growth; in the stationary phase, sulfadiazine markedly inhibited the synthesis of cyclopropane fatty acids. Liposomes prepared from the dried lipids of the three types of organisms were extensively and similarly disrupted by the polymyxin. These findings suggest that polymyxin B resistance in P. mirabilis is determined by the cell envelope which prevents access of the antibiotic to the susceptible lipid target sites.