DECREASED MEMBRANE-PERMEABILITY IN A POLYMYXIN-B-RESISTANT ESCHERICHIA-COLI MUTANT EXHIBITING MULTIPLE RESISTANCE TO BETA-LACTAMS AS WELL AS AMINOGLYCOSIDES

A laboratory mutant of Escherichia coli stably resistant to more than 36 000 U ml(-1) of polymyxin B was isolated. The mutant exhibited mode rate increases in minimum inhibitory concentration to fluoroquinolones and bacitracin but high levels of cross-resistance to beta-lactams an d aminoglycosides. However, it remained susceptible to tetracycline, n alidixic acid and novobiocin. Changes were observed in the outer membr ane proteins and lipopolysaccharide profile leading to a decrease in p ermeability as evident from reduction in the following: (i) minimum in hibitory concentration values in the presence of Tween 80, (ii) uptake of 1-N-phenyl naphthylamine and norfloxacin, (iii) hydrolysis of beta -lactams and (iv) diffusion of lactose and cefazolin into proteoliposo mes reconstituted with outer membrane proteins. We therefore suggest t hat the novel pattern of cross-resistance of our isolate is due to the decrease in its permeability. (C) 1998 Federation of European Microbi ological Societies. Published by Elsevier Science B.V.