Divalent cation dependent resistance in E. coli LMR-26 to the broad spectrum antimicrobial agent irgasan

Strains of E. coli found in nature are commonly sensitive to irgasan at concentrations of 1-3 ppm. However, E. coli LMR-26 has been shown to be resistant in media up to a maximum of 35 ppm irgasan. The presence of a divalent cation (Mg++ or Ca++) for this resistance is required in the medium. Resistance to similar concentrations of hexachlorophene with Mg++ was also observed with E. coli LMR-26. Comparative analysis of total envelope proteins gel electrophoresis showed no significant differences between an irgasan-sensitive wild strain and the resistant organism. However, gas liquid chromatography did indicate significant differences in the envelope fatty acids of the two organisms. Electron microscopy revealed major morphological modifications in the resistant organism subjected to an irgasan stress. A possible mechanism of resistance is discussed.