Decreased sensitivity to polymyxin B in colicin K tolerant cells of Escherichia coli K-12 in the presence of colicin K

Cells refractory to the lethal effect of colicin K were selected from a sensitive Escherichia coli K-12 strain. They could still be lysed by bacteriophage T6 which is thought to share with colicin K common receptor sites on the bacterial cell wall. Incubation of these cells for 30 min in a colicin K preparation considerably reduced its subsequent activity on the sensitive parent strain. When these tolerant cells were exposed to polymyxin B in the presence of colicin K, the lethal activity of the polymyxin was decreased significantly as compared with that exerted by the antibiotic in the absence of colicin K. In the colicin K sensitive wild strain, the combined lethal effect of the bacteriocin and of the antibiotic was not smaller than that of either inhibitor alone. Examination with the fluorescence microscope of cells stained with acridine orange showed that colicin K tolerant cells incubated with polymyxin B had undergone greater morphological changes than similar cells treated with polymyxin B in the presence of colicin K. Leakage of materials absorbing at 260 nm was greatest for the cells exposed to polymyxin B alone. The results suggest that in colicin K tolerant cells of E. coli derived from a sensitive strain, the bacteriocin might compete with polymyxin B for sites of attachment on the bacterial cell wall or mask some of the envelope phospholipids which are believed to react with polymyxin B.