Pretreatment with colistin and Proteus sensitivity to other agents

The effects of pretreatment with colistin (polymyxin E) on the sensitivity of Proteus mirabilis, P. vulgaris and P. morganii strains to tris and sodium deoxycholate (DOC) have been studied. Pretreatment of two P. mirabilis strains (NCTC 60 and 4199) with low concentrations (0.25 approximately 1 microgram/ml) of colistin rendered them sensitive to lysis by tris (0.05M) or DOC (250 approximately 1,000 microgram/ml) although DOC induced lysis of control (non-colistin-treated) suspensions also. In contrast, the other P. mirabilis strains, as well as the P. vulgaris and P. morganii strains were little affected by tris (0.2M) or DOC (10,000 microgram/ml) even after exposure of the cells to high colistin concentrations (up to 500 microgram/ml). Colistin-pretreated or control cells of P. mirabilis NCTC 60 rapidly lost viability when suspended in water but not when held in 0.16M sodium chloride solution. Ethylenediamine tetraacetate-pretreated cells of strains 60 and 4199 were fairly sensitive to tris, although the extent of the lysis was less than when colistin was used as pretreating agent. One strain of P. vulgaris (NCTC 4175) became sensitive to tris and to DOC following exposure of the cells to ampicillin.