Further studies on colistin (Polymyxin E)-induced cell leakage in Mycobacteria: Mg ++ Efflux in mycobacterium avium and its effects on drug-susceptibility

Abstract

Action of Colistin (polymyxin E) was investigated on the opportunistic pathogenic species Mycobacterium avium ATCC 15769 (resistant strain, MIC > 100 μg/ml). Mycobacterium intracellulare ATCC 13950: a colistin-susceptible strain (MIC 25 μg/ml), was used as a parallel control for Mg ++ efflux experiments. After the addition of 100 μg/ml of Colistin to bacteria suspended in a buffer, both loss of viability and Mg ++ efflux were followed for one week. Although there was an initial Mg ++ efflux in both strains, it soon attained a plateau in case of the resistant strain without any loss of viability until 48 h, whereas in case of the susceptible strain, Mg ++ efflux was about 3 times higher than former, a plateau was attained only 24 h after the drug addition, and the loss of viability started only 6 h after the drug addition. Consequently, the loss of viability of M. avium with later incubation times (6 days) was not due to the action of Colistin on the cytoplasmic Mg ++ efflux solely, but some additional effect was implicated. The drug-susceptibility of extracellularly-growing and intracellularly-growing (in the J-774 macrophage cell line) M. avium to the antibiotics tested could not be potentiated when they were used in combination with 5 μg/ml of Colistin (maximal obtainable serum level in man) or the polymyxin B nonapeptide (PMBN), nor in the case of bacteria pretreated with these molecules.