Antibacterial action of colistin (polymyxin E) against Mycobacterium aurum

Mycobacterium aurum was susceptible to the antibiotic colistin (polymyxin E),which had an MIC of 5 micrograms/ml and an apparent bactericidal effect at concentrations above 50 micrograms/ml. Treatment of actively growing cells with sublethal concentrations of colistin (15 micrograms/ml) resulted in synchronized cell division once the antibiotic was removed. Under conditions of synchronized cell growth, one cycle of DNA replication lasted 120 min and one cycle of cell division lasted about 180 min. Although the antibiotic treatment during synchronization experiments did not produce apparent changes in the bacterial envelope, it was accompanied by the accumulation of a polysaccharide-like substance in the bacterial cytoplasm which gradually decreased after the removal of antibiotic and by an increase in the number of mesosomes at 3 h after antibiotic removal. This step was closely linked to the doubling time of bacteria. Lethal concentrations of colistin of 50 and 100 micrograms/ml, which caused about 90 and 99% cell death, respectively, produced significant cytoplasmic membrane injuries, patchy appearance of the cell wall outer polysaccharide layer, and little cell lysis. These data indicate that the cytoplasmic membrane is a site of action of colistin and raise a question as to whether an outer bilayer exists in mycobacteria, at least functionally.