Origin of antibacterial stasis by polymyxin B in Escherichia coli

We show that blockage of hyperosmotic shock induced plasmolysis by polymyxin B (PxB) is related to its selective antimicrobial action against Gram-negative organisms. The rapid wrinkling of the cytoplasmic membrane induced by the hyperosmotic shrinkage of cytoplasmic volume due to the water efflux is monitored as an increase in the 90 degrees light scattering. The rapid scattering response is complete within 1 min after the addition of hyperosmolar NaCl. PxB decreases the amplitude of the rapid increase in the light scattering due to the shrinkage of the cytoplasmic volume by hyperosmotic shock. The amplitude is highest with cells in the early log phase of growth. The effect of PxB is induced rapidly and the maximum effect is seen within 1 min preincubation of cells. The effect of PxB is concentration dependent, and about 50% decrease in the amplitude is seen in the range of the growth inhibitory concentrations of PxB. The effect of PxB is not seen if added after the onset of the up-shock. As a heuristic model we suggest that PxB forms contacts between the two phospholipid interfaces that enclose the periplasmic space. The plasmolytic response results with osmY(-) mutant suggest that, like PxB, the osmY gene product in the periplasmic space prevents the shrinkage of the cytoplasmic compartment. Since PxB induces osmY transcription, we propose that, as a possible locus for the origin of the PxB induced stress, a contact between the phospholipid interfaces surrounding the periplasmic space triggers the metabolic changes leading to bacterial stasis.