[Heavy metal cation-induced increase in the antimicrobial activity of gramicidin S. Increased sensitivity of metal-resistant mutants of Escherichia coli B to the antibiotic].

Gramicidin S response of metal resistant mutants of E. coli B and the effect of concentrations of Cu2+, Ag+, Co2+ and Cd2+ on the growth and sensitivity of E. coli B to cationic antibiotics, i.e. gramicidin S2+ and streptomycin2+, were studied. It was shown that the metal-cumulating mutants of E. coli B with two different mechanisms of cross resistance to Cu2+, Cd2+ and Ag+ had higher sensitivity to gramicidin S than the initial wild type strain of E. coli B. It was found that in the threshold or higher doses the salts of Cu, Ag, Co and Cd increased the gramicidin S antimicrobial action on actively metabolizing cells of E. coli B. Analysis of the experimental data as well as the literature ones suggested that the synergic action of gramicidin S and the heavy metals stemmed from an increase in the cationic conductivity of the cytoplasma membrane modified by the metals in the threshold doses which induced an increase in the transport and accumulation of the cations in the bacterial cells by the electric field gradient (with the negative sign inside). Withdrawal of Ca2+ and Mg2+ from the E. coli outer structures into the cytoplasm impaired the barrier properties of the outer membrane and promoted binding of the gramicidin S cations to the liberated anionic groups of the E. coli outer structures and potentiation of the gramicidin S antimicrobial activity as was shown in our experiments.