Effect of HS and cysteine homeostasis disturbance on ciprofloxacin sensitivity of Escherichia coli in cystine-free and cystine-fed minimal medium.

Endogenous HS has been proposed to be a universal defense mechanism against different antibiotics. Here, we studied the role of HS transiently generated during ciprofloxacin (CF) treatment in M9 minimal medium with sulfate or produced by E. coli when fed with cystine. The cysM and mstA mutants did not produce HS, while gshA generated more HS in response to ciprofloxacin in cystine-free media. All mutants showed a reduced ability to maintain cysteine homeostasis under these conditions. We found no relationship between HS generation, cysteine concentration and sensitivity to ciprofloxacin. Excess cysteine, which occurred during E. coli growth in cystine-fed media, triggered continuous HS production, accelerated glutathione synthesis and cysteine export. This was accompanied by a twofold increase in ciprofloxacin tolerance in all strains except gshA, whose sensitivity increased 5-8-fold at high CF doses, indicating the importance of GSH in restoring the intracellular redox situation during growth in cystine-fed media.