Effect of H2S and cysteine homeostasis disturbance on ciprofloxacin sensitivity of Escherichia coli in cystine-free and cystine-fed minimal medium

Endogenous H₂S has been proposed to be a universal defense mechanism against different antibiotics. Here, we studied the role of H₂S 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 H₂S, while gshA generated more H₂S 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 H₂S generation, cysteine concentration and sensitivity to ciprofloxacin. Excess cysteine, which occurred during E. coli growth in cystine-fed media, triggered continuous H₂S 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.