Abstract
The ability of hydrogen sulfide (H(2)S) to protect bacteria from bactericidal antibiotics has previously been described. The main source of H(2)S is the desulfurization of cysteine, which is either synthesized by cells from sulfate or transported from the medium, depending on its composition. Applying electrochemical sensors and a complex of biochemical and microbiological methods, changes in growth, respiration, membrane potential, SOS response, H(2)S production and bacterial survival under the action of bactericidal ciprofloxacin and bacteriostatic chloramphenicol in commonly used media were studied. Chloramphenicol caused a sharp inhibition of metabolism in all studied media. The physiological response of bacteria to ciprofloxacin strongly depended on its dose. In rich LB medium, cells retained metabolic activity at higher concentrations of ciprofloxacin than in minimal M9 medium. This decreased number of surviving cells (CFU) by 2-3 orders of magnitude in LB compared to M9 medium, and shifted optimal bactericidal concentration (OBC) from 0.3 µg/mL in M9 to 3 µg/mL in LB. Both drugs induced transient production of H(2)S in M9 medium. In media containing cystine, H(2)S was produced independently of antibiotics. Thus, medium composition significantly modifies physiological response of E. coli to bactericidal antibiotic, which should be taken into account when interpreting data and developing drugs.