Salmonella Typhimurium exhibits fluoroquinolone resistance mediated by the accumulation of the antioxidant molecule H2S in a CysK-dependent manner.

Objectives: To evaluate the contribution of cysK and cysM to the fluoroquinolone (ofloxacin) antibiotic resistance in Salmonella Typhimurium, and their impact on H ₂ S and cysteine production through targeted mutagenesis.
Methods: Salmonella Typhimurium 14028s and its cysK and cysM mutants were tested for their susceptibility to ofloxacin, as determined by a broth microdilution test (to determine the MIC) and survival curves. H ₂ S levels were measured by the Pb(AC) ₂ method and cysteine levels were determined using 5,5-dithio-bis-2-nitrobenzoic acid. DNA damage induced by antibiotic treatment was determined by PFGE. Finally, expression of cysK and cysM genes under antibiotic treatment was determined by real-time reverse transcription PCR.
Results: As determined by MIC, the ΔcysK strain was more resistant to ofloxacin, a reactive oxygen species (ROS)-producing fluoroquinolone, than the WT and ΔcysM strains, which correlates with survival curves. Moreover, the ΔcysK strain exhibited higher H ₂ S levels and lower cysteine levels than the WT strain. Finally, the ΔcysK strain exhibited lower DNA damage upon challenge with ofloxacin than the WT and ΔcysM strains. These results are in accordance with lower expression of cysK under ofloxacin treatment in the WT strain.
Conclusions: This work demonstrated that cysteine metabolism in Salmonella Typhimurium modulated H ₂ S levels, conferring resistance to second-generation fluoroquinolones.
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