Effects of low-level ciprofloxacin challenge in the in vitro development of ciprofloxacin resistance in Campylobacter jejuni.

The effects on MIC values and the selection of different base substitutions in the quinolone resistance determining region (QRDR) of gyrA were studied on initially ciprofloxacin-susceptible Campylobacter jejuni strains by challenging them to 0.125 mg/L of ciprofloxacin. This ciprofloxacin challenge selected variants with ciprofloxacin MIC levels up to 32 mg/L. Repeated experiments under identical conditions resulted in different responses in MIC levels and alterations in the QRDR of gyrA. A characteristic outcome to ciprofloxacin challenges was the appearance of double peaks in the sequencing chromatograms of QRDR. This finding suggested the coexistence of subpopulations possessing Thr86 --> Ile and/or Asp90 --> Asn mutations alongside the unmutated parent population. In some cases, bacterial variants expressing ciprofloxacin-resistant phenotypes possessed no mutations in their QRDR. These variants were prone to regain susceptibility to ciprofloxacin rapidly after the removal of the selection pressure, whereas the QRDR-mutated variants persisted over several subcultivations in a medium without ciprofloxacin. In conclusion, a low ciprofloxacin concentration of 0.125 mg/L selects a variety of QRDR mutations and also a QRDR-independent resistance mechanism, which may coexist with each other in a C. jejuni population. Persistent ciprofloxacin challenge selects Thr86 --> Ile and/or Asp90 --> Asn mutants.