Serial passage of Chlamydia spp. in sub-inhibitory fluoroquinolone concentrations.

We investigated the in vitro development of fluoroquinolone resistance in Chlamydia trachomatis and Chlamydia (Chlamydophila) pneumoniae grown in McCoy cell monolayers in supplemented Eagle's minimum essential medium. With C. trachomatis, initial passages at sub-inhibitory fluoroquinolone concentrations did not affect fluoroquinolone susceptibility. However, after an initial lag of 10-24 passages (depending upon the fluoroquinolone used), fluoroquinolone resistance developed rapidly. The final fluoroquinolone MIC after a total of 30 passages was >256 times the MIC of the original wild-type strain with ofloxacin or ciprofloxacin passage. Analysis of the quinolone-resistance determining regions of two quinolone-resistant C. trachomatis mutants obtained after 30 passages showed that both isolates had a single serine to isoleucine substitution at amino acid position 83 in GyrA. In stark contrast, with C. pneumoniae no reduced fluoroquinolone susceptibility could be sustained, even after 30 passages with moxifloxacin or ofloxacin. With sparfloxacin passage, some indication of resistance was observed but no viable organisms could be isolated for further investigation. It is possible that fluoroquinolone-resistant C. pneumoniae are less able to survive than wild type, which may explain why resistance does not develop readily.