Deleterious effect of ciprofloxacin on Rickettsia conorii-infected cells is linked to toxinantitoxin module up-regulation

To confirm and better understand the deleterious effect of fluoroquinolones reported during Rickettsia conorii infection in humans. We used a new plaque assay to test the effect of ciprofloxacin on cells infected by R. conorii. Controls were mock-treated infected cells and infected cells treated with doxycycline. We used real-time quantitative RTPCR to quantify vapC and vapB transcripts in cells infected by R. conorii treated with ciprofloxacin or mock treated. By plaque assay, at baseline (0 h) there is no difference in lytic area between cells treated with ciprofloxacin (whatever concentration used) and controls. Ciprofloxacin at 0.5 and 50 mg/L induced a significant increase in lytic areas compared with the control at 2 h, 4 h, 6 h (P0.0001) and 24 h (P0.0001 and P0.035, respectively) when not induced with doxycycline. By real-time quantitative RTPCR, ciprofloxacin was found to cause an up-regulation of toxinantitoxin (TA) module transcription. Indeed, the mRNA levels of vapC and vapB were significantly increased at 2 h and 4 h in cells treated with 50 mg/L ciprofloxacin (not with 0.5 mg/L ciprofloxacin) compared with control levels (fold change 2.9). These in vitro findings correlated with our previous clinical findings: fluoroquinolones have a deleterious effect during R. conorii infection, not found with doxycycline. We speculate that the toxic effect of fluoroquinolones on R. conorii-infected cells is mediated by the overexpression of TA, possibly followed by their release into the host cytoplasm as described in Rickettsia felis.