In vitro susceptibility of Rickettsia conorii to ciprofloxacin as determined by suppressing lethality in chicken embryos and by plaque assay.

We tested the susceptibility of Rickettsia conorii to ciprofloxacin, a new quinolone antibiotic. A final concentration of 1 microgram/g of egg was effective in suppressing chicken embryo lethality, and a concentration of 0.25 micrograms/ml inhibited plaque formation in a plaque assay; however, a concentration of 0.5 microgram/ml was necessary to obtain rickettsiacidal activity. These results support the idea that ciprofloxacin could be of clinical use in treating Mediterranean spotted fever.