In vitro evaluation of josamycin, spiramycin, and erythromycin against Rickettsia rickettsii and R. conorii.

The antimicrobial activities of josamycin, erythromycin, and spiramycin against Rickettsia conorii and R. rickettsii were evaluated in two tests: a dye-uptake assay and a plaque assay. The MIC of josamycin was 1 microgram/ml for both species; the MICs of erythromycin and spiramycin were 4 to 8 and 16 to 32 micrograms/ml, respectively, for both species. Only josamycin may be of clinical use in treating spotted fever rickettsiosis. It may be useful in treating pregnant women and young children.