In vitro antibiotic susceptibility of the newly recognized agent of ehrlichiosis in humans, Ehrlichia chaffeensis.

Ehrlichiosis in humans, a rickettsial disease recently discovered in the United States, is generally treated successfully with tetracyclines; however treatment with these agents is usually avoided with children and pregnant women. The in vitro susceptibility of Ehrlichia chaffeensis, the agent of human ehrlichiosis in the United States, was assessed by a quantitative evaluation of infected DH82 cells cultivated in 96-well microtiter plates in the presence of different concentrations of selected antibiotics. Extracellular MICs and MBCs were evaluated after 72 h of exposure to the antibiotics. Doxycycline and rifampin were found to exert rapidly bactericidal effects, with MBCs in the extracellular culture medium of less than 0.5 and 0.125 microgram/ml, respectively. E. chaffeensis was resistant to chloramphenicol, ciprofloxacin, erythromycin, co-trimoxazole, penicillin, and gentamicin, which had MICs greater than 16, 4, 8, 4, 40, and 32 micrograms/ml, respectively. These observations are consistent with the finding that human ehrlichiosis appears to respond to tetracycline therapy, which has been the therapy of first choice. Further clinical investigations are necessary to evaluate the role of rifampin in the treatment of human ehrlichiosis, especially in children.