Antibiotic susceptibility of the newly cultivated agent of human granulocytic ehrlichiosis: Promising activity of quinolones and rifamycins

Human granulocytic ehrlichiosis (HGE) is a rapidly emerging tick-borne infection which presents as an acute febrile illness and is associated with hematologic abnormalities, elevated hepatic transaminase levels, and characteristic intracellular organisms in peripheral blood granulocytes. Although HGE has been successfully treated with tetracyclines, its susceptibility to other antibiotics remains unknown, No clear treatment alternatives exist for young children, pregnant women, or allergic individuals, in whom tetracyclines are contraindicated, We performed in vitro antibiotic susceptibility tests with this recently isolated agent groan in the human promyelocytic leukemia cell line HL-60. Doxycycline (MIC, 0.25 mu g/ml), rifampin (MIC, 0.5 mu g/ml), rifabutin (MIC, less than or equal to 0.125 mu g/ml), ciprofloxacin and ofloxacin (both with MICs of 2 mu g/ml), and trovafloxacin (MIG, less than or equal to 0.125 mu g/ml) demonstrated significant activity against the HGE agent, These agents were also bactericidal, The HGE agent was resistant to clindamycin, trimethoprim-sulfamethoxazole, and imipenem-cilastatin, as well as to ampicillin, ceftriaxone, erythromycin, and azithromycin, antibiotics commonly used to tl eat Lyme disease, Both chloramphenicol and gentamicin had weak inhibitory activities hut were not bactericidal. Our findings confirm the observed clinical efficacy of doxycycline and further suggest that the rifamycins and quinolones, particularly trovafloxacin, hold promise as alternative agents for treating this new infection.