In vitro susceptibility of Bartonella species to 17 antimicrobial compounds:: comparison of Etest and agar dilution

Objectives: In vitro susceptibility testing of 31 Bartonella spp. strains including 21 Bartonella henselae isolates was performed for 17 antimicrobial agents (telithromycin, four macrolides, five fluoroquinolones, five aminoglycosides, doxycycline and rifampicin). Methods: MICs were determined by agar dilution and Etest using chocolate agar containing 5% defibrinated sheep blood as assay medium. Longer incubation periods of 3-5 days in a humid atmosphere with 5% CO2 were required until bacterial growth became visible and MICs could be read. Results: The ketolide telithromycin was the most active agent exhibiting the lowest MICs. The Bartonella spp. were also highly susceptible to macrolides, particularly clarithromycin, and to doxycycline and rifampicin, with MICs of <= 0.12 mg/L. Gatifloxacin, gemifloxacin and moxifloxacin were the most potent fluoroquinolones, with MICs ranging from 0.06 to 2 mg/L. Netilmicin was the most active agent among the aminoglycosides. Etest MICs correlated well with MICs determined by agar dilution. Conclusions: Telithromycin, macrolides, doxycycline and rifampicin were the most effective agents against Bartonella spp. Our data confirm that Etest may be a reliable method for determining susceptibility of Bartonella spp.