In vitro susceptibility of European human Borrelia burgdorferi sensu stricto strains to antimicrobial agents.

Broth microdilution and macrodilution assays were used to determine minimum inhibitory concentrations (MICs) and minimum bactericidal concentrations (MBCs) of six antimicrobial agents (ceftriaxone, cefuroxime sodium, azithromycin, amoxicillin, doxycycline and amikacin) for nine European human isolates of Borrelia burgdorferi sensu stricto (s.s.). Strains were obtained from patients diagnosed with Lyme borreliosis in Slovenia. Modified Kelly-Pettenkofer medium with a final inoculum of 10⁵ Borrelia cells/mL and incubation periods of 72 h and of 3 weeks and 6 weeks were used in the determination of MICs and MBCs, respectively. Observed MICs indicated that all isolates were susceptible to all the tested antimicrobial agents with the exception of amikacin. Cefuroxime sodium (MIC₉₀=0.063 mg/L), azithromycin (MIC₉₀=0.22 mg/L) and ceftriaxone (MIC₉₀=0.25 mg/L) displayed the lowest MICs, followed by amoxicillin (MIC₉₀=1 mg/L) and doxycycline (MIC₉₀=2 mg/L); no strain was susceptible to amikacin (MIC₉₀=256 mg/L). MBCs after incubation for 3 weeks and 6 weeks were determined for amoxicillin (MBC₉₀=32 mg/L), doxycycline (MBC₉₀)=32 mg/L) and amikacin (MBC₉₀=1024 mg/L) and were found to be high (but not defined) for azithromycin (MBC₉₀>0.88 mg/L), cefuroxime sodium (MBC₉₀>4 mg/L) and ceftriaxone (MBC₉₀>4 mg/L). In determination of borrelial susceptibility to antimicrobial agents, intrinsic low susceptibility or methodological factors could result in low in vitro susceptibility of individual strains. This study is the first report on the antibiotic susceptibility of a series of European human isolates of B. burgdorferi s.s.