IN-VITRO ANTIMICROBIAL SUSCEPTIBILITY TESTING OF BORRELIA-BURGDORFERI- INFLUENCE OF TEST CONDITIONS ON MINIMAL INHIBITORY CONCENTRATION (MIC) VALUES

A broth microdilution assay was performed to determine the minimal inh ibitory concentrations (MICs) of 25 antimicrobial agents for two strai ns of Borrelia (B.) burgdorferi sensu late and one strain of B. hermsi i. The method comprised BSK II medium lacking gelatin and an incubatio n period of 72 hours. To investigate the influence of reading mode and density of inoculum on MIC values, microscopical as well as macroscop ical MIC reading was performed using standardized final inocula of 10( 6) and 10(7) borreliae/ml. Data were processed by two-way analysis of variance. In the microdilution assay, MIC values were significantly in fluenced either by the inoculum density or reading mode. However, usin g clearly defined criteria for macroscopical endpoint determination, M ICs from macroscopical and microscopical reading were found to be in c lose agreement. B. burgdorferi sensu lato strains tested were highly s usceptible to azithromycin, erythromycin, mezlocillin, piperacillin as well as ceftriaxone, with MICs ranging from less than or equal to 0.1 6 to 0.125 mu g/ml. B. hermsii was highly susceptible to azithromycin and erythromycin. In comparison to B. hermsii, the p-lactam antibiotic s revealed a significantly higher activity and gentamicin, ofloxacin, and rifampin revealed a significantly lower activity against B. burgdo rferi sensu lato strains. To further investigate interactions between BSK II medium, incubation time, and antibiotic efficacy, an agar diffu sion bioassay was performed. Our of seven antibiotics tested, the acti vities of mezlocillin, penicillin G, and piperacillin were significant ly influenced by BSK II medium and incubation period and showed a mark ed decrease of on average 84.0% within 72 hours of incubation.