Trends in the minimum inhibitory concentrations of erythromycin, clarithromycin, azithromycin, ciprofloxacin, and trimethoprim/sulfamethoxazole for strains of Bordetella pertussis isolated in the Czech Republic, 1967-2015

Objective: The aim of the study was to determine trends in the minimum inhibitory concentrations (MICs) of erythromycin used as first-line therapy and alternative antibiotics against Bordetella pertussis (B. pertussis) strains isolated from patients with whooping cough in the Czech Republic (CR) in three periods from 1967 to 2015. Methods: In total, 135 isolates from the years 1967-2015 were analysed. The strains were divided into three groups by the year of isolation: 1967-1999 (42 strains), 2004-2010 (43 strains), and 2011-2015 (50 strains). MIC of selected antibiotics (erythromycin, clarithromycin, azithromycin, ciprofloxacin, and trimethoprim/sulfamethoxazole) were obtained by the reference agar dilution method on Bordet Gengou Agar with 15% defibrinated sheep blood. Results: The study set included 70 strains previously tested for MICs of erythromycin and four other antibiotics. In the three study periods, the MICs of the tested antibiotics for B. pertussis were nearly identical. All but a single strain, inhibited by erythromycin at a concentration of 0.03 mg/l, were inhibited by two concentrations of erythromycin and azithromycin (0.06 and 0.125 mg/l). Clarithromycin inhibited the strains from all three study periods at the following concentrations: 0.03, 0.06, and 0.125 mg/l. Any of the 135 strains was inhibited by ciprofloxacin at a single concentration of 0.06 mg/l and by trimethoprim/sulfamethoxazole at three concentrations (0.125, 0.25, and 0.5 mg/l). Conclusion: The study set of 135 Czech strains of B. pertussis isolated in 1967-2015 appears to be homogeneous in terms of the MICs for five antimicrobials. The MICs remained in a narrow range of two to three low concentrations; the unimodal distribution of the MICs suggests the absence of resistance mechanisms. The highest MICs of erythromycin, clarithromycin, and azithromycin were equally 0.125 mg/l, that of ciprofloxacin was 0.06 mg/l, and that of trimethoprim/sulfamethoxazole was 0.5 mg/l. Over the study period of 55 years, the MICs of the study antibiotics remained in the same ranges.