Antimicrobial susceptibility testing of Finnish Bordetella pertussis isolates collected during 2006–2017

Macrolides, such as azithromycin and erythromycin, are first-line drugs for the (prophylactic) treatment of pertussis. This study aimed to screen for macrolide-, quinolone- or trimethoprim/sulfamethoxazole (SXT)-resistant strains among Finnish Bordetella pertussis isolates. Antimicrobial susceptibility testing was performed on 148 B. pertussis strains isolated during 2006–2017. Isolates were analysed by allele-specific PCR for detection of the macrolide resistance-associated mutation A2047G in the 23S rRNA gene. The gyrA gene was sequenced for detection of the A260G mutation associated with quinolone resistance. For phenotyping, a random selection was made by selecting every third isolate (n = 50) to determine the minimum inhibitory concentrations (MICs) for erythromycin and azithromycin by Etest and the inhibition zone size for nalidixic acid (NAL) and SXT by single disk diffusion assay. Neither the macrolide resistance-associated mutation A2047G nor the quinolone resistance-associated mutation A260G was detected in any of the B. pertussis isolates. MICs of azithromycin and erythromycin ranged between 0.016–0.19 μg/mL and 0.016–0.25 μg/mL, respectively. The size of the inhibition zone surrounding the NAL disk ranged between 22–27 mm in diameter. The inhibition zone surrounding the SXT disk ranged between 24–37 mm in diameter. No isolates resistant to any of the tested antimicrobials were identified. The allele-specific PCR is a simple and useful tool for screening B. pertussis resistance to macrolides. All Finnish isolates tested were susceptible to macrolides, quinolones and SXT.