Methods: Banked B. pertussis clinical isolates at St. Christopher’s Hospital for Children from 2007-2014 were studied. Characterization of PRN was determined by Western blot, and PRN genes were analyzed by amplification and sequencing of the entire coding region for pertactin (Queenan et al. NEJM 2013;368:583-84). Antimicrobial susceptibility testing was performed using the agar gradient diffusion method (Etest, bioMerieux) by inoculating Regan-Lowe agar without cephalexin (Hardy Diagnostics) with B. pertussis suspensions equivalent to a 0.5 McFarland standard prepared in cation-adjusted Mueller-Hinton broth (Thermo Scientific). 5 antimicrobial agents were tested: ampicillin, azithromycin, erythromycin, levofloxacin and trimethoprim-sulfamethoxazole. Etests were read after 72 hours incubation at 350C with 5% CO2. ATCC strain #49619 of S. pneumoniae was used as standard control.
Results: Etest MICs of all 5 antimicrobial agents tested were ≤ 0.25 µg/mL for 13 PRN+ and 10 PRN_ isolates, which are interpreted as susceptible based on HACEK breakpoints in CLSI document M45-A2 (2010). Among both PRN+and PRN_ strains, macrolide MICs increased from 2007 to 2014. For PRN+ isolates, azithromycin MICs increased 2-fold from 0.016 to 0.047 µg/mL from 2007 to 2014, respectively. For PRN_ isolates, azithromycin MICs increased 4-fold from 0.023 to 0.094 µg/mL from 2007 to 2014. For both PRN+ and PRN_strains, erythromycin MICs increased 2-fold from 0.016 to 0.047 µg/mL from 2007 to 2014. No significant changes in MICs were noted for ampicillin (0.125-0.25), levofloxacin (0.047-0.064), or trimethoprim/sulfamethoxazole (0.19-0.25) over the 8-year period.
Conclusion: PRN+ and PRN_ B. pertussis isolates are susceptible to the 5 antimicrobial agents tested from 2007 to 2014. A trend towards higher MICs for azithromycin and erythromycin was observed for both PRN+ and PRN_ strains over the 8-year period.
A. M. Queenan, None
S. S. Long, None
A. T. Evangelista, None