Emergence of Macrolide-Resistant Bordetella pertussis Harboring the A2047G Mutation During the 2024 Pertussis Outbreak in Korea

Background: Bordetella pertussis remains a reemerging pathogen despite widespread vaccination.Macrolide resistance, driven by an A2047G mutation in the 23S rRNA gene, has been reported in several Asian countries but not previously confirmed in Korea.This study aimed to characterize the antimicrobial susceptibility and molecular resistance mechanisms of B. pertussis isolates obtained during the 2024 national pertussis epidemic.Methods: Nasopharyngeal swabs were collected from 180 pediatric patients meeting the Centers for Disease Control and Prevention (2024) clinical case definition for pertussis in Gyeongnam Province.Cultured isolates were identified by polymerase chain reaction (PCR) targeting IS481, IS1001, IS1002, tracheal colonization factor A, and porin.Antimicrobial susceptibility was assessed by Etest for erythromycin, clarithromycin, azithromycin, and trimethoprim-sulfamethoxazole (TMP/SMX).Detection of the 23S rRNA A2047G mutation was performed using a modified allele-specific PCR and confirmed by Sanger sequencing.Results: Six viable B. pertussis isolates (3.3%) were recovered from 180 pediatric patients.Three isolates (H1, H2, H4) demonstrated clear inhibition zones for all macrolides, with minimum inhibitory concentrations (MICs) < 0.016 µg/mL, confirming susceptibility.In contrast, isolates H3, H5, and H6 showed no visible inhibition zones for erythromycin, clarithromycin, or azithromycin, corresponding to high-level resistance (MIC ≥ 256 µg/mL).All isolates remained susceptible to TMP/SMX.The A2047G substitution was detected in all macrolide-resistant isolates (H3, H5, H6) and was absent in all macrolide-susceptible isolates (H1, H2, H4), showing complete concordance between genotype and phenotypic resistance.Conclusion: This study provides the first phenotypic confirmation of macrolide-resistant B. pertussis in Korea.