Epidemiology and antimicrobial resistance of pathogens in granulomatous mastitis: A multicenter study in Guangdong, China.

Granulomatous lobular mastitis (GLM) and non-GLM occur worldwide; however, the pathogen prevalence and resistance vary regionally. Therefore, we conducted a retrospective analysis of 1,535 women with mastitis from five hospitals in Guangdong, China (2018-2023). GLM comprised 37.3%, concentrated at ages 25-34; nearly all GLM occurred in non-lactating women (555/573), whereas 52.4% of non-GLM presented during lactation. The GLM proportion rose from 29.2% to 47.5% during 2018-2023. The predominant pathogen in non-GLM was Staphylococcus aureus (S. aureus) (57.3%), whereas in GLM it was Corynebacterium kroppenstedtii (C. kroppenstedtii) (79.8%). C. kroppenstedtii showed rising penicillin resistance (25.0-71.0%) and high clindamycin resistance (66.7-90.5%), but remained largely susceptible to gentamicin (78.7-94.3%) and rifampicin (>90%). S. aureus exhibited a remarkable resistance to penicillin, erythromycin, and clindamycin, and showed susceptibility to levofloxacin, gentamicin, TMP-SMZ (trimethoprim-sulfamethoxazole), moxifloxacin and rifampicin (>90%). None of the strains showed resistance to linezolid or vancomycin. These findings support personalized treatment, antibiotic stewardship, and regional surveillance.