Clinical and microbiological characteristics of granulomatous mastitis caused by Corynebacterium species: A case series.

Corynebacterium species are an important cause of granulomatous mastitis (GM). Although there have been worldwide reports of infections caused by Corynebacterium kroppenstedtii, few cases of C. tuberculostearicum GM have been reported. We conducted a retrospective study to investigate the clinical and microbiological characteristics of GM caused by Corynebacterium spp. between January 1, 2014, and May 31, 2024. During the study period, six patients with GM were identified, including four cases of C. kroppenstedtii GM and two cases of C. tuberculostearicum GM. All patients were female with a median age of 32 years (range: 18-50 years). All patients underwent surgical drainage and were treated with clarithromycin. The minimum inhibitory concentration (MIC) values for various antimicrobial agents, including azithromycin, clarithromycin, ciprofloxacin, clindamycin, and trimethoprim-sulfamethoxazole, were higher for C. tuberculostearicum than for C. kroppenstedtii. Additionally, MALDI-TOF MS and 16s rRNA gene sequencing accurately identified C. kroppenstedtii and C. tuberculostearicum. Accurate identification of lipophilic Corynebacterium spp. is important for antibiotic treatment selection. If clinicians suspect lipophilic Corynebacterium spp. infection, particularly GM, they should ask the microbiology laboratory to test for lipophilic Corynebacterium spp.