Clinical and Microbial Features of Granulomatous Mastitis: Comparative Analysis of Breast and Gut Microbiota

Granulomatous mastitis (GM) is a rare benign inflammatory breast disease that may mimic carcinoma. Etiology remains unclear, but emerging evidence suggests a role for both breast and gut microbiota.

Between June 2022 and June 2024, 32 women with histopathology-confirmed GM and 44 non-GM controls (benign and non-granulomatous inflammatory conditions) were enrolled. Granulomatous tissue, adjacent normal breast tissue, and stool samples were collected. Microbiota composition was analyzed using 16S rRNA sequencing. Clinical variables were documented. Diversity analyses and receiver operating characteristic (ROC) curves assessed microbial differences and diagnostic potential.

The GM cohort had a mean age of 36.8 years and BMI of 27.6 ± 3.9 kg/m²; 86.2% were postpartum, with no autoimmune or malignant conditions. GM lesions showed enrichment of Corynebacterium (notably C. kroppenstedtii), Staphylococcus, and Microbacterium, whereas Lactobacillus and Bifidobacterium were higher in normal tissue. Fecal samples exhibited reduced microbial diversity in GM compared with controls but did not replicate breast microbial profiles. ROC analysis indicated that Corynebacterium discriminated GM from controls with excellent accuracy (AUC = 0.87), while Staphylococcus was moderate (AUC = 0.78) and Lactobacillus inversely associated (AUC = 0.11).

GM demonstrates distinct microbial signatures, including pro-inflammatory taxa enrichment and protective commensal depletion, along with systemic dysbiosis. These findings support a microbiota-driven pathogenesis and highlight the potential of microbial biomarkers and therapeutic modulation.