Corynebacterium kroppenstedtii in granulomatous mastitis: Analysis of formalin‐fixed, paraffin‐embedded biopsy specimens by immunostaining using low‐specificity bacterial antisera and real‐time polymerase chain reaction

Granulomatous mastitis (GM) is a rare inflammatory disease of the post‐lactation breast, clinically mimicking breast cancer. GM is microscopically characterized by formation of epithelioid granulomas and abscess (suppurative granulomas) with lipid droplet‐centered inflammation. Corynebacterium kroppenstedtii (Ck) is known as a causative bacterium of GM, and identification of Ck infection within the lesion should thus be essential for confirming the diagnosis. In the present study, we analyzed formalin‐fixed, paraffin‐embedded (FFPE) biopsy specimens of a total of 18 GM lesions with immunostaining and real‐time PCR for Ck genome. Widely cross‐reactive rabbit antisera against Bacillus Calmette‐Guerin (BCG), Bacillus cereus, Treponema pallidum and Escherichia coli were chosen. With real‐time PCR, Ck genome was demonstrated in 7 of 18 GM lesions. Immunohistochemically, the low‐specificity antisera reacted with the cytoplasm of phagocytes and/or granuloma‐engulfed lipid droplets in 12 of 18 GM lesions. Antigenic positivity was observed in the following order: BCG > B. cereus > T. pallidum > E. coli. Real‐time PCR using DNA extracted from FFPE sections was useful but not consistent for identifying the Ck genome in GM, while immunostaining using cross‐reactive antisera against four kinds of bacteria was not Ck‐specific but was applicable to visualizing bacterial infection within the GM lesions.