Corynebacterium species are known constituents of normal skin flora and prominent contaminants of clinical specimens. However, several species are now recognised as opportunistic pathogens that cause significant disease in humans.
Corynebacterium kroppenstedtii is a lipophilic Gram-positive bacterium that is unique to other members of the taxon by its lack of cell envelope mycolic acids. Although commonly associated with breast abscesses and granulomatous mastitis, C. kroppenstedtii has recently been implicated in more uncommon cystic neutrophilic granulomatous mastitis (CNGM).
Here we describe a rare case of C. kroppenstedtii isolated from breast tissue of CNGM in a patient with previously suspected malignancy. This case is unusual due to the initial presentation of infection associated with an accessory nipple. Accessory breast tissue can be found in up to 6% of the population, however cases of associated infection are rarely reported.
A 34-year-old woman of Asian descent presented with pain and hardening in a lower right breast lump located posterior to an axillary accessory nipple. The patient had no history of breast disease but based on initial presentation and mammogram, malignancy was suspected. Biopsy and surgical excision of the lump and accessory nipple showed histology consistent with CNGM. Gram positive bacilli were seen within cystic spaces in Gram-stained tissue.
Bacterial culture and antibiotic sensitivity testing (European Committee on Antimicrobial Susceptibility Testing) were performed on breast samples following significant post-surgical wound deterioration despite empiric beta-lactam treatment. C. kroppenstedtii was isolated after extended incubation and identified by MALDI-ToF (Vitek MS). This isolate was resistant to tetracyclines and treated successfully with linezolid 600mg.
This case highlights the importance of excluding bacterial infection when investigating suspected malignancy in breast samples, particularly when diagnosis is consistent with CNGM.