A 56-year-old female presented with right breast pain and a palpable mass, initially suspected to be a granulomatous mastitis. Imaging revealed a complex cystic mass with internal debris. Drainage yielded purulent fluid, and initial cultures were negative. However, 16S rRNA gene sequencing identified Lawsonella clevelandensis, a fastidious anaerobic Gram-positive bacillus, as the monomicrobial cause of the breast abscess. The patient responded well to incision and drainage combined with amoxicillin-clavulanate. This case highlights the importance of advanced molecular diagnostics for identifying uncommon, fastidious pathogens in culture-negative abscesses.
Key note: Lawsonella clevelandensis can cause monomicrobial breast abscesses, requiring molecular methods for identification.