Cutibacterium acnes is an intracellular and intraarticular commensal of the human shoulder joint

Abstract Background Cutibacterium acnes is a mysterious member of the shoulder microbiome and is associated with chronic postoperative complications and low-grade infections. Nevertheless, it is unclear whether it represents a contaminant or whether it accounts for true infections. Because it can persist intracellularly in macrophages at several body sites, it might in fact be an intraarticular commensal of the shoulder joint. Methods In 23 consecutive, otherwise healthy patients (17 m, 6 f; 58 y) who had no previous injections, multiple specimens were taken from the intraarticular tissue during first-time arthroscopic and open shoulder surgery. The samples were investigated by cultivation, genetic phylotyping and immunohistochemistry using C. acnes-specific antibodies and confocal laser -scanning microscopy. Results In 10 patients (43%), cultures were C. acnes-positive. Phylotype IA1 dominated the subcutaneous samples (71%), whereas type II dominated the deep tissue samples (57%). Sixteen of 23 patients (69.9%) were C. acnes-positive by immunohistochemistry; in total, 25 of 40 samples were positive (62.5%). In total, 56.3% of glenohumeral immunohistochemical samples, 62.5% of subacromial samples and 75% of AC joint samples were positive. In 62.5% of the tested patients, C. acnes was detected immunohistochemically to reside intracellularly within stromal cells and macrophages. Discussion These data indicate that C. acnes is a commensal of the human shoulder joint where it persists within macrophages and stromal cells. Compared to culture-based methods, immunohistochemical staining can increase C. acnes detection. Phylotype II seems to be most prevalent in the deep shoulder tissue. The high detection rate of C. acnes in osteoarthritic AC joints might link its intraarticular presence to the initiation of osteoarthritis.