Intracellular infection of <i>Cutibacterium acnes</i> in macrophages of extensive peri‐implantitis lesions: A clinical case series

Cutibacterium acnes is a facultative anaerobic, gram-positive rod, and a commensal bacterium of the body surface including oral cavity. A causal relationship between C. acnes and chronic granulomatous diseases, such as sarcoidosis and orthopedic implant-associated infections, has been previously reported. Typically, C. acnes has been observed inside macrophages, allowing evasion of host immunity, and triggering a persistent inflammatory response. However, such findings have not been reported in peri-implantitis lesions. In this case series, we collected inflamed tissues from extensive peri-implantitis lesions of eight patients. Out of the eight samples, seven tested positive for the 16 s rRNA gene of C. acnes by polymerase chain reaction, and six were positive by immunohistochemistry. Immunohistochemical staining revealed the presence of C. acnes in the cytoplasm of macrophages, suggesting a role in lesion formation. This finding may enhance our understanding of the pathophysiology of persistent peri-implantitis lesions and provide implications for future therapy.