Strain-dependent internalization and persistence of Cutibacterium acnes differentially alter biofilm formation

Cutibacterium acnes is increasingly recognized as a major pathogen in orthopedic implant-associated infections, particularly in shoulder prostheses. Its ability to form biofilm and interact with bone cells may contribute to chronicity and diagnostic challenges. This study aimed to investigate the strain-dependent internalization and persistence of C. acnes within osteoblasts and their impact on biofilm formation and bone cell function. Seven C. acnes clinical strains were studied. Internalization assays were performed using murine osteoblastic cell lines modified for β1 integrin (OB β1 +/+ and OB β1 −/− ). Internalization and intracellular persistence, biofilm formation before and after internalization, bone mineralization (alizarin red staining), gene expression (RT-qPCR), and IL-6 secretion were evaluated. Internalization capacity varied significantly between strains, with low overall rates (<0.5%). β1 integrin appeared to contribute partially to bacterial entry (decrease internalization rate from 50 to 97% depending on the strain in β1 −/− cells). Most of the strains persisted intracellularly up to 4–10 days. Biofilm production was strain-dependent and modified after internalization. No significant effect on osteoblastic mineralization or gene expression were observed. IL-6 secretion showed minimal variation in our conditions. These findings confirm that C. acnes can be internalized and persist within osteoblasts, by using partially β1 integrin pathway, contributing to chronic implant infections. Internalization influences bacterial phenotype, biofilm production ability but does not significantly impair osteoblastic function in this model. The role of intracellular reservoirs and alternative entry pathways warrants further investigation.