Intervertebral disc puncture predisposes the disc to Cutibacterium acnes infection via hematogenous and contiguous routes: a rat model study

BACKGROUND: Intervertebral disc degeneration (IVDD) is a primary cause of chronic low back pain. Recent evidence suggests a potential association between low-virulence infections, particularly by Cutibacterium acnes, and IVDD. However, the pathways and mechanisms by which this commensal skin bacterium infects intervertebral discs remain poorly understood. OBJECTIVE: The purpose of this study was to determine whether Cutibacterium acnes can enter the intervertebral disc through hematogenous or contiguous routes, and whether injury-induced damage to the annulus fibrosus and IVDD increases susceptibility to infection, using an animal model. METHODS: To investigate Cutibacterium acnes infection pathways, Sprague-Dawley rats underwent caudal disc annulus fibrosus puncture or remained intact (confirmed by 28-day MRI T2WI). After MRI, hematogenous and contiguous infections were modeled by daily Cutibacterium acnes injections for 7 days. Tail-vein injections were given in Groups A (intact discs) and C (punctured discs) to simulate hematogenous spread, while subcutaneous injections were given in Groups B (intact discs) and D (punctured discs) to simulate contiguous spread. Intervertebral disc samples were taken under very strict aseptic conditions after the intervention. Gram staining and Cutibacterium acnes-specific 16S rDNA PCR were used to confirm the presence of Cutibacterium acnes in these disc specimens cultured under aerobic and anaerobic conditions. The remaining segment was subjected to histological evaluation (H&E staining). RESULTS: Intervertebral disc puncture successfully induced IVDD, as evidenced by MRI and histology. Punctured discs exhibited significantly higher susceptibility to Cutibacterium acnes infection via both hematogenous (Group C: 17.5% vs. Group A: 0%; P = 0.012) and contiguous spread (Group D: 20% vs. Group B: 0%; P = 0.005), as confirmed by anaerobic culture. No bacteria were cultured in an aerobic environment, and the PBS group. CONCLUSIONS: Annulus fibrosus injury and IVDD significantly increase the risk of intervertebral disc Cutibacterium acnes infection through hematogenous and contiguous spread pathways.