Sheep as a Potential Model of Intradiscal Infection by the Bacterium Cutibacterium acnes

Abstract

The anaerobic bacterium Cutibacterium acnes has been increasingly linked to the development of degenerative disc disease (DDD), although causality is yet to be conclusively proven. To better study how this organism could contribute to the aetiology of DDD, improved animal models that are more reflective of human disc anatomy, biology and mechanical properties are required. Against this background, our proof-of concept study aimed to be the first demonstration that C. acnes could be safely administered percutaneously into sheep intervertebral discs (IVDs) for in vivo study. Following our protocol, two sheep were successfully injected with a strain of C. acnes (8.3 × 10⁶ CFU/disc) previously recovered from a human degenerative disc. No adverse reactions were noted, and at one-month post inoculation all triplicate infected discs in our first animal grew C. acnes, albeit at a reduced load (5.12 × 10⁴ to 6.67 × 10⁴ CFU/disc). At six months, no growth was detected in discs from our second animal indicating bacterial clearance. This pilot study has demonstrated the feasibility of safe percutaneous injection of C. acnes into sheep IVDs under fluoroscopic guidance. The design of follow-up sheep studies to investigate the potential of C. acnes to drive pathological changes within infected discs should now be pursued.

Keywords: Cutibacterium acnes; bacterial discitis; percutaneous injections; sheep model; spinal intervertebral discs.

Figure 1

(A) 25-gauge needles within the 20-gauge introducer needles, serially spaced into the lumbar discs. (B) Fluoroscopic radiograph verifying needle placement in the AP plane.

Figure 2

Growth of C. acnes from 3 × discs of the same sheep (USDA#1283) four weeks after initial inoculation. Data is presented as mean ± SEM of log₁₀ CFU per disc. Statistical analysis was performed for paired samples.

Figure 3

One month MRI scans for sheep USDA#1283. (A) T1 weighted baseline (B) One month T1 weighted (C) T2 weighted FS baseline (D) one month T2 weighted FS.