INTRODUCTION

Back pain is a major problem worldwide and is linked to intervertebral disc degeneration and Modic change (MC). Several studies report bacterial growth, commonly Cutibacterium acnes (previously called Propionibacterium acnes), following extraction of degenerate discs at spine surgery. A pathophysiological role for infection in back pain has been proposed. We aimed to clarify if C. acnes is indeed the predominant species in degenerate discs and assess the utility of current laboratory and research practices in the detection and quantification of bacteria in disc material.

METHODS

We conducted a PRISMA systematic review. MEDLINE, PubMed, Scopus and Web of Science were searched with the terms Modic change, intervertebral dis*, bacteria, microb*, and infect*. Date limits of 2001-2021 were set. Human studies investigating the presence or role of bacteria in disc degeneration or MC adjacent discs were included. We also investigated whether discs adjacent to vertebral MC are at increased risk of bacterial proliferation.

RESULTS

Thirty-six articles from 34 research investigations relating to bacteria in human degenerate discs were found. Twenty-seven studies reported finding disc bacteria and nine concluded contamination findings. C. acnes was identified in pathological disc material. A ‘candidate bacterium’ approach was repeatedly adopted likely biasing results to find C. acnes. Targeted microbial culture precludes the opportunity to isolate non-C. acnes bacteria, making accurate determination of a range of species impossible. Only 6/36 studies used universal PCR or genome-wide sequencing techniques to capture all bacterial DNA present in disc samples.

We identified 12 studies reporting MC v. non-MC adjacent disc bacteria. Eight found MC adjacent discs had higher bacterial loads, yet only one reported that MC1 drove differences. More commonly, MC1 & MC2 cases were pooled when increased bacterial proliferation compared with control discs was found. Five studies reported multiple bacterial species in the disc. Three found bacterial species in healthy disc tissue; interestingly one study showed C. acnes was present in similar proportions, but not predominant, in healthy, degenerated and herniated discs.  

DISCUSSION

Evidence to date implicates C. acnes identified through culture, microscopy and sequencing, with some suggestion of diverse bacterial colonisation in the disc. This review found studies which used culture methods and conventional PCR for bacterial detection. Further agnostic investigation using new genomic methods which identify bacteria by their genetic material rather than by targeted culture and may well identify novel organisms, or combinations of organisms, as yet uncatalogued. Including omics analyses and advanced histological techniques will strengthen disc degeneration research.