In recent years, multiple studies have reported low-virulent anaerobic bacteria in disc material, suggesting that such infection may contribute to degenerate spine conditions, including sciatica, neck pain, and back pain. In this study, 32.5% of patients had low-virulence bacteria in the IVDs, which was similar to previous reports of approximately 8%~ 53% . In addition, the surrounding muscles and ligaments served as contamination markers, so the isolated bacteria could be attributed to original growth inside the IVDs rather than possible contamination. Therefore, we concluded that there was latent infection of low-virulence bacteria in IVDs with a prevalence of 28.75% after the exclusion of three suspicious cases.
In addition, the bacteria we isolated and identified were also similar to those in previous reports. In our study, P. acnes accounted for 26.25%, and CNS accounted for 6.25%. Stirling et al. first reported that the prevalence of P. acnes and CNS was 44.4% (16/36) and 5.5% (2/36) in degenerated IVDs, respectively . Subsequently, several papers also reported the existence of low-virulence anaerobic bacteria in IVDs, especially P. acnes, with the prevalence ranging from 45 to 84%. More importantly, the survival, reproduction and pathogenicity of the bacterium was further validated in animal studies according to previous reports [11, 12]. Hence, low-virulence anaerobic bacteria, especially P. acnes, have been considered a new pathogenic factor for disc disease by increasing numbers of scholars.
Despite this evidence, the hypothesis that latent infection of anaerobic low-virulence bacteria resided in IVDs remains controversial. Several studies have suggested that bacteria detected in disc cultures may originate from the patient’s skin, the air, and laminar flow, especially that P. acnes or CNS are normal flora residing on the human skin [13, 14]. For example, McLorinan et al. demonstrated that P. acnes would contaminate the incision during the operation, indicating that the bacteria was detected in 29.1% of skin samples, 21.5% of tissue samples, and 16.5% of washings . However, it is difficult to attribute all the isolated bacteria to contamination because several papers have demonstrated the existence of bacteria within the IVDs, and the phylogroup patterns of bacteria were different from those of skin .
Reports by Stirling and others suggest that low-grade infection may play a role in inflammation and herniation [3, 15,16,17]. In our study, positive cultures were significantly associated with Modic changes in the adjacent vertebrae, further supporting the theory that MCs in the vertebrae adjacent to a previously herniated disc may be related to oedema surrounding an infected disc. Animal studies also demonstrated that inoculation of anaerobic low-virulence bacteria into IVDs would cause degenerative discogenic disease, such as Modic changes or disc degeneration, which is different from the pyogenic discitis caused by Staphylococcus aureus (S. aureus) [11, 12]. Therefore, low-virulence anaerobic bacteria have been considered one of the reasons for Modic changes.
Unfortunately, there were still several limitations in this study. First, all patients had to receive preoperative prophylactic antibiotics according to the protocol of our Institutional Ethics Board, which may have decreased the rate of positive culture. Additionally, we acknowledge that the use of antibiotics might also have reduced the number of positive cultures obtained from the ligament and muscle material, which was interpreted in this study as a control for contamination. Additionally, the polymerase chain reaction was not performed directly on tissue to detect the bacterial DNA. Although laboratory culture is the gold standard for the diagnosis of bacterial pathogens, polymerase chain reaction has advantages and disadvantages. Finally, more samples should have been included in the study to increase the accuracy of the conclusions.