INTRODUCTION
Chronic low back pain (CLBP) is a multifactorial condition with various potential mechanisms and causes leading to its common debilitating outcome. Systemic inflammation is a potential mechanism for CLBP. 1 Gut microbiome (GM) has been increasingly studied as a potential contributor to systemic inflammation through neuro-immune, endocrine, and metabolic mechanisms. 2–4 Alpha diversity, the measure of microbiome diversity within a subject, has been associated with health status, with lower diversity generally linked to worse health outcomes. 5–7 In the present study, we explored associations between GM, pain, and function, hypothesizing that the GM of CLBP subjects with higher pain severity and disability would be associated with decreased relative abundance of specific species linked to positive health outcomes and decreased diversity measures.
METHODS
419 participants were recruited from a larger study of CLBP participants. The clinical variables of pain and disability were collected using the Numeric Pain Rating Scale (NPRS) and the Oswestry Disability Index (ODI). The GM from stool was classified using 16S rRNA gene sequences and investigated through abundance-based and distribution-based methods built on individuals' taxonomic profiles. The relative species abundances (Abundance-based method) utilized the additive-log ratio (alr) in linear association models against NPRS and ODI. Separate statistical models were constructed using alr while controlling for appropriate covariates (sex, age, Body Mass Index, education, income, race) and compared to models with covariates only. The distribution-based methods utilized two alpha diversity indexes: the Shannon index and Tail statistic. Linear association models were run between alpha diversity indices, NPRS, and ODI.
RESULTS
Significant associations between the relative taxa abundance and ODI were highlighted in the case of Lachnospira (p=.007) and Anaerostipes (p=.010) (negative associations), and Subdoligranulum (p=.005), Ruminococcaceae (p=.022), and Christensenellacea (p=.010) (positive associations). Blautia was the only taxon found to have a significant positive association (p=.002) with pain. Comparisons of reduced models (covariates only, R2=0.29) against full models (covariates+alr, R2=0.25) determined that inclusion of the microbiota significantly improved the prediction of ODI (p=0.0198), but not pain. Both diversity measurements were significantly associated with ODI (positive association, Tail coeff=4.12, p<.0001; Shannon coeff=1.74, p<.001), but not with pain.
DISCUSSION
These results begin to define GM characteristics for a CLBP population. While each altered taxa has been associated with one or more chronic conditions, this pattern has not been reported previously. For example, decreased Lachnospira has been observed in various diseases, though its role remains unclear. 8 An increase of one or more of Ruminococcaceae, Alistipes, and Blautia has been observed in different inflammation-based conditions, including arthritis, 9 obesity and diabetes, 10,11 Parkinson and Alzheimer's Diseases, 12 polyneuropathy, 13 depression, 14 and irritable bowel syndrome. 15 The significant R2 improvement in the model including alr compared to the model excluding it suggests that GM significantly contributes to explaining the variability of ODI, but not that of pain. Interestingly, we observed a positive association between both diversity measures and ODI. This finding contradicts most of the literature on GM, citing higher diversity as a contributor to health, and requires further analysis.