In this MR study, we initially explored potential causal associations between gut microbiota and osteomyelitis risk by utilizing large-scale summary statistics of gut microbiota GWAs and osteomyelitis GWAs. Remarkably, a total of seven bacterial traits were identified as having a noteworthy causal association with osteomyelitis risk.
We delineated three main pathways through which gut flora affects distant organs: modulation of nutrient absorption, alteration of the immune system in intestinal endothelial cells, and translocation across the endothelial barrier into the systemic circulation by gut flora or their metabolites [28]. Evidence indicates that diet significantly influences the composition and abundance of intestinal flora in mice. By evaluating the specific responses to inflammation and immunity in mice subjected to varied diets, researchers have deduced that the intestinal flora correspondingly modulates its biological functions [29]. This hypothesis gained further support from a study by Phillips et al. They observed that osteomyelitis mice fed a high-fat diet exhibited a higher proportion of Lactobacillus and Prevotella in their gut microbiome. In contrast, a greater presence of Prevotella was documented in the cohort on a standard diet, which subsequently developed osteomyelitis. Furthermore, high-fat diet mice demonstrated reduced IL-1b levels in their pedis relative to those on a standard diet. Crucially, fecal microflora transplantation from high-fat-diet mice to standard-diet mice prior to disease onset not only inhibited Prevotella proliferation but also decreased IL-1b synthesis, thereby averting osteomyelitis. This underscores the proposition that diet-induced shifts in gut microbiota, through interactive modulation, can influence both the initiation and advancement of osteomyelitis [16]. Within the human gastrointestinal tract, the intestinal epithelial barrier serves as a crucial defense against detrimental antigens and pathogens, concurrently housing the gut microbiota. This barrier is pivotal for nutrient absorption and immune defense. Under typical conditions, tight junction (TJ) proteins securely seal the intestinal epithelium, providing a protective function. However, during intestinal dysbiosis, the gut microbiota can disrupt TJ proteins' expression and localization, consequently increasing intestinal permeability and precipitating extensive inflammatory reactions, including severe skeletal inflammations like osteomyelitis [30]. Moreover, research focusing on the gut-bone axis has revealed that short-chain fatty acids, gut microbiota metabolites, are instrumental in fostering regulatory T cells (Tregs) development, impeding Th17 cell production, diminishing inflammatory cytokines synthesis, encompassing cellular inflammatory mediators such as IL-6, IL-17, and IL-23, and preserving the systemic immune equilibrium [31].
Bacilli represent a category of Gram-positive bacteria, notable for their production of resistant endospores, exhibiting broad-spectrum bacillary activity, and synthesizing bacteriocins. They are prevalently found in various biological environments and constitute a significant component of the human intestinal flora. While most Bacilli are non-invasive, exceptions exist, with Bacillus cereus being a prime example. According to a report by Veysseyre et al., Bacillus cereus was responsible for 57 infections, of which 10 involved bones and joints. This phenomenon leads us to conjecture that such infections might arise when intestinal flora or their metabolic byproducts breach the endothelial barrier, entering the bloodstream, initiating an inflammatory response, and eventually precipitating a systemic inflammatory reaction [32]. In a related study, Bradley et al. discovered that Bacillus cereus can compromise the human intestinal barrier via a binary protein mechanism. This mechanism entails bacteria and their toxins disrupting the human intestinal barrier by achieving receptor-mediated endocytosis and employing the AB toxin complex for cellular intoxication, composed of ADP-ribosyltransferase (A) and cell-binding (B) components. This finding lends credence to the hypothesis that such disruptions could theoretically underpin the pathogenesis of osteomyelitis [33].
Our findings demonstrate that both Bacteroidia and Bacteroidales are positively associated with the risk of developing osteomyelitis. Furthermore, Bacteroidales, a member of the Gram-negative bacteria, represent an important cornerstone of the intestinal flora and boast one of the most complex polysaccharide-pod systems known in bacteria, comprising at least eight different polysaccharides (PSA- PSH). In a different spectrum, the lipopolysaccharides of the genus Anabaena, which lack O antigens, exhibit a virulence approximately 1000 times greater than that of E. coli lipopolysaccharides. Contrastingly, the fragile anaplasma toxin, a critical virulence factor within the Anaplasma genus, manifests in three isoforms (BFT-1, BFT-2, BFT-3), with BFT-2 identified as the prime facilitator of tissue damage. This super-potent toxin, BFT-2, has the potential to traverse the intestinal epithelial barrier, enter the bloodstream, and consequentially initiate hematogenous osteomyelitis. Moreover, the proficiency in withstanding oxidative stress significantly contributes to pathogenic virulence. Through the regulation of various oxidoreductases production and transport, including catalase, peroxidase, and thioredoxin under the governance of the transcription factor OxyR, bacterial flora acquire resilience against the host's oxidative responses, thus achieving enhanced tolerance [34].
In an intriguing case study, Panagiota et al. documented two instances of vertebral osteomyelitis attributable to Bacillus fragilis, with no detectable infection foci elsewhere in the body. This observation propels the hypothesis of a potential correlation with enterobacteria, offering valuable insights for forthcoming investigations [34]. The disproportion, characterized by the overgrowth of certain microorganisms and the depletion of others, disrupts the intestinal microbial ecosystem, leading to the forfeiture of crucial physiological functions, a condition termed gut microbiota dysbiosis [35]. This aberration in the gut flora composition is speculated to influence the progression and manifestation of osteomyelitis significantly. It does so primarily by orchestrating the production of short-chain fatty acids (SCFAs), modulating intestinal permeability, and adjusting immune and inflammatory responses. Among the SCFAs, butyrate stands out as a predominant component, chiefly generated through the enzymatic breakdown of dietary fibers within the intestinal conduit and serving as a principal energy substrate for intestinal epithelial cells [21]. Noteworthy producers of butyrate encompass bacteria like rumen Coccidioides, Vibrio butyronucleicus, and Aeromonas butyricola. Butyrate executes a diverse array of functions by engaging with G protein-coupled receptors (GPACs) and partaking in both local and extensive signaling networks. Such interactions facilitate the fortification of intestinal barrier function, mucosal immunity, and intestinal homeostasis, potentially optimizing energy metabolism, fostering weight reduction, attenuating inflammation, and restoring the equilibrium of the gut-brain axis. However, a decline or alteration in the population of Aeromonas butyric acidophilus can compromise the body's defensive mechanisms, resulting in an imbalanced immune system and heightened inflammatory processes.
Furthermore, our research identified a potential link between the genera Tyzzerella3 and Coprococcus3 and the susceptibility to osteomyelitis. Particularly, Coprococcus3, an inhabitant of the human intestinal flora and recognized commensal bacterium, has drawn attention due to the escalating imbalance within the genus triggered by antibiotic misuse. This imbalance has been associated with elevated inflammatory levels in some Coprococcus3 species, with emerging evidence suggesting their role in virulence and increasing antibiotic resistance. In the context of early-stage osteomyelitis, where clinical symptoms may be inconspicuous or where there is indiscriminate administration of anti-infective antibiotics, there's a postulation that these factors may exert a pronounced effect on this genus, thereby intensifying the onset and progression of osteomyelitis.
Conversely, Lachnospira, the sole genus identified as a potential protective agent in our study, falls under the phylum of Thick-walled Bacteria. It's posited that Trichoderma, a likely beneficial bacterium, engages in the metabolism of diverse carbohydrates, producing essential substances like acetic acid and butyric acid to energize the host [36]. Identified primarily in early infant populations, Lachnospira's prevalence has been linked to increased concentrations of short-chain fatty acids (SCFAs). These SCFAs play pivotal roles, particularly in sustaining the integrity of the intestinal barrier and harboring anti-inflammatory attributes. This not only guarantees efficient nutrient absorption but also acts as a safeguard against the infiltration of deleterious substances (such as bacterial endotoxins) into the systemic circulation, consequently mitigating the risk of hematogenous osteomyelitis [37].
The innovative methodology of this study lays the groundwork for the first Mendelian Randomization (MR) analysis to ascertain a causal relationship between gut microbiota and osteomyelitis. By eradicating confounding elements, it earmarks candidate bacteria for future functional explorations. Furthermore, the implications of the MR findings are far-reaching in the realm of public health. They supplement previous research on the gut microbiota-osteomyelitis nexus by introducing a novel viewpoint at the genetic stratum. From a preventive medicine perspective, these insights could guide osteomyelitis prevention strategies through timely gut microbiota modulation. Diagnostically, they underscore the necessity for heightened vigilance in osteomyelitis screening and early detection among individuals with gut microbiota anomalies.
However, this study presents several limitations. Firstly, the pool of SNPs accessible for analysis was constrained, a result of adhering to a stringent genome-wide significance criterion (P < 5×10 − 8). We included only those SNPs that fulfilled the stipulated significance benchmark (P < 1×10 − 5), a factor that might undermine the robustness and exactitude of our outcomes. Additionally, the gut microbiota GWAS data employed are in their preliminary phases in terms of sample magnitude and the extent of currentness. Moreover, the dataset utilized was derived solely from European ancestries, not accounting for variables such as gender and ethnicity, thereby curtailing the extrapolation of our insights to broader populations. It's also vital to acknowledge that our research narrowly concentrated on bacteria, overlooking the substantial diversity of eukaryotic viruses and prokaryotic phages within the human microbiota [38]. This necessitates further inquiries to delve into their potential implication in osteomyelitis' onset and evolution. Ultimately, the connection between the human microbiota and its host, in states of both health and disease, doesn't adhere to a simplistic unidirectional "cause and effect" paradigm [39]. Consequently, ensuing research ought to take into account the intricate interplay and intercommunication amidst the host and gut microbiota to deepen our grasp of their interrelation with various diseases.