Analysis of the fecal and oral microbiota in chronic recurrent multifocal osteomyelitis
Arthritis Research & Therapy volume 24, Article number: 54 (2022)
Chronic recurrent multifocal osteomyelitis (CRMO) is a rare autoinflammatory bone disease for which a lack of bacterial involvement is a key diagnostic feature to distinguish it from other symptomatically related diseases. However, the growing evidence suggesting an involvement of the host-associated microbiota in rheumatic disorders together with the now wide accessibility of modern culture-independent methods warrant a closer examination of CRMO.
In this study, we show through bacterial 16S rRNA gene profiling that numerous features of the oral- and fecal microbial communities differentiate children with and without CRMO.
Notably, communities in diseased children are characterized by a lack of potential probiotic bacteria in the fecal community and an overabundance of known pathobionts in the oral microbial communities. Of special interest is the HACEK group, a set of commonly known oral pathogens that are implicated in the development of several acute and chronic diseases such as osteitis and rheumatoid arthritis. Furthermore, we observe that gut bacterial communities in the diseased children appear to reflect an altered host physiology more strongly than the oral community, which could suggest an oral disease origin followed by propagation and/or responses beyond the oral cavity.
Bacterial communities, in particular the oral microbiota, may serve as an indicator of underlying susceptibility to CRMO, or play a yet undefined role in its development.
Chronic recurrent multifocal osteomyelitis was first described by Giedion et al.  and is ascribed to the very heterogeneous disease pattern of non-bacterial osteomyelitis , affecting mainly children and adolescents. The cause of this disease complex remains unknown, although chronicity, recurrence, and involvement of the innate immune system speak for an origin in the auto-inflammatory disease spectrum [3, 4]. A sterile bone inflammation (culture negative blood sample or bone biopsy) is the hallmark of auto-inflammatory bone disorders with clinical representations ranging from a mostly mild and limited unifocal disease such as chronic non-bacterial osteomyelitis (CNO) to severe diseases with multifocal chronic active and recurrent inflammation known as chronic recurrent multifocal osteomyelitis (CRMO) . Classic clinical signs are pain in the affected bones, sometimes swelling or warmth and erythema, and rarely fever . In a few cases, chronic bone inflammation is associated with inflammatory skin disease, psoriasis, palmoplantar pustulosis, or acne fulminans and in 10% of CRMO cases inflammatory bowel diseases can occur . Specific laboratory parameters do not exist and often no or only very mild changes in the known inflammation parameters (CRP and BSR/ESR) are observed. The inflammation can be self-limiting and is primarily localized in the metaphyses of tubular long bones, but can occur in every bone, including the clavicle, mandible, and vertebrae . In most cases, chronic osteomyelitis does not lead to changes in the macroscopic bone structure, but occasionally bone thickening (hyperostosis) and spinal involvement can occur . Histologically, the affected bones show immune cell infiltration and bone resorption processes, but also a partly exceeding formation of bone material. Disease patterns with simultaneous occurrence of acne fulminans and arthritis are very close to the SAPHO disease complex (synovitis, acne, pustulosis, hyperostosis, osteitis), from which CRMO is believed to be the pediatric equivalent based on shared inflammatory features [9, 10]. However, CRMO is a low incidence disease with only 0.43 cases per 100,000 children (chronic non-bacterial osteomyelitis) and predominantly occurs in girls .
An immediate cause of CRMO and a central pillar of its pathogenesis appears to be an imbalance of pro-inflammatory monocytes (increased expression of IL-1β, IL6, TNFα) and reduced levels of anti-inflammatory cytokines (IL10) . Genetic factors are also implicated, including a potential risk locus for CRMO on chromosome 18 , observed disease clustering among family groups, and high- and low-IL10 expression promotor haplotypes [14, 15]. The activation of typical bacterial sensors like TLR2 and TLR4 may also be involved in the signaling cascade, which leads to reduced IL10 expression, dysregulated immune reactions, and eventually bone damage via RANK activation and osteoclast formation [6, 16].
Although a critical feature of CRMO is the apparent lack of a specific infectious cause, there are a number of indications for potential microbial involvement, not as a primary infection, but as triggers or as contributing factors. First, an established mouse model of osteomyelitis carries a genetic risk locus known as Pstpip2 (Proline-serine-threonine phosphatase-interacting protein 2) , whose human ortholog PSTPIP1 (CD2BP1) is an autosomal dominant cause of the hereditary autoinflammatory PAPA syndrome (pyogenic arthritis, pyoderma gangrenosum, acne) , a disease characterized by autoinflammatory bone inflammation [12, 19]. PSTPIP1 also lies within the genomic region of a previously described CRMO risk locus in humans . The murine Pstpip2cmo model shows spontaneous osteomyelitis, which resembles the CRMO phenotype in humans. Interestingly, this CRMO model displays evidence of interaction with the microbiota, as Pstpip2cmo mice show a marked increase of intestinal Prevotella abundance, which can be modified by dietary and antibiotic intervention with direct disease ameliorating effects (e.g., decreasing IL-1β levels) . So far CRMO and related comorbidities have been treated successfully by the administration of IL-1-Receptor antagonists like anakinra, emphasizing the importance of IL-1β and neutrophil activation [21,22,23,24,25,26]. Furthermore clear associations/comorbidities with autoinflammatory diseases like psoriasis or inflammatory bowel disease were reported, which speak for an overlap in genetic and/or environmental risk factors, including potential bacterial triggers [5, 27]. Similarly, the association of host innate immune receptors TLR2 and TLR4 in CRMO pathogenesis [14, 28] or NLRP3  imply microbial involvement. Finally, recent data suggest that mast cells appear to be an additional factor in sterile bone inflammation . For inflammatory bowel disease, it was shown that mast cell activities altered by the host microbiome probably play a role in its pathogenesis .
Although living bacteria were thus far not isolated from biopsies or blood samples of CRMO patients, it remains possible that yet unknown slow growing and/or fastidious bacteria could be undetected triggers and not necessarily restricted to the site of inflammation . Thus, microbial community composition may hold valuable diagnostic information as in other inflammatory disorders such as inflammatory bowel disease  and could represent a therapeutic target. We here describe the oral and fecal microbiota in a panel of CRMO patients and matched controls derived from Northern Germany, which reveals numerous disease-related signatures at both body sites and implicates several bacterial taxa with known pathogenic properties.
Human sample collection and clinical data
Fecal and saliva samples were taken with consent over the course of regular clinical control examinations in the pediatric rheumatology outpatient clinic at the “Klinik für Kinder- und Jugendmedizin I”, Universitätsklinikum Schleswig Holstein, Campus Kiel (UKSH, Kiel campus, Germany; ethical permit: D 438/13). All procedures related to patients and healthy subjects followed the guidelines of the Declaration of Helsinki. For an overview of cohort specific meta-data, like age, gender, physiological measurements, and medication, see Table 1. The inclusion of the patients in the study did not require a predefined time of disease or treatment history. We recruited 25 CRMO patients in clinical remission, defined as having no ailment with or without treatment at the time of sampling, and a group of 24 healthy children matched by age and gender with no evidence of inflammatory disease as controls.
Fecal and saliva samples were collected after written consent, following the manufacturer’s instructions (PSP Saliva Gene DNA Kit, PSP Spin Stool DNA Kit; STRATEC Molecular GmbH, Germany) and stored in stabilizing solution at −20°C. As part of regular examinations, we assessed several clinical blood parameters, including among others the concentration of C-reactive protein (CRP mg/l) (see Table 1, Figure S1).
DNA extraction and 16S rRNA gene sequencing
DNA was extracted from 47 saliva and 42 stool samples from 49 individuals using the PSP Saliva Gene DNA Kit and PSP Spin Stool DNA Kit respectively following the manufacturer’s protocol (STRATEC Molecular GmbH, Germany). The 16S rRNA gene was amplified using uniquely barcoded primers flanking the V1 and V2 hypervariable regions (27F-338R) with fused MiSeq adapters and sequencing was performed on the Illumina MiSeq platform with v3 chemistry. See supplemental methods for additional details.
Sequence processing and quality control
FASTQ files were filtered and merged via USEARCH v8.0.1623  and FASTX tool v0.0.13 . Chimeric sequences were removed [35, 37] and sequences were classified and confirmed as bacterial using the RDP classifier and the RDP16 database in mothur 1.39.5 [38, 39]. For all downstream analyses of diversity and habitat association, we took a random subset of 13641 sequences per sample to normalize the read distribution. OTU binning was performed by the opticlust algorithm to ensure high quality OTU clusters . Further details can be found in the supplemental methods.
Species diversity indices (ACE approximated species richness, Shannon-Weaver number equivalent) were calculated in R 3.3.2 using vegan [41,42,43,44]. For beta diversity analyses, we included OTU-based metrics that highlight shared presence (Jaccard distance) or abundance (Bray-Curtis distance). Distances were visualized via principal coordinate analysis (PCoA) and centroids and vectors were fit via an iterative goodness of fit test. Testing for community separation between health conditions or by age was performed via distance-based redundancy analyses and permutative ANOVA [45, 46]. Univariate analyses were carried out with negative binomial generalized linear models and linear models using prior model selection by AIC criterion and likelihood ratio tests in MASS  and permutative Wilcoxon tests as implemented in coin . Indicator species analysis (IndVal.g) was used to assess the predictive value of a taxon for each respective host phenotype/category as implemented in indicspecies . P-values of the genera and OTU associations were adjusted by the Benjamini-Hochberg procedure . Additionally, machine learning via RandomForest was employed for supervised classification and derivation of important taxa to distinguish healthy individuals and CRMO patients [51, 52].
Evaluating correlations between bacterial taxa and clinical parameters was done using the Euclidean distance correlation , where only associations with P ≤ 0.010 were chosen for further analysis, as this method is able to detect a wide variety of correlations. Further, SparCC was used to generate co-abundance networks of consensus genera and species level OTUs in fecal and oral communities at a P-value cutoff of PFDR ≤ 0.05 and ║R║ ≥ 0.20 . Node-based importance measures (degree, betweenness, PageRank-index) were calculated as implemented in igraph [55,56,57] and significance of importance was obtained via a one-sided Z-test against a set of permuted networks (10,000 permutations).
We recruited 49 individuals (25 CRMO patients, 24 healthy controls) and collected fecal and saliva samples. A summary of patient characteristics is given in Table 1. Two patients were enrolled in the study at the time of diagnosis, before the start of treatment. In 22 patients, the collection of stool and saliva samples was between 1 and 90 months after first diagnosis. Eight of these patients had no treatment at the time of sample collection; 17 patients were under therapy with an NSAID. Seven patients were given corticosteroids, one was given Etanercept, and two were treated with bisphosphonates during their course of disease and prior to the time of sample collection. All patients were previously diagnosed with CRMO, with signs of active bone inflammation (clinical signs of inflammation, inflammation biomarkers, detection of bone inflammation via MRI), but at the time of sampling were in a state of clinical remission without signs of discomfort or ailment with or without treatment. Thus, with the exception of BSR, CRP, monocyte, and basophile number, most clinical parameters showed no clear deviation from their respective normal reference ranges (see Table 1; Figure S1) .
In total, we successfully sequenced the V1/V2 region of the bacterial 16S rRNA gene in 42 fecal samples (20 healthy, 22 diseased) and 47 oral samples (23 healthy, 24 diseased). This resulted in a sampling depth of 13641 sequences per sample, corresponding to a Good’s coverage at the species level (97% sequence identity OTUs) of 0.9498 ± 0.0264 (fecal) and 0.9876 ± 0.0049 (oral; mean ± sd). Throughout the subsequent analyses, the potential influence of anti-inflammatory treatment, which varied among patients, was tested in all instances where feasible/appropriate (i.e., for taxon abundances and alpha- and beta diversity analyses).
Differential phylum abundances between controls and CRMO patients
In the primary analysis, we investigated the influence of patient characteristics (Table 1) on the abundance of the major taxonomic groups in the fecal and oral microbiome. Sex and age are the main contributors to abundance differences in the major phyla of the fecal microbiota, such as Firmicutes and Bacteroidetes. Actinobacteria are also influenced by sex and are underrepresented among CRMO patients (see Fig. 1A–F; Table 2). In the oral communities, Firmicutes are influenced by age and sex, while Bacteroidetes show a slight abundance increase in CRMO patients compared to healthy controls, which increases with age. However, oral Actinobacteria and Fusobacteria significantly increase in samples of diseased children with age (Fig. 1F and Figure S2C & S2D; Table 2). Members of the phylum Proteobacteria in the fecal and oral microbial communities do not show any association to health condition, age, or sex in our study (Figure S2A & S2B; Table 2).
Bacterial associations with disease state
A closer inspection of taxon associations to CRMO disease state reveals many interesting genus- and species-level associations, which mainly overlap. Among the healthy children, several potential probiotic bacteria are overrepresented in feces including several members of the Bacteroidales (Paraprevotella, Coprobacter, Porphyromonadaceae, Barnesiella) and others (Fig. 1G and Figure S2E; Table 3 and Table S1). Interestingly, Christensenella, a bacterium shown to be highly heritable and previously shown to be associated with decreased weight gain , is overrepresented in the fecal samples of healthy children. Surprisingly, Faecalibacterium, which is a taxon typically positively associated with a healthy status in the context of other inflammatory disorders such as inflammatory bowel disease , rather shows an association to CRMO, similar to another member of the Clostridia Cluster Fusicatenibacter (see Fig. 1G and Figure S2E; Table 3 and Table S1).
Next, we focused on abundance differences in oral bacteria according to disease status, whereby Veillonella, Granullicatella, members of the Bacteroidales (Prevotellaceae, Porphyromonadaceae), and others are associated to healthy subjects (Fig. 1H and Figure S2F). Granulicatella is a common member of the oral microbial community known to cause nosocomial infections and depends on metabolic byproducts of other bacteria . Similarly Veillonella is a common fastidious and mostly mutualistic bacterium of the human oral and gut microbiota that rarely causes disease [61,62,63]. In contrast, oral bacteria associated to CRMO include among others Actinomyces, Cardiobacterium, Campylobacter, Eikenella, Leptotrichia, and Kingella. Eikenella is known to cause osteomyelitis and endocarditis particularly in immunocompromised individuals [64,65,66]. Kingella was also recently recognized as a widely distributed and important pathogen for children, with disease manifestations in joints and bones . Haemophilus, Aggregatibacter, Cardiobacterium, Eikenella, and Kingella together comprise the HACEK group (Haemophilus parainfluenzae, Aggregatibacter actinomycetemcomitans, Aggregatibacter aphrophilus, Aggregatibacter paraphrophilus, Cardiobacterium spp., Eikenella corrodens, Kingella spp.; see Fig. 1H and Figure S2F; Table 3 and Table S1), an assembly of slowly growing, fastidious bacteria that are often involved in unusual infections with cardiac complications, as well as strong periodontal and skeletal involvement [68,69,70]. Bacteria grouped as unclassified Pasteurellaceae were included in the HACEK group, due to the difficult to resolve taxonomy among members of this taxonomic group, e.g., Haemophilus and Aggregatibacter . Furthermore, Actinomyces and Leptotrichia were reported to cause oral abscesses, systemic inflammation, bacteremia, and even bone degradation [72,73,74]. Additional supervised classification of disease states via RandomForest further validated many of the indicator species and genera, including several members of the HACEK group, which strongly differentiate CRMO patients and healthy individuals (see Table S2) .
Taxon co-abundance network analysis highlights the importance of opportunistic pathobionts
Through the analysis of genus and species level co-abundance networks (Fig. 2A–F; Figure S3, S4, S5), we identified highly interactive groups of oral and fecal bacteria, whose abundances are closely correlated and may have important community wide and potentially disease-modifying effects. Indicator bacteria for CRMO (Fig. 2A, B; OTU level: Figure S4A-S4D) are on average more highly interconnected than other bacteria, as determined by their number of “interaction partners” (node degree; fecal-genera: P=0.9822, oral-genera: P=0.0005, fecal-OTU: P<0.0001, oral-OTU: P<0.0001; Wilcoxon test), but also by their positional importance, specifically in the oral community network (PageRank; fecal-genera: P=0.2168, oral-genera: P=0.0010, fecal-OTU: P<0.0001, oral-OTU: P<0.0001; Wilcoxon test). Similarly, a higher relative number of connections among HACEK bacteria (Fig. 2C, D; OTU level: Figure S4E & S4F) compared to others is observed (node degree; fecal-genera: P=0.1733, oral-genera: P=0.05131, fecal-OTU: P=0.6716, oral-OTU: P=0.0049; Wilcoxon test) as well as a higher positional importance specifically in the oral communities (PageRank; fecal-genera: P=0.0185, oral-genera: P=0.09001, fecal-OTU: P=0.6914, oral-OTU: P=0.0391; Wilcoxon test). By permuting the network nodes in an untargeted approach, we identified a significantly higher than average taxon importance of, among others, indicator genera and species for CRMO in the oral community (i.e., Kingella, Capnocytophaga, Campylobacter, Eikenella), as well as several other potentially pathogenic bacteria like Fusobacteria or Streptococcus (Fig. 2E; Figure S3A & 3C; OTU level: Figure S5A, C, D; Table S3 and S4). In contrast, several of the most important bacteria in the fecal co-abundance networks are indicator bacteria associated to a healthy condition or absence of disease relapse, with the exception of Faecalibacterium (associated to CRMO; see Fig. 2F; Figure S3B & S3D; OTU level: Figure S5B, S5E, S5F; Table S3 and S4). These analyses imply for one, a central role of the detected indicator taxa in the respective communities, but also the importance of disease-associated bacteria, in particular members of the HACEK group, in the oral microbial communities.
Bacterial associations to clinical parameters
The host-associated microbiome is known to be intimately associated to a broad range of physiological and immune parameters . Thus, given the availability of clinical measurements for CRMO patients, we conducted further exploratory analyses to gain further insight in this group. Interestingly, several consensus genera and OTUs in the fecal and oral samples show strong and mostly directionally consistent associations to clinical data in CRMO patients (Tables S5, S6, S7), although clinical measures are mainly in the normal and age-appropriate physiological range (Figure S1). As an example, fecal OTUs belonging to the uncl. Ruminococcaceae mostly positively correlate with BSR, CRP, MCHC, counts of eosinophils, neutrophils, and thrombocytes, while consistently negatively correlate to levels of hemoglobin, hematocrit and MCH, and counts of erythrocytes and lymphocytes. Particularly, indicator bacteria for health or disease show a similar direction of association with the respective clinical measure. In the fecal samples, OTUs belonging to the unclassified Lachnospiraceae and Faecalibacterium are associated with decreased calcium levels as well as being indicators for CRMO (see Table S6), while indicators of healthy individuals in the Clostridiales positively associate to MCH, neutrophils, and BSR. Health-associated fecal Proteobacteria are positively correlated to MCH and the levels of eosinophils, but negatively correlated to lymphocyte counts (Table S6). An indicator in feces for having experienced disease relapse (OTU-697: uncl. Ruminococcaceae) is also positively associated to CRP levels, and negatively to hemoglobin concentration and hematocrit. In contrast, indicators of no relapse show opposing associations to hematocrit, erythrocytes, and thrombocytes (e.g., OTU-1104: Faecalibacterium; OTU-2811: Lachnospiraceae uncl.).
Oral indicator species for CRMO are associated with MCHC (OTU-512: Kingella; OTU-835: Cardiobacterium), lymphocyte number (OTU-255: Campylobacter), and LDH activity (OTU-548: uncl. Prevotellaceae) (Table S7). In general, most members of the HACEK group display a comparable directionality of their association to clinical parameters, although with heterogeneity at the OTU level (e.g., see Table S6). This separation between potentially pathogenic or potentially probiotic bacterial groups in the way they collectively correlate to the clinical measures implies a similar relationship to host physiology among those bacterial groups.
Community diversity and its relationship to disease condition
To characterize community complexity, we employed different and complementary measures of alpha diversity based on the OTU presence and abundance distribution (ACE species richness, Shannon Entropy number equivalent) [44, 76] (Fig. 3). We could not find differences in alpha diversity measures derived from fecal communities, according to health status, age, or gender of the sampled individuals (Fig. 3A, B; Table 4). Interestingly, the oral microbial communities showed a pronounced association with subject age, as communities appear to become more diverse with age, particularly in male subjects (Fig. 3C). Further, the oral communities of CRMO patients become more diverse with increasing subject age, as compared to the healthy controls (Fig. 3D; Table 4). The treatment of CRMO patients had no significant effect on any measure of alpha diversity in our patient cohort, in contrast to a recently published study (fecal- ACE: Z=0.8208, P=0.4446; Shannon Entropy: Z=0.8208, P=0.4410; oral- ACE: Z=0.0869, P=0.9546; Shannon Entropy: Z=−0.5504, P=0.6049; Wilcoxon test) . Whether patients suffered a disease relapse since their diagnosis also did not show an influence on community diversity among patients (fecal- ACE: Z=−1.2285, P=0.2372; Shannon Entropy: Z=−1.6381, P=0.1057; oral- ACE: Z=−0.0894, P=0.9501; Shannon Entropy: Z=0.0298, P=1.0000; Wilcoxon test). However, among CRMO patients, we also identified significant associations between clinical measurements and alpha diversity, which are mostly restricted to either the fecal or oral microbial community (Fig. 3E; Table S8). Lymphocyte number decreases with a more diverse community (ACE species richness, Shannon Entropy) in the gut microbial communities and with increasing Shannon diversity in the oral microbial communities. In contrast, the number of neutrophils increases with increasingly diverse fecal communities (ACE species richness, Shannon Entropy). Only in the oral microbial community did increasing diversity correlate with increasing blood calcium levels (Table S8).
Community differentiation between disease conditions
To characterize differences between microbial communities, we used beta diversity measures based on qualitative and quantitative differences in OTU composition (Jaccard [J], Bray-Curtis [BC]). This revealed significant differences in structure and composition between healthy and diseased subjects in both fecal and oral microbial communities (Fig. 4A–D). A correlation of community differences with age was exclusive to the oral microbial communities, similar to what we observed in the alpha diversity of oral communities. This implies that the composition of the oral microbial community also changes successively with age (Fig. 4B, D). Further, we could also identify significant correlations between clinical measurements and community differences in the fecal- and oral microbial communities of CRMO patients (Fig. 4E–H). Calcium concentration and eosinophil number are exclusively correlated to differences in the oral microbial communities (Fig. 4F, H). In contrast, only hematocrit is correlated to community differences in the fecal microbial communities of CRMO patients, further highlighting the different influence of the fecal and oral microbial communities on patient physiology (see Table S9; Fig. 4E, G). However, we failed to identify any significant community differentiation between patients with or without a previous disease relapse (fecal- J: F1,20=1.0205, P=0.1420; BC: F1,20=1.0615, P=0.2961; oral- J: F1,22=0.9732, P=0.6868; BC: F1,22=0.9793, P=0.4472). To further account for potential influences of anti-inflammatory treatment, we performed analyses between treated and untreated individuals, for which no influence on fecal or oral communities was detected (fecal- J: F1,20=0.9843, P=0.6001; BC: F1,20=1.0642, P=0.3469; oral- J: F1,22=0.8851, P=0.9336; BC: F1,22=0.3966, P=0.9961).
In this study, we identify a number of intriguing microbial signatures in CRMO, including significant signals of disease manifestation and a spectrum of indicator species that were previously found to be associated with other diseases, such as rheumatoid arthritis (RA) . The reduced diversity in the feces of diseased individuals and somewhat opposing community dynamics in the oral community (increased diversity) suggests potentially higher oral colonization in diseased individuals, possibly driven by reduced colonization resistance. Further, the numerous associations between clinical parameters and community composition and complexity among CRMO patients indicate possible interactions relevant to disease development and progression.
Several other aspects of the microbiota between CRMO patients and controls are, of note, particularly the consistent and significant compositional differences of fecal and oral microbiota between healthy controls and CRMO patients, which were not yet reported for this disease. These patterns are another clear sign of a community wide influence of CRMO on the microbiota, or vice versa. Upon characterizing these differences to the individual taxonomic level in the fecal microbiota, we find an overrepresentation of potentially probiotic bacteria among controls such as Barnesiella or Clostridial groups (Cluster IV / Clostridium leptum group, Cluster XIVa), which are well-known butyrate producers with anti-inflammatory effects [59, 79, 80]. However, Faecalibacterium strain competition (multiple species level OTUs are observed in this cohort) can negatively influence the spectrum of short chain fatty acids, which might explain the association of this usually beneficial bacterial group to CRMO, which is similar to what was observed in cases of atopic dermatitis or juvenile idiopathic arthritis [81, 82].
In contrast, oral taxa indicative of CRMO such as Corynebacterium, Actinomyces, Propionibacteria, and Leptotrichia are generally associated to nosocomial infections, which can develop into local and systemic inflammation with skeletal manifestations. Infections by Corynebacterium, Actinomyces, and Propionibacteria in particular show similar etiologies (e.g., actinomycosis like symptoms: swelling, suppuration, abscess, fibrosis) and are known for associations to bacteremia, endocarditis, systemic inflammation, and even bone degradation [72,73,74, 83]. Although we must emphasize that no indication of infection was present in our CRMO patient cohort, associations involving the HACEK group (Haemophilus parainfluenzae, Aggregatibacter actinomycetemcomitans, A.aphrophilus, A.paraphrophilus, Cardiobacterium spp., Eikenella corrodens, Kingella spp.) are among the most interesting findings of our study. HACEK members are slow growing fastidious bacteria mainly of oral origin and involved in unusual infections with cardiac and skeletal complications [68,69,70], including children with inflammation in joints and bones . Eikenella in particular is known to cause osteomyelitis and endocarditis if it is able to invade the respective body sites [64,65,66]. Further, Aggregatibacter was shown to induce prostaglandin, TNFα, and IL-6 expression , which are central drivers of CRMO pathogenesis . The concerted occurrence of HACEK members and their close correlation are strong signs that this bacterial assembly has a stable co-occurrence pattern, with potential synergistic interactions with other bacteria, and is relevant to disease. Interestingly, slow growing fastidious bacteria have been suspected to trigger or associate with CNO/CRMO, but were not yet detected in biopsies of diseased individuals via culture or PCR [32, 85]. Although antibiotic treatment has no significant influence on CRMO progression , most HACEK members are resistant against several commonly used antibiotics, which may contribute to the missing effect of antibiotics on CRMO [86, 87].
Bone marrow-derived hematopoietic cells were identified as highly associated to CMO/CRMO . Neutrophils in particular appear to be a causative agent in animal models and humans acting via (pro-)IL-1β-related pathways [6, 20, 88] and are further associated to the microbial community in our analyses. Experimental ablation of neutrophils confers significant protection from disease progression and tissue damage in Pstpip2cmo mice . It has been reported that LPS leads to a stronger inflammasome activation (NLRP3) and subsequently a stronger imbalance between pro-inflammatory (increase in IL-20) and anti-inflammatory signals (reduced IL-1, IL-10 expression) in CRMO patient samples [14, 29]. Thus, oral activation of the inflammasome via bacteria, like members of the HACEK group or Fusobacteria, may lead to a systemic induction or amplification of inflammatory processes in CRMO [89,90,91]. However, there is also evidence for inflammasome independent inflammation in the Pstpip2cmo mouse model for CRMO . Accordingly, members of the fecal microbiota mainly correlate negatively to AP activity, which is in stark contrast to the oral communities, which appear to increase in regard to AP activity and blood calcium levels, potentially signifying inflammatory processes . In the fecal communities, we observe a lack of potential beneficial bacteria, while an increase of potential pathobionts is observed in the oral communities in CRMO cases. Thus, it is possible that a lack of beneficial bacterial signals (e.g., SCFAs) in the gut together with increasing the reactivity/inflammatory potential of the oral microbiota may result in some children having a higher probability to develop or sustain systemic autoinflammatory processes (i.e., CRMO).
In our study, the concentration of various immune cell types such as lymphocytes, monocytes, basophiles, and eosinophils and characteristics of erythrocytes also correlate to structural characteristics and single members of the microbial communities and thus indicate a modulating effect of the microbiota on the immune system of children with CRMO. For CRP, we identified positive correlations with oral Rothia, while fecal Ruminococcaceae, Lachnospiraceae, and others associate with increased CRP. Interestingly, the HACEK bacteria group was also implicated in a metagenomic study of RA in Chinese individuals , where they were abundant in healthy controls, but correlated negatively with certain clinical disease markers such as C-reactive protein (CRP) and RA-specific autoantibodies (anti-CCP). Indicator taxa also differ in their association to disease like Holdemania (Zhang: RA associated, current: health associated), Leptotrichia (Zhang: control associated, current: CRMO associated), while others overlap (e.g., Rothia-control associated, Campylobacter-control associated, Actinomyces-disease associated). Another notable group identified in our study is Prevotella, which was previously identified in the context of a mouse model of CNO by Lukens et al. . In the current study, fecal and oral Prevotella/Prevotellaceae OTUs associate to healthy individuals. The oral Prevotellaceae OTU displays a positive correlation to LDH activity CRMO patients. Prevotella is a heterogeneous group with a range of different metabolic capacities [93, 94], and may contain genomic variation changing their physiological and pathological behavior in relation to their host and disease status [20, 95].
Although CRMO fundamentally differs from RA in many underlying principles of pathophysiology, where RA is a classical autoimmune disease in comparison to CRMO, which is autoinflammatory, our results display interesting overlap with recent studies of RA. Specifically, Aggregatibacter actinomycetemcomitans as a representative of the HACEK group was found to lead to the hypercitrullination of autoantigens in RA, linking this disease to periodontal infection with this bacterium . With regard to CRMO, Aggregatibacter is known to disrupt neutrophil membrane integrity via Leukotoxin A (LtxA) and thereby reduces and dysregulates neutrophils, a cell population under strong suspicion to be a causative agent for autoinflammation in CRMO animal models via (pro-)IL-1β [20, 88]. Another HACEK member identified in this study, Cardiobacterium, was shown to increase IL-1β levels , as well as Actinomyces  and Propionibacteria (now Cutibacterium) , which is often isolated “concomitantly” with HACEK bacteria and is associated to the related SAPHO syndrome, although controversially discussed . Thus, HACEK bacteria, Corynebacterium, Actinomyces, Propionibacteria, and Leptotrichia may together form a disease-modulating consortium, and their single and/or combined disease associations may lead to skeletal manifestations [83, 98, 101, 102]. For other autoinflammatory diseases, links between dysbioses of the gut microbiome and disease activity were also previously discussed. Individual authors report that clinical symptoms in Familial Mediterranean Fever, a classic autoinflammatory syndrome, are influenced by an altered microbiome [103, 104]. However, Ozen et al. failed to provide evidence of microbiome alterations in an international study population. Further, gut microbial dysbiosis in systemic juvenile idiopathic arthritis (Still syndrome) is also suspected to be associated to the health condition and disease activity in different cohorts [82, 105].
Despite the numerous significant observations of our study, it suffers from a number of limitations. First, the patient cohort was sampled at different time points over the course of disease, leading some samples to be from treated and others from untreated individuals. Although we failed to observe significant differences between these subgroups at several levels of the analysis, this heterogeneity could mask some disease-related signals. Further, due to the relative small sample size of this rare disease, we could not further distinguish between the types of anti-inflammatory therapy.
In conclusion, our culture-independent analysis of microbial communities draws new attention to the potential role of members of the microbiota in the pathology of CRMO. We reveal a clear signal of known pathogenic groups such as HACEK or Actinomyces. Importantly, however, these observations may not reflect a direct causal role, but could also be a consequence of other primary microbial community alterations and/or host immunological responses, resulting in an imbalance of cytokine expression . Thus, future studies employing longitudinal sampling strategies, larger sample sizes, and deeper molecular and phenotypic characterization of patients and controls are warranted to better understand the etiology of CRMO.
Availability of data and materials
The datasets generated and analyzed (raw sequencing data, meta data) during the current study are available at the European Nucleotide Archive (https://www.ebi.ac.uk/ena/) under the accession number PRJEB22984 or ERP104714.
- 16S rRNA:
16S ribosomal RNA
Akaike Information Criterion
Blood sedimentation rate (1 hour) / Erythrocyte sedimentation rate
Chronic multifocal osteomyelitis
Chronic non-bacterial osteomyelitis
Chronic recurrent multifocal osteomyelitis
Bacterial group consisting of Haemophilus spp., Aggregatibacter actinomycetemcomitans, Aggregatibacter aphrophilus, Aggregatibacter paraphrophilus, Cardiobacterium spp., Eikenella corrodens, Kingella spp
Mean corpuscular hemoglobin
Mean corpuscular hemoglobin concentration
Mean cell volume
Magnetic Resonance Imaging
Non-steroidal anti-inflammatory drug
Operational taxonomic unit
Principal Coordinate Analysis
Tumor necrosis factor α
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We thank all study participants and parents, Katja Cloppenborg-Schmidt for her excellent technical assistance, and the Deutsche Forschungsgemeinschaft (DFG) ExC 306 Excellence Cluster “Inflammation at Interfaces”.
Open Access funding enabled and organized by Projekt DEAL. This study was funded by the Deutsche Forschungsgemeinschaft (DFG) Excellence Cluster “Precision Medicine in Chronic Inflammation (PMI; Grant no. EXC2167)”.
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The samples were taken with consent over the course of regular clinical control examinations in the pediatric rheumatology outpatient clinic at the “Klinik für Kinder- und Jugendmedizin I”, Universitäts klinikum Schleswig Holstein, Campus Kiel (UKSH, Kiel campus, Germany, ethical permit: D 438/13). All procedures related to patients and healthy subjects followed the guidelines of the Declaration of Helsinki.
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Rausch, P., Hartmann, M., Baines, J.F. et al. Analysis of the fecal and oral microbiota in chronic recurrent multifocal osteomyelitis. Arthritis Res Ther 24, 54 (2022). https://doi.org/10.1186/s13075-021-02711-8