Shared microbiological and immunological patterns in periodontitis and IBD: A scoping review

Abstract Objectives To extract the microbiological and immunological evidence underpinning the association between periodontitis and inflammatory bowel disease (IBD). Methods Relevant articles were sorted through a systematic search on PubMed, Embase, Scopus and Web of Science up to October 2020. Available evidence was grouped in three different clusters: (a) studies that examined oral microbial alterations in IBD patients; (b) studies that investigated intestinal dysbiosis in patients with periodontitis; and (c) evidence for a shared immunological pattern between the two conditions. Results A total of 15 studies involving 1,171 patients were included. Oral microbiome, either subgingival or salivary, was consistently altered in patients with IBD compared to healthy subjects (a) Additionally, gut dysbiotic microbiota of IBD patients was colonized by pathobionts from oral origin, either via haematogenous or enteric route. Suffering from periodontitis is associated with lower alpha diversity in the gut microbiome (b) Lastly, both IBD and periodontitis are characterized by similar expression patterns of inflammatory cytokines at the gingival and gut levels that are exacerbated when both diseases are present (c). Conclusions Periodontitis and IBD share common dysbiotic and immunological traits. Well‐designed preclinical models and longitudinal cohort studies are required to better explore the causal pathways between the two conditions (PROSPERO CRD42020194379).

in Europe (2.2 million) and North America (1.5 million), with the number of incident cases on the rise (GBD, 2017). The clinical aspect is represented by abdominal pain, diarrhoea, rectal bleeding, anaemia and weight loss including extraintestinal manifestations (Gomollòn et al., 2017). Knowledge about the aetiopathogenesis of IBD is still limited, but it is supposed that its onset originates from genetic and environmental factors, together with impaired intestinal permeability, gut dysbiosis and immunological dysregulation (de Souza & Fiocchi, 2016).
Periodontitis is a biofilm-induced chronic inflammatory disease of tooth supporting structures that leads to progressive loss of attachment and alveolar bone destruction (Tonetti et al., 2018). Periodontitis is the major cause of tooth loss in adults, with an estimated prevalence of 30%-50% of the population (Frencken et al., 2017). At the initial stages, clinical signs and symptoms can be lacking or very mildsuch as gingival inflammation and bleeding. When periodontal tissue destruction proceeds, the disease results in tooth mobility, drifting, flaring and finally loss of the affected tooth (Könönen et al., 2019).
Regarding its pathogenesis, it is currently considered that microbial aetiologic factors induce a series of host responses that mediate inflammatory events in genetically susceptible patients, with environmental risk factors playing a modifying role (Van Dyke et al., 2020).
Periodontitis has been linked with various systemic diseases/conditions and their negative consequences, mainly through bacteraemia and low-grade chronic inflammation (Romandini et al., 2021).
Recent studies have provided significant evidence for the association between inflammatory bowel disease and periodontitis (Papageorgiou et al., 2017). However, the underlying connection mechanisms remain unclear. Hypotheses mainly focus on a common bacterial aetiology and a connection between the immune pathways involved in both diseases. In fact, dysbiosis occurs in periodontitis such as in IBD, and oral microbiome is likely to move to the gut and affect the gastrointestinal microbiome . Moreover, both diseases are characterized by an excessive and non-self-limiting immune response towards the dysbiotic microbiome (Kaser et al., 2010).
Although meta-epidemiological evidence is now available (She et al., 2020), the study of the biological association between the two diseases still presents remarkable heterogeneity in methods and findings (Agossa et al., 2017). To explore such a composite body of knowledge, scoping reviews may be preferred over systematic reviews because of the broadness of their coverage and their ability of mapping the evidence to discuss advancements and limitations (Munn et al., 2018). Therefore, the present scoping review was conducted to investigate the relationship between periodontitis and IBD at both immunological and microbiological levels, and to identify gaps in the available evidence to be implemented by future research.

| Study design and reporting
The present scoping review was structured according to the PRISMA (Preferred Reporting Items for Systematic Reviews and Meta-Analyses) Extension for Scoping Reviews checklist. A detailed protocol has been registered on PROSPERO before the commencement of the work (CRD42020194379).

| Search strategy
The following electronic databases were searched in duplicate by 2 review authors (GB and AM): MEDLINE (via PubMed), Embase, Scopus and Cochrane Library. No restrictions on language or date of publication were employed when searching the electronic databases. General controlled vocabulary (MeSH terms) and keywords were chosen, including "bacteria", "microbiome", "microbiota", "dysbiosis", "inflammatory bowel", "ulcerative colitis", "Crohn's disease", "periodontal disease", "periodontitis" and "cytokines". In addition, two review authors double-checked the bibliographies of all included studies and previous reviews.

| Eligibility criteria and selection of the manuscripts
The primary scope of this review was sorting studies presenting microbiological and immunological analysis in patients with both periodontitis and IBD. Due to the paucity of entries retrieved, studies investigating oral microbial/immunological changes in IBD patients and gut microbial/immunological variations in periodontitis patients were also included. Thus, only original comparative studies of human populations, case-control studies, cohort studies or intervention studies with the concomitant analysis of clinical and immunological or microbiological parameters at oral and gut levels were selected for this review. Case reports, literature reviews, editorials, animal studies and in vitro experiments were excluded, and so were studies dealing with probiotic supplements.
Titles and abstracts of all reports identified through the searches were screened independently by the two review authors.
For studies appearing to fulfil the inclusion criteria, the complete manuscript was obtained. The full texts obtained from various electronic sources and manual searching were thereby analysed independently for eligibility check. The reasons for exclusion of studies were recorded.

| Data extraction and synthesis
A two-step procedure was used to extract data from the included studies independently by two reviewers. Initially, descriptive data were tabulated with respect to population, exposures, controls and outcomes. Second, each selected study was re-analysed and main findings were critically reviewed in the context of the available literature. The studies were then grouped in three different sections with respect to the aim and the initial population recruited. A first pool of articles set the spotlight on the analysis of oral microbiome in patients suffering from IBD. A second group focused on the intestinal microbial alteration that can be encountered among periodontal patients. Due to paucity of evidence, we included the studies in which gut microbiome in IBD patients was compared to oral flora regardless the presence of periodontitis. A third cluster was aimed at gathering the evidence about common immunopathological features between the two diseases.

| RE SULTS
The literature search resulted in 688 manuscripts, and after the removal of duplicates and those that did not meet the inclusion criteria, 28 were considered for full-text review. After full-text reading, 13 articles were excluded (Table S1) and Figure S1 presents a PRISMA flow diagram outlining the study selection process. The search method finally yielded 15 articles relevant to the research questions. Analysis of the geographic distribution revealed that 5 studies were carried out in Brazil, 4 in the United States and 2 in Germany, while the others originated from Sweden, China and Japan.

| Oral microbial alterations in IBD patients
Studies investigating the impact of IBD on oral or periodontal microbiota are summarized in Table 1. Nine studies were selected, with a sample size ranging from 40 to 147. Xun et al., (2018)  Saliva was also sampled by Said et al., (2014), in a cohort of 21 CD, 14 UC patients, and 24 healthy controls (HCs). Phylum Bacteroidetes was significantly higher in both of the CD and UC groups compared with the HC; the same trend was observed for genera Prevotella and Veillonella, while Proteobacteria and Streptococcus and Haemophilus followed the opposite direction. Two other genera, Neisseria and Gemella, were also found to be significantly lower in the CD group than the HC. The same approach was adopted in an early study by Meurman et al., (1994), which employed bacterial cultures to identify salivary microbiological features in CD patients with active or inactive disease. It was found that salivary yeast, Lactobacilli and Streptococcus mutans counts were higher in patients with active than those with inactive disease (Menegat et al., 2016).
Another subset of articles examined microbial alterations in subgingival plaque. In the study of Schmidt et al., (2018) The cross-sectional investigation by Kelsen et al., (2015) characterized the subgingival microbiota in paediatric patients with active or non-active CD. 16S rRNA sequencing identified 17 genera as candidate biomarkers: Capnocytophaga, Rothia and TM7 were more abundant in CD patients than the healthy controls. It was also found that both antibiotic exposure and disease state were associated with differences in bacterial community composition.
Likewise, Docktor et al., (2012) studied the oral microbiome of children and young adults with IBD in comparison with HC, by sampling tongue and buccal mucosal brushings. Next-generation sequencing highlighted that tongue samples in CD showed a significant loss of probe activity from the two phyla, Fusobacteria and Firmicutes. For UC patients, similar significant loss of probe signal was noted in Fusobacteria, whereas those of Spirochaetes, Synergistetes and Bacteroidetes were all increased as compared with the controls.
Lastly, Van Dyke et al., (1986) observed differences in oral microbiome in IBD patients compared to both periodontitis patients and HC. Using cultural method and dark-field microscopy, the authors reported predominance of a Gram-negative rod consistent with genus Wolinella from IBD patients. Interestingly, in the periodontal disease-free IBD patients the bacterial load in the gingival sulcus was significantly less than that of the IBD group with periodontal disease. Table 2 summarizes the evidence about intestinal microbial alterations in relation to oral-derived bacteria, while Figure 1 depicts the prevalent bacterial clusters encountered in oral and gut environments in IBD and periodontitis. Only one study comparing the gut microbiome of individuals with different periodontal conditions was found. Lourenςo et al., (2018) analysed the gut microbiome in stool samples of 7 individuals with clinically healthy periodontium, 14 presenting gingivitis and 23 presenting chronic periodontitis. They found a tendency to lower alpha diversity in the gut microbiome of patients with chronic periodontitis when compared to HCs or gingivitis.

Firmicutes, Proteobacteria, Verrucomicrobia and Euryarchaeota
were increased, whereas Bacteroidetes were decreased in patients with periodontitis compared to periodontally healthy individuals with non-statistically significant differences. 16S rRNA gene sequencing Enrichment of Capnocytophaga, Rothia, and TM7 in CD at baseline. 8 weeks of therapy were sufficient to return these taxa to healthy levels Alloprevotella, Fusobacterium, Porphyromonas and Prevotella decreased in the antibiotic-exposed CD group, compared to CD subjects not using antibiotics, at baseline and after 8 weeks of therapy

| Evidence for a shared immunological pattern
Five of the included studies evaluated possible common immunological features between IBD and periodontitis as outlined in Table 3.

| D ISCUSS I ON
Recent systematic reviews have shown that patients with IBD are at a significantly higher risk of having periodontitis compared to non-IBD patients, although the linking mechanisms need to be further explored (Papageorgiou et al., 2017;She et al., 2020). This paper aimed to evaluate the current level of evidence in human studies about biological mechanisms underlying the epidemiological association between the two diseases. Due to the high heterogeneity in objectives and methods across the selected studies, the findings are presented under the form of a scoping review. It was attempted to provide an overview of the state of art using a systematic search method, differing from systematic reviews in broadness of the coverage.

| Oral microbial alterations in IBD patients
A distinct salivary and subgingival ecotype is detectable in patients with IBD patients compared to HCs. In the presence of UC and CD, physiologic oral microbiota is altered with characteristic features of dysbiotic states: bloom of pathobionts, depletion of commensals and overall loss of diversity (Korem et al., 2015). Remarkable changes in numerous oral microbial residents were found consistent with changes in the intestine of patients with IBD. Recent evidence has revealed a strong concordance between oral and gut microbiota in association with diseases such as liver cirrhosis and rheumatoid

F I G U R E 1 Prevalent oral and gut microbials in inflammatory bowel disease (IBD) and PD
arthritis (Qin et al., 2014;Zhang et al., 2015). Furthermore, patients suffering from gut diseases display an aberrant enrichment of characteristic oral bacteria in both lumen and gut mucosa. Therefore, it can be hypothesized that oral cavity acts as a reservoir of pathobionts whose oral-gut translocation plays a role in the pathogenesis of IBD. On the other hand, periodontitis is likely to exacerbate gut inflammation and mediate inflammatory conditions at the systemic level (Kitamoto, Nagao-Kitamoto, Hein, et al., 2020). Indeed, periodontitis and its chronic low-grade inflammatory burden have been associated with several systemic conditions ultimately increasing the risk of mortality (Romandini et al., 2018(Romandini et al., , 2021. Subgingival plaque from IBD patients revealed a higher prevalence of opportunistic bacteria, including P. micra, T. denticola in IBD patients and Campylobacter species (Wolinella in the old taxonomy).
Campylobacter species are among the most frequent causes of bacterial gastroenteritis and were detected with a high prevalence both in intestinal biopsies and faecal samples of IBD patients compared to healthy subjects (Man, 2011).
Despite the evidence of taxonomic and functional dysbiosis, data on specific bacteria were controversial. A possible explanation may be found in the high inter-individual variance when dealing with physiologic or pathologic microbiome, plausibly due to the differences in consumed food, age, and ethnic background (Dethlefsen et al., 2007). Furthermore, different detection techniques and different sequencing platform used in different studies might have generated discrepancies in the outcomes. Significant differences were also encountered between CD and UC patients, and even though CD and UC have important characteristics (such as symptoms, structural damage and therapy) in common, emerging evidence suggest that they represent two distinct pathophysiological entities (de Souza & Fiocchi, 2016).
Another subset of papers accounted for oral microbial alterations in paediatric patients with IBD. Although periodontitis was absent and no study presented a longitudinal follow-up, the findings revealed oral dysbiotic alterations in children consistent with the ones found in adults (Kelsen et al., 2015).

| Intestinal dysbiosis in periodontitis patients
Recent data have indicated that resident oral microbiota is able to colonize the gastrointestinal tract through haematogenous and enteral routes (Kitamoto, Nagao-Kitamoto, Hein, et al., 2020). This intestinal translocation of bacteria from oral origin triggers inflammation and has been correlated with several non-communicable diseases, including IBD, irritable bowel syndrome and colorectal cancer (Forbes et al., 2016). P. gingivalis is an important oral pathobiont, capable of cell invasion and resistant to gastric acidity (Hajishengallis et al., 2020). Oral administration of P. gingivalis to mice is reported to disrupt the gut epithelial integrity and to significantly alter microbial composition, both via outgrowing in the gut and systemic endotoxemia (Nakajima et al., 2015). Furthermore, Klebsiella spp., mainly isolated from the oral microbiota, are known to be strong inducers of T helper 1 (TH1) when they translocate to the gut (Atarashi et al., 2017).

Notably, genera as Fusobacterium, Prevotella, Streptococcus and
Porphyromonas not only dominate the gut microbiome of patients with IBD but are also significantly more prevalent in diseased compared to adjacent healthy colon specimens (Dinakaran et al., 2019).
Plausibly, these oral bacteria can act as keystone pathobionts when they colonize the gut native microbiota, with the effect of promoting a reduction in alpha diversity, which is considered as a reliable indicator of disease-associated dysbiosis (Duvallet et al., 2017). In relation to beta diversity, it was not possible to clearly distinguish individuals with distinct clinical status based on the composition of the gut microbiota, as it resulted from the study of Lourenςo et al., (2018).
However, this latter was a pilot investigation and presented a limited sample size. Not specific pathogens but rather a synergy of bacteria is implicated in IBD pathogenesis, and a unique dysbiotic microbiome cannot explain the aetiology of this disease (Duvallet et al., 2017). Notably, many consortia of bacteria are present in both healthy and diseased state. Also, gut mucosal microbial colonization may be significantly different from the one found in the lumen, with the former being more likely to interact and trigger the immune system (Li et al., 2015). Sampling from different parts of the intestine may provide more accurate reflection of the whole microbiome.

Tissue sampled Laboratory analysis Key findings
Gingival tissue (21) and intestinal tissue (21) Multiplex assay IL−4, IL−10 and IL−21 expression significantly increased in gingival tissue of patients with active IBD Inflammation score (mean value of IL−1 β, IL−6, IL−21 and sCD40L) significantly higher in gingival tissue of patients with IBD activity Significant correlation between gingival and intestinal inflammation scores Significantly higher IL−23 and IFNγ levels and lower IL−31 and TNFα levels in gingival tissues rather than in gut The relationship between intestinal dysbiosis and periodontitis may have a bidirectional character. The richness in the variety of gut bacteria can potentially affect the immune fitness of the entire body, by epigenetic remodelling and altered gene expression (Levy et al., 2017). The recent pandemic emergence of some multifactorial inflammatory, autoimmune, neurodegenerative and metabolic diseases has been consistently associated with overabundance of opportunistic pathogens in the gut . Also, periodontitis is now investigated as an extraintestinal manifestation of enteric dysbiosis (She et al., 2020). Most studies emphasized the role of Th17 cytokines, such as IL-17A, IL-21, IL-22 and IL-23 that sustain chronic periodontal lesions and are implicated in the aetiopathogenesis of IBD (Fujino et al., 2003;Gaffen et al., 2008). The similarity between IBD and periodontitis in terms of cytokine expression pattern may indicate an integrated mucosal immunological system and support the hypothesis that the two diseases share some common pathogenic mechanisms (Gill et al., 2010;.

| Evidence for a shared immunological pattern
Accordingly, a recent preclinical study demonstrated generation of oral pathobiont-reactive Th17 cells in the oral cavity by periodontitis and these cells may migrate to the gut via lymphatic routes. Once in the intestine, these cells can be activated by translocated oral pathobionts and trigger colitis .

| Limitations and future directions
The present work presents some inherent limitations, mainly as-  (Levy et al., 2017). immunological markers of disease activity should be implemented to make significant advances in this field.
In conclusion, findings of the present review indicate that oral microbiome of patients with IBD is characterized by a lower diversity and higher dysbiotic traits compared to healthy subjects. Moreover, the data show that gut lumen and mucosal tissues of patients with IBD harbour periodontal pathogens that may have translocated via haematogenous and/or enteral route. There are animal studies showing that oral-gut translocation of pathobionts may cause intestinal dysbiosis but the clinical evidence is limited to observational studies. At present, a relationship in terms of an association seems to be likely and further epidemiologic cohort studies are warranted to determine whether a causal relationship exists between the two diseases. Considering the available immunological data, MMP-8, IL-17A, and INFγ seem to be promising biomarkers for an interaction between IBD and periodontitis and possible role of Th17 cells deserves further research.

CO N FLI C T O F I NTE R E S T
The authors declare that they have no competing interests.

PE E R R E V I E W
The peer review history for this article is available at https://publo ns.com/publo n/10.1111/odi.13843.

DATA AVA I L A B I L I T Y S TAT E M E N T
All data generated or analysed during this study are included in this published article (and its supplementary information files).