Commensalism of Fusobacterium nucleatum - The dilemma

Abstract

Fusobacterium nucleatum is a Gram-negative, anaerobic bacterium that serves as a periodontal pathogen and plays a key role in linking Gram-positive and Gram-negative bacteria within the periodontal biofilm. It was shown that Fusobacterium produces significant amounts of butyric acid, which is a great source of energy for anti-inflammatory cells. On the other hand, it is associated with the destruction of periodontal structures. This bacterium can enter the blood circulation as a result of periodontal infection. It could cause numerous conditions such as halitosis, dental pulp infection, oral cancer, and systemic diseases. The present review discusses the virulence mechanisms involved in the diseases, with emphasis on its colonization, systemic dissemination, and induction of host inflammatory and tumorigenic responses. This would motivate future research on the role of this bacterium on periodontal pathology as well as its influence on the evolution of systemic diseases.

INTRODUCTION

The opportunistic pathogen Fusobacterium nucleatum, which resides in the human oral cavity and digestive tract, is implicated in a range of infectious disorders in the oropharynx and other regions of the oral cavity.[1] When periodontal diseases were first identified, F. nucleatum was considered a potential etiological agent.[2] Advancements in microbial detection technologies have led to the identification of numerous previously overlooked organisms that play integral roles in various conditions. Recent studies have revealed a link between F. nucleatum and cancers beyond the oral cavity, including those of the breast, stomach, esophagus, and colon.[1] Periodontal infections can allow these bacteria to enter the bloodstream, enabling them to disseminate to other parts of the body from their primary site of colonization in the oral cavity.[3]

STRUCTURE

The name Fusobacterium is derived from the bacterium’s shape; “fusus” is the Latin word for spindle, and “bacterion” means a small rod, together implying “spindle-shaped rod.” The second part of the name, nucleatum, refers to the bacterium’s appearance under light and electron microscopes, where it takes on a spherical shape resembling a nucleus.[3]

The organism is a highly diversified species, classified into five subgroups based on complex phenotypic traits and DNA–DNA hybridization patterns: nucleatum, vincentii, animalis, polymorphum, and fusiforme.[4] Each strain of F. nucleatum possesses a unique rRNA gene sequence.[3]

CULTURE CHARACTERISTICS

F. nucleatum requires nutrient-rich media for optimal growth, such as trypticase, peptone, or yeast extract. It primarily utilizes amino acids and peptides as energy sources, with a particular preference for glutamate, histidine, and aspartate. While all strains metabolize amino acids, some also depend on peptides, and the bacterium shows a marked preference for peptides over free amino acids. F. nucleatum ferments glutamate and lysine, producing metabolic byproducts such as acetate and butyrate, which are involved in its energy metabolism.

The organism exhibits limited use of glucose for energy, instead relying on glucose for biosynthetic processes and the storage of intracellular polymers. Under conditions of amino acid deprivation, F. nucleatum degrades these stored polymers to support survival. Its metabolic byproducts, including butyrate, propionate, and ammonium ions, are known to inhibit gingival fibroblast proliferation, potentially contributing to the pathogenesis of periodontal disease. In addition, F. nucleatum is involved in the production of volatile sulfur compounds, such as hydrogen sulfide and methyl mercaptan, which are major contributors to halitosis.[3]

Fusobacterium species are Gram-negative, slender, and pointed rods, typically occurring in pairs aligned end-to-end. On blood agar, they form small, pinpoint colonies, with some species, such as F. nucleatum, developing distinctive “fried egg” colonies after 3–5 days of incubation. Depending on the strain, they may exhibit hemolytic activity. Fusobacterium shows variability in Gram staining and cellular morphology, ranging from coccoid or pleomorphic forms to rods with either rounded or pointed ends, often arranged end-to-end.

BENEFICIAL ROLE

F. nucleatum produces significant amounts of butyric acid, which serves as a valuable energy source for anti-inflammatory cells. This metabolite may also promote the transcription of protease inhibitors, which neutrophils release in response to bacterial presence, thereby helping to maintain healthy mucosal tissue. In addition, several oral bacteria, including Porphyromonas gingivalis, Treponema denticola, and Tannerella forsythia, produce serine or cysteine proteases, which are considered as virulence factors. The harmful effects of these proteases are mitigated by the protease inhibitors produced by F. nucleatum.

Furthermore, F. nucleatum stimulates mucosal epithelial cells for the production of cationic peptides, specifically human β-defensin-2 and, to a lesser extent, human β-defensin-3. These defensins can effectively target and eliminate both Gram-positive and Gram-negative bacteria, as well as certain viruses. Their cationic charge allows them to rapidly interact with teichoic acids and lipopolysaccharides on bacterial cell membranes. Through this interaction, defensins help regulate pathogen levels and protect the host from infections. Thus, it can be concluded that F. nucleatum plays a crucial role in activating the immune system.[3]

PATHOGENICITY

F. nucleatum is implicated in the etiology of periodontal disease and plays a key role in the formation of dental biofilm. Acting as a bridge between symbiotic and pathogenic microorganisms presents on the teeth and epithelial surfaces, F. nucleatum is a critical species in facilitating direct interactions between both Gram-negative and Gram-positive bacteria.[5]

Adhesins on the surface of F. nucleatum enable the bacterium to bind to surrounding cells and other bacteria, thereby enhancing its pathogenicity. The primary virulence factor in F. nucleatum is the Fusobacterium adhesion A (FadA) protein.[6] Pre-FadA and mature Fusobacterium adhesion A combine to form an active FadA complex, a functional compound that facilitates the attachment to and invasion of host cells.[7] In oral Fusobacterium, FadA is highly conserved, in contrast to Fusobacterium species found in extraoral sites, where FadA exhibits greater variability.[8] As a result, FadA could serve as a potential diagnostic marker for identifying oral Fusobacterium species.

The following biological traits are closely associated with the pathogenicity of F. nucleatum: First, various surface adhesins, including Aid1, FomA, and RadD which facilitate the coaggregation of microorganisms, promoting biofilm formation.[9] In addition, F. nucleatum has the ability to infect a variety of host cells, including endothelial cells, fibroblasts, and epithelial cells.[10] Third, F. nucleatum produces a range of metabolites, including butyrate, hydrogen sulfide, and endotoxins, which are released upon the death of host cells.[11] Finally, like most Gram-negative organisms, F. nucleatum can release extracellular vesicles containing various bioactive compounds, which play a key role in bacterial communication as well as in interactions between bacteria and host cells.[12] As a result, F. nucleatum is strongly linked to the development of several conditions, including halitosis, pulp infections, periodontal diseases, and cancers outside the oral cavity.

SIGNIFICANCE OF FUSOBACTERIUM NUCLEATUM IN PERIODONTAL DISEASE

In periodontal disease, F. nucleatum is known for its destructive role. In an animal model of experimental periodontitis, this bacterium alone is capable of causing an abscess or the loss of alveolar bone.[13] T. forsythia, P. gingivalis, and F. nucleatum can cause concurrent infections that activate the body’s immune response, potentially leading to the loss of alveolar bone.[14] “Epithelial–mesenchymal transition” (EMT) of gingival cells, the immunological environment at the site of infection, and the virulence factors of the organism are all linked to the periodontal pathogenicity of F. nucleatum.

Virulence factors

Proteins present in the outer membrane of F. nucleatum, including FomA, RadD, Fap2, Aid1, CmpA, lipopolysaccharides (LPS), FadA, as well as metabolites like butyric acid and serine proteases, function as virulence factors that contribute to the development of periodontitis. These outer membrane proteins and sensory molecules enable F. nucleatum to coaggregate with both early and late colonizers during dental plaque formation, thereby promoting the progression of periodontal disease.[15] This aggregation acts as a crucial scaffold for microbial growth and development, facilitating the formation and maturation of the dental biofilm.[16]

Previous studies have established a link between the periodontal pathogenicity of F. nucleatum and its virulence factor FadA, which functions both as an adhesin and as an invasive protein.[17] FadA binds to epithelial cadherin, facilitating the invasion of host cells and simultaneously disrupting cell adhesion and intercellular communication. This alteration enables the infiltration of other pathogens into gingival tissue. Once within the epithelium, F. nucleatum interacts with the retinoic acid-inducible gene I receptor through FadA, activating the nuclear factor kappa-B (NF-κB) pathway. This activation triggers an inflammatory response, which can result in tissue damage.[18]

F. nucleatum produces amyloid FadA during disease or under stress conditions, but not in healthy states. This amyloid plays a key role in biofilm formation, enhances acid tolerance, and promotes bacterial attachment to host tissues. In addition, amyloid FadA has been shown to induce bone loss in the periodontal tissues of rats, while amyloid-binding agents have been found to reduce its detrimental effects.[19]

LPS, released during the death of F. nucleatum, are recognized by toll-like receptors on gingival epithelial cells and fibroblasts. The release of internal danger signals activates the NLRP3 inflammasome, which triggers the production of cytokines such as interleukin-1 β (IL-1 β). This cytokine cascade amplifies inflammation in periodontal tissues and contributes to bone resorption.[20] In addition, the 65 kDa serine protease released by F. nucleatum can degrade host tissues, providing essential nutrients for bacterial survival. This protease also breaks down extracellular matrix proteins, leading to the degradation of host immune components and connective tissue. Specifically, it cleaves the IgA chain, aiding the bacteria in evading the host’s immune defenses.[21]

Another metabolite of F. nucleatum, butyric acid, can influence both the degeneration and healing of periodontal tissues. Elevated levels of butyric acid promote the production of reactive oxygen species (ROS) in bone-forming cells, which subsequently stimulates the release of 8-isoprostaglandin and MMP-2. These substances contribute to bone damage and impair the healing process.[22]

Epithelial–mesenchymal transition

F. nucleatum, along with other Gram-negative periodontal pathogens, can induce EMT in gingival epithelial cells. This process promotes the expression of Snail-1, suppresses E-cadherin expression, and disrupts the adhesion between epithelial cells. As the gingival epithelium loses its structural integrity, pathogens are more likely to invade deeper tissues, facilitating further tissue damage and disease progression.[23]

Immune microenvironment

F. nucleatum can create a local immunological environment conducive to the development of periodontal disease. Kurgan et al.[15] found that F. nucleatum subspecies nucleatum and polymorphum can inhibit neutrophils from producing superoxide, thereby preventing their oxidative death. The bacterium can induce both necrosis and apoptosis in neutrophils, effectively reducing their numbers at the infection site. By diminishing the protective role of neutrophils during the early stages of periodontal disease, F. nucleatum may promote the accumulation of late colonizers, such as P. gingivalis, at the lesion site, thus accelerating the progression of periodontal conditions.

By increasing the production of IL-1 β, IL-6, tumor necrosis factor-α (TNF-α), and HMGB1 and promoting macrophage infiltration in BALB/c mice, F. nucleatum can rapidly induce gingival inflammation.[24] The infection also contributes to the recruitment of osteoclasts. Meanwhile, the growth of osteoclasts may be further stimulated by IL-1 β and TNF-α.[25] Consequently, the heightened production of pro-inflammatory cytokines induced by F. nucleatum infections is associated with the activation of osteoclasts and subsequent bone destruction.

In addition, F. nucleatum can hinder tissue healing by activating the protein kinase B (Akt) and NF-κB signaling pathways. This leads to increased fibroblast apoptosis, enhanced ROS production, and the secretion of pro-inflammatory cytokines, all of which contribute to impaired tissue repair and inflammation.[26]

Other oral conditions

Recent research has demonstrated a strong correlation between halitosis and the oral bacteria associated with periodontal disease, with F. nucleatum being more prevalent in the tongue coating of patients with halitosis. Found primarily in the posterior region of the tongue, F. nucleatum is a significant contributor to the increased production of volatile sulfur compounds, which are the primary intraoral cause of halitosis.[27] In addition, endodontic conditions such as pulp necrosis and periapical periodontitis are frequently associated with F. nucleatum.[28]

Systemic involvement

Hematogenous transmission has been proposed as the mechanism by which F. nucleatum is transmitted from the mother’s oral cavity to the intrauterine environment. When introduced through hematogenous injection, F. nucleatum specifically colonizes the fetoplacental unit without infecting the entire body. The bacterium initially establishes colonies in the decidua by penetrating the endothelium, then spreads to the amniotic fluid, fetal membranes, and fetus, simulating chorioamnionitis, which can ultimately lead to preterm or term fetal death.

In addition, individuals with colorectal cancer have been shown to have higher concentrations of F. nucleatum in their carcinomas and rectal swabs. The levels of F. nucleatum are also elevated in adenomas, as well as in the feces of individuals with colorectal carcinoma and adenoma and are associated with the stages of colorectal neoplasia development. Finally, F. nucleatum is implicated in a wide range of infections and abscesses, including those affecting the head and neck, bloodstream, lungs, abdomen, pelvis, bones, and joints.[1]

CONCLUSION

F. nucleatum has proven to be a challenging anaerobe to cultivate, making its detection difficult in clinical settings. Therefore, advancing microbial detection technologies is crucial for identifying individuals at risk and ensuring accurate diagnoses. Moreover, given the high prevalence of F. nucleatum in periodontal disease, it is essential to explore the relationship between periodontal health and various systemic medical conditions. With continued research, we can deepen our understanding of F. nucleatum, laying the foundation for the development of preventive and therapeutic treatments for related diseases.

Conflicts of interest

There are no conflicts of interest.

Funding Statement

Nil.