Peptidoglycan synthesis in Tannerella forsythia: Scavenging is the modus operandi

Sialic acid metabolism of oral bacteria and its potential role in colorectal cancer and Alzheimer's disease

Sialic acid metabolism in oral bacteria is a complex process involving nutrient acquisition, immune evasion, cell surface modification, and the production of metabolites that contribute to bacterial persistence and virulence in the oral cavity. In addition to causing various periodontal diseases, certain oral pathogenic bacteria, such as Porphyromonas gingivalis, Tannerella forsythia, and Fusobacterium nucleatum, can induce inflammatory reactions and influence the immunity of host cells. These associations with host cells are linked to various diseases, particularly colorectal cancer and Alzheimer's disease. Sialic acid can be found in the host oral mucosa, saliva, or food residues in the oral cavity, and it may promote the colonization of oral bacteria and contribute to disease development. This review aims to summarize the role of sialic acid metabolism in oral bacteria and discuss its effect on the pathogenesis of colorectal cancer and Alzheimer's disease.

Tannerella forsythia scavenges Fusobacterium nucleatum secreted NOD2 stimulatory molecules to dampen oral epithelial cell inflammatory response

The oral organism Tannerella forsythia is auxotrophic for peptidoglycan amino sugar N-acetylmuramic acid (MurNAc). It survives in the oral cavity by scavenging MurNAc- and MurNAc-linked peptidoglycan fragments (muropeptides) secreted by co-habiting bacteria such as Fusobacterium nucleatum with which it forms synergistic biofilms. Muropeptides, MurNAc-l-Ala-d-isoGln (MDP, muramyl dipeptide) and d-γ-glutamyl-meso-DAP (iE-DAP dipeptide), are strong immunostimulatory molecules that activate nucleotide oligomerization domain (NOD)-like innate immune receptors and induce the expression of inflammatory cytokines and antimicrobial peptides. In this study, we utilized an in vitro T. forsythia-F. nucleatum co-culture model to determine if T. forsythia can selectively scavenge NOD ligands from the environment and impact NOD-mediated inflammation. The results showed that NOD-stimulatory molecules were secreted by F. nucleatum in the spent culture broth, which subsequently induced cytokine and antimicrobial peptide expression in oral epithelial cells. In the spent broth from T. forsythia-F. nucleatum co-cultures, the NOD-stimulatory activity was significantly reduced. These data indicated that F. nucleatum releases NOD2-stimulatory muropeptides in the environment, and T. forsythia can effectively scavenge the muropeptides released by co-habiting bacteria to dampen NOD-mediated host responses. This proof-of-principle study demonstrated that peptidoglycan scavenging by T. forsythia can impact the innate immunity of oral epithelium by dampening NOD activation.

N-acetylmuramic acid recognition by MurK kinase from the MurNAc auxotrophic oral pathogen Tannerella forsythia

The bacterial cell wall consists of a three-dimensional peptidoglycan layer, composed of peptides linked to the sugars N-acetylmuramic acid (MurNAc) and GlcNAc. Unlike other bacteria, the pathogenic Tannerella forsythia, a member of the red complex group of bacteria associated with the late stages of periodontitis, lacks biosynthetic pathways for MurNAc production and therefore obtains MurNAc from the environment. Sugar kinases play a crucial role in the MurNAc recycling process, activating the sugar molecules by phosphorylation. In this study, we present the first crystal structures of a MurNAc kinase, called murein sugar kinase (MurK), in its unbound state as well as in complexes with the ATP analog β-γ-methylene adenosine triphosphate (AMP-PCP) and with MurNAc. We also determined the crystal structures of K1058, a paralogous MurNAc kinase of T. forsythia, in its unbound state and in complex with MurNAc. We identified the active site and residues crucial for MurNAc specificity as the less bulky side chains of S133, P134, and L135, which enlarge the binding cavity for the lactyl ether group, unlike the glutamate or histidine residues present in structural homologs. In establishing the apparent kinetic parameters for both enzymes, we showed a comparable affinity for MurNAc (Km 180 μM and 30 μM for MurK and K1058, respectively), with MurK being over two hundred times faster than K1058 (Vmax 80 and 0.34 μmol min-1 mg-1, respectively). These data might support a structure-guided approach to development of inhibitory MurNAc analogs for pathogen MurK enzymes.

Investigating Peptidoglycan Recycling Pathways in Tannerella forsythia with N-Acetylmuramic Acid Bioorthogonal Probes

The human oral microbiome is the second largest microbial community in humans, harboring over 700 bacterial species, which aid in digestion and protect from growth of disease-causing pathogens. One such oral pathogen, Tannerella forsythia, along with other species, contributes to the pathogenesis of periodontitis. T. forsythia is unable to produce its own N-acetylmuramic acid (NAM) sugar, essential for peptidoglycan biosynthesis and therefore must scavenge NAM from other species with which it cohabitates. Here, we explore the recycling potential of T. forsythia for NAM uptake with a bioorthogonal modification into its peptidoglycan, allowing for click-chemistry-based visualization of the cell wall structure. Additionally, we identified NAM recycling enzyme homologues in T. forsythia that are similar to the enzymes found in Pseudomonas putida. These homologues were then genetically transformed into a laboratory safe Escherichia coli strain, resulting in the efficient incorporation of unnatural NAM analogues into the peptidoglycan backbone and its visualization, alone or in the presence of human macrophages. This strain will be useful in further studies to probe NAM recycling and peptidoglycan scavenging pathways of T. forsythia and other cohabiting bacteria.

Metabolomics Research in Periodontal Disease by Mass Spectrometry

Periodontology is a newer field relative to other areas of dentistry. Remarkable progress has been made in recent years in periodontology in terms of both research and clinical applications, with researchers worldwide now focusing on periodontology. With recent advances in mass spectrometry technology, metabolomics research is now widely conducted in various research fields. Metabolomics, which is also termed metabolomic analysis, is a technology that enables the comprehensive analysis of small-molecule metabolites in living organisms. With the development of metabolite analysis, methods using gas chromatography–mass spectrometry, liquid chromatography–mass spectrometry, capillary electrophoresis–mass spectrometry, etc. have progressed, making it possible to analyze a wider range of metabolites and to detect metabolites at lower concentrations. Metabolomics is widely used for research in the food, plant, microbial, and medical fields. This paper provides an introduction to metabolomic analysis and a review of the increasing applications of metabolomic analysis in periodontal disease research using mass spectrometry technology.

NamZ1 and NamZ2 from the oral pathogen Tannerella forsythia are peptidoglycan processing exo-β-N-acetylmuramidases with distinct substrate specificity

The Gram-negative periodontal pathogen Tannerella forsythia is inherently auxotrophic for N-acetylmuramic acid (MurNAc), which is an essential carbohydrate constituent of the peptidoglycan (PGN) of the bacterial cell wall. Thus, to build up its cell wall, T. forsythia strictly depends on the salvage of exogenous MurNAc or sources of MurNAc, such as polymeric or fragmentary PGN, derived from cohabiting bacteria within the oral microbiome. In our effort to elucidate how T. forsythia satisfies its demand for MurNAc, we recognized that the organism possesses three putative orthologs of the exo-β-N-acetylmuramidase BsNamZ from Bacillus subtilis, which cleaves non-reducing end, terminal MurNAc entities from the artificial substrate pNP-MurNAc and the naturally-occurring disaccharide substrate MurNAc-N-acetylglucosamine (GlcNAc). TfNamZ1 and TfNamZ2 were successfully purified as soluble, pure recombinant His₆-fusions and characterized as exo-lytic β-N-acetylmuramidases with distinct substrate specificities. The activity of TfNamZ1 was considerably lower compared to TfNamZ2 and BsNamZ, in the cleavage of MurNAc-GlcNAc. When peptide-free PGN glycans were used as substrates, we revealed striking differences in the specificity and mode of action of these enzymes, as analyzed by mass spectrometry. TfNamZ1, but not TfNamZ2 or BsNamZ, released GlcNAc-MurNAc disaccharides from these glycans. In addition, glucosamine (GlcN)-MurNAc disaccharides were generated when partially N-deacetylated PGN glycans from B. subtilis 168 were applied. This characterizes TfNamZ1 as a unique disaccharide-forming exo-lytic β-N-acetylmuramidase (exo-disaccharidase), and, TfNamZ2 and BsNamZ as sole MurNAc monosaccharide-lytic exo-β-N-acetylmuramidases. IMPORTANCE Two exo-N-acetylmuramidases from T. forsythia belonging to glycosidase family GH171 (www.cazy.org) were shown to differ in their activities, thus revealing a functional diversity within this family: NamZ1 releases disaccharides (GlcNAc-MurNAc/GlcN-MurNAc) from the non-reducing ends of PGN glycans, whereas NamZ2 releases terminal MurNAc monosaccharides. This work provides a better understanding of how T. forsythia may acquire the essential growth factor MurNAc by the salvage of PGN from cohabiting bacteria in the oral microbiome, which may pave avenues for the development of anti-periodontal drugs. On a broad scale, our study indicates that the utilization of PGN as a nutrient source, involving exo-lytic N-acetylmuramidases with different modes of action, appears to be a general feature of bacteria, particularly among the phylum Bacteroidetes.

Peptidoglycan Salvage Enables the Periodontal Pathogen Tannerella forsythia to Survive within the Oral Microbial Community

Tannerella forsythia is an anaerobic, fusiform Gram-negative oral pathogen strongly associated with periodontitis, a multibacterial inflammatory disease that leads to the destruction of the teeth-supporting tissue, ultimately causing tooth loss. To survive in the oral habitat, T. forsythia depends on cohabiting bacteria for the provision of nutrients. For axenic growth under laboratory conditions, it specifically relies on the external supply of N-acetylmuramic acid (MurNAc), which is an essential constituent of the peptidoglycan (PGN) of bacterial cell walls. T. forsythia comprises a typical Gram-negative PGN; however, as evidenced by genome sequence analysis, the organism lacks common enzymes required for the de novo synthesis of precursors of PGN, which rationalizes its MurNAc auxotrophy. Only recently insights were obtained into how T. forsythia gains access to MurNAc in its oral habitat, enabling synthesis of the own PGN cell wall. This report summarizes T. forsythia's strategies to survive in the oral habitat by means of PGN salvage pathways, including recovery of exogenous MurNAc and PGN-derived fragments but also polymeric PGN, which are all derived from cohabiting bacteria either via cell wall turnover or decay of cells. Salvage of polymeric PGN presumably requires the removal of peptides from PGN by an unknown amidase, concomitantly with the translocation of the polymer across the outer membrane. Two recently identified exo-lytic N-acetylmuramidases (Tf_NamZ1 and Tf_NamZ2) specifically cleave the peptide-free, exogenous (nutrition source) PGN in the periplasm and release the MurNAc and disaccharide substrates for the transporters Tf_MurT and Tf_AmpG, respectively, whereas the peptide-containing, endogenous (the self-cell wall) PGN stays unattached. This review also outlines how T. forsythia synthesises the PGN precursors UDP-MurNAc and UDP-N-acetylglucosamine (UDP-GlcNAc), involving homologs of the Pseudomonas sp. recycling enzymes AmgK/MurU and a monofunctional uridylyl transferase (named Tf_GlmU*), respectively.

Resource sharing between central metabolism and cell envelope synthesis

Synthesis of the bacterial cell envelope requires a regulated partitioning of resources from central metabolism. Here, we consider the key metabolic junctions that provide the precursors needed to assemble the cell envelope. Peptidoglycan synthesis requires redirection of a glycolytic intermediate, fructose-6-phosphate, into aminosugar biosynthesis by the highly regulated branchpoint enzyme GlmS. MurA directs the downstream product, UDP-GlcNAc, specifically into peptidoglycan synthesis. Other shared resources required for cell envelope synthesis include the isoprenoid carrier lipid undecaprenyl phosphate and amino acids required for peptidoglycan cross-bridges. Assembly of the envelope requires a sharing of limited resources between competing cellular pathways and may additionally benefit from scavenging of metabolites released from neighboring cells or the formation of symbiotic relationships with a host.

Persistence of Tannerella forsythia and Fusobacterium nucleatum in dental plaque: a strategic alliance

PURPOSE OF REVIEW

The Gram-negative oral pathogen Tannerella forsythia is implicated in the pathogenesis of periodontitis, an inflammatory disease characterized by progressive destruction of the tooth supporting structures affecting over 700 million people worldwide. This review highlights the basis of why and how T. forsythia interacts with Fusobacterium nucleatum, a bacterium considered to be a bridge between the early and late colonizing bacteria of the dental plaque.

RECENT FINDINGS

The recent findings indicate that these two organisms have a strong mutualistic relationship that involves foraging by T. forsythia on F. nucleatum peptidoglycan and utilization of glucose, released by the hydrolytic activity of T. forsythia glucanase, as a nutrient by F. nucleatum. In addition, T. forsythia has the unique ability to generate a toxic and inflammogenic compound, methylglyoxal, from glucose. This compound can induce inflammation, leading to the degradation of periodontal tissues and release of host components as nutrients for bacteria to further exacerbate the disease.

SUMMARY

In summary, this article will present our current understanding of mechanisms underpinning T. forsythia-F. nucleatum mutualism, and how this mutualism might impact periodontal disease progression.

Peptidoglycan-type analysis of the N-acetylmuramic acid auxotrophic oral pathogen Tannerella forsythia and reclassification of the peptidoglycan-type of Porphyromonas gingivalis

BACKGROUND

Tannerella forsythia is a Gram-negative oral pathogen. Together with Porphyromonas gingivalis and Treponema denticola it constitutes the "red complex" of bacteria, which is crucially associated with periodontitis, an inflammatory disease of the tooth supporting tissues that poses a health burden worldwide. Due to the absence of common peptidoglycan biosynthesis genes, the unique bacterial cell wall sugar N-acetylmuramic acid (MurNAc) is an essential growth factor of T. forsythia to build up its peptidoglycan cell wall. Peptidoglycan is typically composed of a glycan backbone of alternating N-acetylglucosamine (GlcNAc) and MurNAc residues that terminates with anhydroMurNAc (anhMurNAc), and short peptides via which the sugar backbones are cross-linked to build up a bag-shaped network.

RESULTS

We investigated T. forsythia's peptidoglycan structure, which is an essential step towards anti-infective strategies against this pathogen. A new sensitive radioassay was developed which verified the presence of MurNAc and anhMurNAc in the cell wall of the bacterium. Upon digest of isolated peptidoglycan with endo-N-acetylmuramidase, exo-N-acetylglucosaminidase and muramyl-L-alanine amidase, respectively, peptidoglycan fragments were obtained. HPLC and mass spectrometry (MS) analyses revealed the presence of GlcNAc-MurNAc-peptides and the cross-linked dimer with retention-times and masses, respectively, equalling those of control digests of Escherichia coli and P. gingivalis peptidoglycan. Data were confirmed by tandem mass spectrometry (MS2) analysis, revealing the GlcNAc-MurNAc-tetra-tetra-MurNAc-GlcNAc dimer to contain the sequence of the amino acids alanine, glutamic acid, diaminopimelic acid (DAP) and alanine, as well as a direct cross-link between DAP on the third and alanine on the fourth position of the two opposite stem peptides. The stereochemistry of DAP was determined by reversed-phase HPLC after dabsylation of hydrolysed peptidoglycan to be of the meso-type.

CONCLUSION

T. forsythia peptidoglycan is of the A1γ-type like that of E. coli. Additionally, the classification of P. gingivalis peptidoglycan as A3γ needs to be revised to A1γ, due to the presence of meso-DAP instead of LL-DAP, as reported previously.