Identification of a Novel N-Acetylmuramic Acid Transporter in Tannerella forsythia

Bioorthogonal Labeling and Click-Chemistry-Based Visualization of the Tannerella forsythia Cell Wall

The objective of this chapter is to provide a detailed protocol for the peptidoglycan (cell wall) labeling of the periodontal pathogen Tannerella forsythia and the development of a laboratory-safe Escherichia coli strain utilizing the N-acetylmuramic acid recycling enzymes AmgK, N-acetylmuramate/N-acetylglucosamine kinase, and MurU, N-acetylmuramate alpha-1-phosphate uridylyltransferase, from T. forsythia. The procedure involves bioorthogonal labeling of bacterial cells with an azido-modified analog of the amino sugar, N-acetylmuramic acid, through "click chemistry" with a fluorescent dye. The protocol is suitable for the generation of fluorescently labeled peptidoglycan molecules for applications in the study of bacterial and peptidoglycan trafficking in the host cells and cell wall recycling in complex microbiomes.

Impact of Environmental Sub-Inhibitory Concentrations of Antibiotics, Heavy Metals, and Biocides on the Emergence of Tolerance and Effects on the Mutant Selection Window in E. coli

Bacteria's ability to withstand the detrimental effects of antimicrobials could occur as resistance or tolerance with the minimum inhibitory concentration, the mutant prevention concentration, and the mutant selection window as salient concepts. Thus, this study assessed the impact of exposure to extremely high doses of ampicillin on the level of persistence and tolerance development in isolates previously exposed to different concentrations of selected antibiotics, biocides, and heavy metals. These isolates were previously exposed to oxytetracycline (OXYTET), amoxicillin (AMX), copper (Cu), zinc (Zn), benzalkonium chloride (BAC) 10, dimethylammonium chloride (DADMAC) 12 and a combination of all the individual pollutants (ALL). The isolates were exposed to very high concentrations (25 × MIC) of ampicillin, and their tolerance was calculated as the time required to kill 99.9% of the bacterial population (MDK99.9). The MDK99.9 increased by 30 to 50% in test isolates (DADMAC, OXYTET, Zinc = 28 h; BAC, Copper = 30 h; amoxycillin, ALL = 26 h) compared to the untreated control. BAC-exposed isolates decreased from 2.5 × 108 CFU/mL to 2.5 × 104 CFU/mL on the second day, displaying the highest tolerance increase. The tolerance appeared to originate from two sources, i.e., stochastic persistence and genetic-induced persistence, involving multiple genes with diverse mechanisms. The mutant selection window of the isolates to ampicillin, amoxicillin, and oxytetracycline also slightly increased compared to the control, indicating the selective survival of persister cells during the 30-day exposure. These findings indicate that bacterial exposure to sub-inhibitory concentrations of environmental chemical stressors may not always result in the development of antimicrobial resistance but could initiate this process by selecting persisters that could evolve into resistant isolates.

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.

Functional characterization of the phosphotransferase system in Parageobacillus thermoglucosidasius

Parageobacillus thermoglucosidasius is a thermophilic bacterium characterized by rapid growth, low nutrient requirements, and amenability to genetic manipulation. These characteristics along with its ability to ferment a broad range of carbohydrates make P. thermoglucosidasius a potential workhorse in whole-cell biocatalysis. The phosphoenolpyruvate:carbohydrate phosphotransferase system (PTS) catalyzes the transport and phosphorylation of carbohydrates and sugar derivatives in bacteria, making it important for their physiological characterization. In this study, the role of PTS elements on the catabolism of PTS and non-PTS substrates was investigated for P. thermoglucosidasius DSM 2542. Knockout of the common enzyme I, part of all PTSs, showed that arbutin, cellobiose, fructose, glucose, glycerol, mannitol, mannose, N-acetylglucosamine, N-acetylmuramic acid, sorbitol, salicin, sucrose, and trehalose were PTS-dependent on translocation and coupled to phosphorylation. The role of each putative PTS was investigated and six PTS-deletion variants could not grow on arbutin, mannitol, N-acetylglucosamine, sorbitol, and trehalose as the main carbon source, or showed diminished growth on N-acetylmuramic acid. We concluded that PTS is a pivotal factor in the sugar metabolism of P. thermoglucosidasius and established six PTS variants important for the translocation of specific carbohydrates. This study lays the groundwork for engineering efforts with P. thermoglucosidasius towards efficient utilization of diverse carbon substrates for whole-cell biocatalysis.

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.

Filling gaps in bacterial catabolic pathways with computation and high-throughput genetics

To discover novel catabolic enzymes and transporters, we combined high-throughput genetic data from 29 bacteria with an automated tool to find gaps in their catabolic pathways. GapMind for carbon sources automatically annotates the uptake and catabolism of 62 compounds in bacterial and archaeal genomes. For the compounds that are utilized by the 29 bacteria, we systematically examined the gaps in GapMind's predicted pathways, and we used the mutant fitness data to find additional genes that were involved in their utilization. We identified novel pathways or enzymes for the utilization of glucosamine, citrulline, myo-inositol, lactose, and phenylacetate, and we annotated 299 diverged enzymes and transporters. We also curated 125 proteins from published reports. For the 29 bacteria with genetic data, GapMind finds high-confidence paths for 85% of utilized carbon sources. In diverse bacteria and archaea, 38% of utilized carbon sources have high-confidence paths, which was improved from 27% by incorporating the fitness-based annotations and our curation. GapMind for carbon sources is available as a web server (http://papers.genomics.lbl.gov/carbon) and takes just 30 seconds for the typical genome.

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.

Subgingival pathogens in chronic periodontitis patients affected by type 2 diabetes mellitus: a retrospective case-control study

Purpose

The aim of this study was to compare the prevalence and bacterial load of 6 main periodontal pathogens between pairs of periodontal patients with and without type 2 diabetes mellitus. Porphyromonas gingivalis and Aggregatibacter actinomycetemcomitans genotypes were also investigated.

Methods

Twenty patients affected by chronic periodontitis and type 2 diabetes were retrospectively selected and matched to 20 patients without diabetes on the basis of the degree and severity of periodontal disease. Microbiological data of subgingival biofilms were analysed and compared for the examined pathogens: A. actinomycetemcomitans, P. gingivalis, Prevotella intermedia, Treponema denticola, Fusobacterium nucleatum, and Tannerella forsythia.

Results

The pairs were balanced in terms of demographic and clinical parameters, except for bleeding on probing and suppuration. In the microbiological test sites (4 for each patient), the mean probing pocket depth was 6.34±1.63 mm in patients with diabetes and 6.41±1.78 mm in patients without diabetes. No significant difference between pairs in the prevalence of P. gingivalis or the distribution of its genotypes was recorded. Patients with diabetes had a significantly greater amount of total bacterial load, P. gingivalis, T. denticola, T. forsythia, and F. nucleatum (P<0.05). Moreover, patients with diabetes had a higher number of sites with a greater cell count than patients without diabetes. When compared to the total bacterial load, only T. forsythia maintained its relative load in patients with diabetes (P=0.001).

Conclusions

This retrospective matched study supports the hypothesis that microbiological differences exist among periodontal patients with and without diabetes mellitus.

Trial Registration

ClinicalTrials.gov Identifier: NCT03786133

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.

Utilization of different MurNAc sources by the oral pathogen Tannerella forsythia and role of the inner membrane transporter AmpG

BACKGROUND

The Gram-negative oral pathogen Tannerella forsythia strictly depends on the external supply of the essential bacterial cell wall sugar N-acetylmuramic acid (MurNAc) for survival because of the lack of the common MurNAc biosynthesis enzymes MurA/MurB. The bacterium thrives in a polymicrobial biofilm consortium and, thus, it is plausible that it procures MurNAc from MurNAc-containing peptidoglycan (PGN) fragments (muropeptides) released from cohabiting bacteria during natural PGN turnover or cell death. There is indirect evidence that in T. forsythia, an AmpG-like permease (Tanf_08365) is involved in cytoplasmic muropeptide uptake. In E. coli, AmpG is specific for the import of N-acetylglucosamine (GlcNAc)-anhydroMurNAc(-peptides) which are common PGN turnover products, with the disaccharide portion as a minimal requirement. Currently, it is unclear which natural, complex MurNAc sources T. forsythia can utilize and which role AmpG plays therein.

RESULTS

We performed a screen of various putative MurNAc sources for T. forsythia mimicking the situation in the natural habitat and compared bacterial growth and cell morphology of the wild-type and a mutant lacking AmpG (T. forsythia ΔampG). We showed that supernatants of the oral biofilm bacteria Porphyromonas gingivalis and Fusobacterium nucleatum, and of E. coli ΔampG, as well as isolated PGN and defined PGN fragments obtained after enzymatic digestion, namely GlcNAc-anhydroMurNAc(-peptides) and GlcNAc-MurNAc(-peptides), could sustain growth of T. forsythia wild-type, while T. forsythia ΔampG suffered from growth inhibition. In supernatants of T. forsythia ΔampG, the presence of GlcNAc-anhMurNAc and, unexpectedly, also GlcNAc-MurNAc was revealed by tandem mass spectrometry analysis, indicating that both disaccharides are substrates of AmpG. The importance of AmpG in the utilization of PGN fragments as MurNAc source was substantiated by a significant ampG upregulation in T. forsythia cells cultivated with PGN, as determined by quantitative real-time PCR. Further, our results indicate that PGN-degrading amidase, lytic transglycosylase and muramidase activities in a T. forsythia cell extract are involved in PGN scavenging.

CONCLUSION

T. forsythia metabolizes intact PGN as well as muropeptides released from various bacteria and the bacterium's inner membrane transporter AmpG is essential for growth on these MurNAc sources, and, contrary to the situation in E. coli, imports both, GlcNAc-anhMurNAc and GlcNAc-MurNAc fragments.

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.

Bacteria's different ways to recycle their own cell wall

The ability to recover components of their own cell wall is a common feature of bacteria. This was initially recognized in the Gram-negative bacterium Escherichia coli, which recycles about half of the peptidoglycan of its cell wall during one cell doubling. Moreover, E. coli was shown to grow on peptidoglycan components provided as nutrients. A distinguished recycling enzyme of E. coli required for both, recovery of the cell wall sugar N-acetylmuramic acid (MurNAc) of the own cell wall and for growth on external MurNAc, is the MurNAc 6-phosphate (MurNAc 6P) lactyl ether hydrolase MurQ. We revealed however, that most Gram-negative bacteria lack a murQ ortholog and instead harbor a pathway, absent in E. coli, that channels MurNAc directly to peptidoglycan biosynthesis. This "anabolic recycling pathway" bypasses the initial steps of peptidoglycan de novo synthesis, including the target of the antibiotic fosfomycin, thus providing intrinsic resistance to the antibiotic. The Gram-negative oral pathogen Tannerella forsythia is auxotrophic for MurNAc and apparently depends on the anabolic recycling pathway to synthesize its own cell wall by scavenging cell wall debris of other bacteria. In contrast, Gram-positive bacteria lack the anabolic recycling genes, but mostly contain one or two murQ orthologs. Quantification of MurNAc 6P accumulation in murQ mutant cells by mass spectrometry allowed us to demonstrate for the first time that Gram-positive bacteria do recycle their own peptidoglycan. This had been questioned earlier, since peptidoglycan turnover products accumulate in the spent media of Gram-positives. We showed, that these fragments are recovered during nutrient limitation, which prolongs starvation survival of Bacillus subtilis and Staphylococcus aureus. Peptidoglycan recycling in these bacteria however differs, as the cell wall is either cleaved exhaustively and monosaccharide building blocks are taken up (B. subtilis) or disaccharides are released and recycled involving a novel phosphomuramidase (MupG; S.aureus). In B. subtilis also the teichoic acids, covalently bound to the peptidoglycan (wall teichoic acids; WTAs), are recycled. During phosphate limitation, the sn-glycerol-3-phosphate phosphodiesterase GlpQ specifically degrades WTAs of B. subtilis. In S. aureus, in contrast, GlpQ is used to scavenge external teichoic acid sources. Thus, although bacteria generally recover their own cell wall, they apparently apply distinct strategies for breakdown and reutilization of cell wall fragments. This review summarizes our work on this topic funded between 2011 and 2019 by the DFG within the collaborative research center SFB766.

The hydrolase LpqI primes mycobacterial peptidoglycan recycling

Growth and division by most bacteria requires remodelling and cleavage of their cell wall. A byproduct of this process is the generation of free peptidoglycan (PG) fragments known as muropeptides, which are recycled in many model organisms. Bacteria and hosts can harness the unique nature of muropeptides as a signal for cell wall damage and infection, respectively. Despite this critical role for muropeptides, it has long been thought that pathogenic mycobacteria such as Mycobacterium tuberculosis do not recycle their PG. Herein we show that M. tuberculosis and Mycobacterium bovis BCG are able to recycle components of their PG. We demonstrate that the core mycobacterial gene lpqI, encodes an authentic NagZ β-N-acetylglucosaminidase and that it is essential for PG-derived amino sugar recycling via an unusual pathway. Together these data provide a critical first step in understanding how mycobacteria recycle their peptidoglycan.

Bacterial growth and division require remodelling of the cell wall, which generates free peptidoglycan fragments. Here, Moynihan et al. show that Mycobacterium tuberculosis can recycle components of their peptidoglycan, and characterise a crucial enzyme required for this process.

Peptidoglycan synthesis in Tannerella forsythia: Scavenging is the modus operandi

Tannerella forsythia is a Gram-negative oral pathogen strongly associated with periodontitis. This bacterium has an absolute requirement for exogenous N-acetylmuramic acid (MurNAc), an amino sugar that forms the repeating disaccharide unit with amino sugar N-acetylglucosamine (GlcNAc) of the peptidoglycan backbone. In silico genome analysis indicates that T. forsythia lacks the key biosynthetic enzymes needed for the de novo synthesis of MurNAc, and so relies on alternative ways to meet its requirement for peptidoglycan biosynthesis. In the subgingival niche, the bacterium can acquire MurNAc and peptidoglycan fragments (muropeptides) released by the cohabiting bacteria during their cell wall breakdown associated with cell division. Tannerella forsythia is able to also use host sialic acid (Neu5Ac) in lieu of MurNAc or muropeptides for its survival during the biofilm growth. Evidence suggests that the bacterium might be able to shunt sialic acid into a metabolic pathway leading to peptidoglycan synthesis. In this review, we explore the mechanisms by which T. forsythia is able to scavenge MurNAc, muropeptide and sialic acid for its peptidoglycan synthesis, and the impact of these scavenging activities on pathogenesis.

N-Acetylmuramic Acid (MurNAc) Auxotrophy of the Oral Pathogen Tannerella forsythia: Characterization of a MurNAc Kinase and Analysis of Its Role in Cell Wall Metabolism

Tannerella forsythia is an anaerobic, Gram-negative oral pathogen that thrives in multispecies gingival biofilms associated with periodontitis. The bacterium is auxotrophic for the commonly essential bacterial cell wall sugar N-acetylmuramic acid (MurNAc) and, thus, strictly depends on an exogenous supply of MurNAc for growth and maintenance of cell morphology. A MurNAc transporter (Tf_MurT; Tanf_08375) and an ortholog of the Escherichia coli etherase MurQ (Tf_MurQ; Tanf_08385) converting MurNAc-6-phosphate to GlcNAc-6-phosphate were recently described for T. forsythia. In between the respective genes on the T. forsythia genome, a putative kinase gene is located. In this study, the putative kinase (Tf_MurK; Tanf_08380) was produced as a recombinant protein and biochemically characterized. Kinetic studies revealed Tf_MurK to be a 6-kinase with stringent substrate specificity for MurNAc exhibiting a 6 × 104-fold higher catalytic efficiency (kcat/Km ) for MurNAc than for N-acetylglucosamine (GlcNAc) with kcat values of 10.5 s-1 and 0.1 s-1 and Km values of 200 μM and 116 mM, respectively. The enzyme kinetic data suggest that Tf_MurK is subject to substrate inhibition (Ki[S] = 4.2 mM). To assess the role of Tf_MurK in the cell wall metabolism of T. forsythia, a kinase deletion mutant (ΔTf_murK::erm) was constructed. This mutant accumulated MurNAc intracellularly in the exponential phase, indicating the capability to take up MurNAc, but inability to catabolize MurNAc. In the stationary phase, the MurNAc level was reduced in the mutant, while the level of the peptidoglycan precursor UDP-MurNAc-pentapeptide was highly elevated. Further, according to scanning electron microscopy evidence, the ΔTf_murK::erm mutant was more tolerant toward low MurNAc concentration in the medium (below 0.5 μg/ml) before transition from healthy, rod-shaped to fusiform cells occurred, while the parent strain required > 1 μg/ml MurNAc for optimal growth. These data reveal that T. forsythia readily catabolizes exogenous MurNAc but simultaneously channels a proportion of the sugar into peptidoglycan biosynthesis. Deletion of Tf_murK blocks MurNAc catabolism and allows the direction of MurNAc solely to peptidoglycan biosynthesis, resulting in a growth advantage in MurNAc-depleted medium. This work increases our understanding of the T. forsythia cell wall metabolism and may pave new routes for lead finding in the treatment of periodontitis.

Peptidoglycan in obligate intracellular bacteria

Peptidoglycan is the predominant stress-bearing structure in the cell envelope of most bacteria, and also a potent stimulator of the eukaryotic immune system. Obligate intracellular bacteria replicate exclusively within the interior of living cells, an osmotically protected niche. Under these conditions peptidoglycan is not necessarily needed to maintain the integrity of the bacterial cell. Moreover, the presence of peptidoglycan puts bacteria at risk of detection and destruction by host peptidoglycan recognition factors and downstream effectors. This has resulted in a selective pressure and opportunity to reduce the levels of peptidoglycan. In this review we have analysed the occurrence of genes involved in peptidoglycan metabolism across the major obligate intracellular bacterial species. From this comparative analysis, we have identified a group of predicted 'peptidoglycan-intermediate' organisms that includes the Chlamydiae, Orientia tsutsugamushi, Wolbachia and Anaplasma marginale. This grouping is likely to reflect biological differences in their infection cycle compared with peptidoglycan-negative obligate intracellular bacteria such as Ehrlichia and Anaplasma phagocytophilum, as well as obligate intracellular bacteria with classical peptidoglycan such as Coxiella, Buchnera and members of the Rickettsia genus. The signature gene set of the peptidoglycan-intermediate group reveals insights into minimal enzymatic requirements for building a peptidoglycan-like sacculus and/or division septum.

Regulation and Molecular Basis of Environmental Muropeptide Uptake and Utilization in Fastidious Oral Anaerobe Tannerella forsythia

Tannerella forsythia is a Gram-negative oral anaerobe associated with periodontitis. This bacterium is auxotrophic for the peptidoglycan amino sugar N-acetylmuramic (MurNAc) and likely relies on scavenging peptidoglycan fragments (muropeptides) released by cohabiting bacteria during their cell wall recycling. Many Gram-negative bacteria utilize an inner membrane permease, AmpG, to transport peptidoglycan fragments into their cytoplasm. In the T. forsythia genome, the Tanf_08365 ORF has been identified as a homolog of AmpG permease. In order to confirm the functionality of Tanf_08365, a reporter system in an Escherichia coli host was generated that could detect AmpG-dependent accumulation of cytosolic muropeptides via a muropeptide-inducible β-lactamase reporter gene. In trans complementation of this reporter strain with a Tanf_08365 containing plasmid caused significant induction of β-lactamase activity compared to that with an empty plasmid control. These data indicated that Tanf_08365 acted as a functional muropeptide permease causing accumulation of muropeptides in E. coli and thus suggested that it is a permease involved in muropeptide scavenging in T. forsythia. Furthermore, we showed that the promoter regulating the expression of Tanf_08365 was activated significantly by a hybrid two-component system regulatory protein, GppX. We also showed that compared to the parental T. forsythia strain a mutant lacking GppX in which the expression of AmpG was reduced significantly attenuated in utilizing free muropeptides. In summary, we have uncovered the mechanism by which this nutritionally fastidious microbe accesses released muropeptides in its environment, opening up the possibility of targeting this activity to reduce its numbers in periodontitis patients with potential benefits in the treatment of disease.

Enzymatic synthesis and semi-preparative isolation of N-acetylmuramic acid 6-phosphate

N-acetylmuramic acid 6-phosphate (MurNAc-6P) is a constituent of the bacterial peptidoglycan cell wall, serving as an anchor point of secondary cell wall polymers such as teichoic acids, and it is a key metabolite of the peptidoglycan recycling metabolism. Thus, there is a demand for MurNAc-6P as a standard for cell wall compositional and metabolic analyses and, in addition, as a substrate for peptidoglycan recycling enzymes, e.g. MurNAc-6P etherases (MurQ) and MurNAc-6P phosphatases (MupP), or as an effector molecule of transcriptional MurR regulators. However, MurNAc-6P is commercially not available. We report here the facile enzymatic production of MurNAc-6P in mg-scale from MurNAc and ATP, applying Clostridium acetobutylicum kinase MurK, and purification by semi-preparative HPLC. MurNAc-6P was quantified using a coupled enzyme assay, revealing 75-80% overall product yield, and high purity was confirmed by mass spectrometry and proton NMR.

Draft Genome Sequences of Three Clinical Isolates of Tannerella forsythia Isolated from Subgingival Plaque from Periodontitis Patients in the United States

We report the genome sequences of three clinical isolates of Tannerella forsythia from the subgingival plaque of periodontitis patients attending clinics at the School of Dental Medicine, University at Buffalo. The availability of these genome sequences will aid the understanding of the pathogenesis of periodontitis.