Porphyromonas gingivalisC-terminal signal peptidase PG0026 and HagA interact with outer membrane protein PG27/LptO

TrkA serves as a virulence modulator in Porphyromonas gingivalis by maintaining heme acquisition and pathogenesis

Periodontitis is an inflammatory disease of the supporting tissues of the teeth, with polymicrobial infection serving as the major pathogenic factor. As a periodontitis-related keystone pathogen, Porphyromonas gingivalis can orchestrate polymicrobial biofilm skewing into dysbiosis. Some metatranscriptomic studies have suggested that modulation of potassium ion uptake might serve as a signal enhancing microbiota nososymbiocity and periodontitis progression. Although the relationship between potassium transport and virulence has been elucidated in some bacteria, less is mentioned about the periodontitis-related pathogen. Herein, we centered on the virulence modulation potential of TrkA, the potassium uptake regulatory protein of P. gingivalis, and uncovered TrkA as the modulator in the heme acquisition process and in maintaining optimal pathogenicity in an experimental murine model of periodontitis. Hemagglutination and hemolytic activities were attenuated in the case of trkA gene loss, and the entire transcriptomic profiling revealed that the trkA gene can control the expression of genes in relation to electron transport chain activity and translation, as well as some transcriptional factors, including cdhR, the regulator of the heme uptake system hmuYR. Collectively, these results link the heme acquisition process to the potassium transporter, providing new insights into the role of potassium ion in P. gingivalis pathogenesis.

Calcium ions and vitamin B12 are growth factors for Porphyromonas gingivalis

BACKGROUND

Porphyromonas gingivalis is a causative agent of chronic periodontitis. Standard strains of P. gingivalis, such as W83 and ATCC 33277, proliferate in minimal medium when protein is added as the energy source and hemin and menadione are added as growth factors. Nevertheless, minimal medium containing bovine serum albumin sometimes fails to support growth.

HIGHLIGHTS

The proliferation of two W83 strains and seven ATCC 33277 strains in various minimal media was investigated. Previously, we determined that calcium chloride (CaCl₂) was a growth factor for W83NM, a W83 strain. In this study, we found that vitamin B₁₂ enhanced the proliferation of W83NM in a minimal medium with cultures from the fourth passage but not from the first to the third passage. Therefore, using fourth-passage cultures, we assessed the proliferation of two W83 and seven ATCC 33277 strains in minimal media and the effects of CaCl₂ and vitamin B₁₂. Surprisingly, the nine P. gingivalis strains all differed with respect to their proliferation in minimal media, and protein products used as energy sources showed product-to-product and lot-to-lot heterogeneity. Even though strains or protein products were different, we found CaCl₂-dependent growth in nine strains and vitamin B₁₂-dependent growth in seven strains.

CONCLUSION

These results suggest that calcium ions and vitamin B₁₂ are novel growth factors for P. gingivalis.

Intermolecular latency regulates the essential C-terminal signal peptidase and sortase of the Porphyromonas gingivalis type-IX secretion system

Porphyromonas gingivalis is a keystone pathogen of the human dysbiotic oral microbiome that causes severe periodontitis. It employs a type-IX secretion system (T9SS) to shuttle proteins across the outer membrane (OM) for virulence. Uniquely, T9SS cargoes carry a C-terminal domain (CTD) as a secretion signal, which is cleaved and replaced with anionic lipopolysaccharide by transpeptidation for extracellular anchorage to the OM. Both reactions are carried out by PorU, the only known dual-function, C-terminal signal peptidase and sortase. PorU is itself secreted by the T9SS, but its CTD is not removed; instead, intact PorU combines with PorQ, PorV, and PorZ in the OM-inserted "attachment complex." Herein, we revealed that PorU transits between active monomers and latent dimers and solved the crystal structure of the ∼260-kDa dimer. PorU has an elongated shape ∼130 Å in length and consists of seven domains. The first three form an intertwined N-terminal cluster likely engaged in substrate binding. They are followed by a gingipain-type catalytic domain (CD), two immunoglobulin-like domains (IGL), and the CTD. In the first IGL, a long "latency β-hairpin" protrudes ∼30 Å from the surface to form an intermolecular β-barrel with β-strands from the symmetric CD, which is in a latent conformation. Homology modeling of the competent CD followed by in vivo validation through a cohort of mutant strains revealed that PorU is transported and functions as a monomer through a C690/H657 catalytic dyad. Thus, dimerization is an intermolecular mechanism for PorU regulation to prevent untimely activity until joining the attachment complex.

Molecular Strategies Underlying Porphyromonas gingivalis Virulence

The anaerobic Gram-negative bacterium Porphyromonas gingivalis is considered the keystone of periodontitis diseases, a set of inflammatory conditions that affects the tissues surrounding the teeth. In the recent years, the major virulence factors exploited by P. gingivalis have been identified and characterized, including a cocktail of toxins, mainly proteases called gingipains, which promote gingival tissue invasion. These effectors use the Sec pathway to cross the inner membrane and are then recruited and transported across the outer membrane by the type IX secretion system (T9SS). In P. gingivalis, most secreted effectors are attached to anionic lipopolysaccharides (A-LPS), and hence form a virulence coat at the cell surface. P. gingivalis produces additional virulence factors to evade host immune responses, such as capsular polysaccharide, fimbriae and outer membrane vesicles. In addition to periodontitis, it is proposed that this broad repertoire of virulence factors enable P. gingivalis to be involved in diverse human diseases such as rheumatoid arthritis, and neurodegenerative, Alzheimer, and cardiovascular disorders. Here, we review the major virulence determinants of P. gingivalis and discuss future directions to better understand their mechanisms of action.

PorA, a conserved C-terminal domain-containing protein, impacts the PorXY-SigP signaling of the type IX secretion system

Porphyromonas gingivalis, a periodontal pathogen, translocates many virulence factors including the cysteine proteases referred to as gingipains to the cell surface via the type IX secretion system (T9SS). Expression of the T9SS component proteins is regulated by the tandem signaling of the PorXY two-component system and the ECF sigma factor SigP. However, the details of this regulatory pathway are still unknown. We found that one of the T9SS conserved C-terminal domain-containing proteins, PGN_0123, which we have designated PorA, is involved in regulating expression of genes encoding T9SS structural proteins and that PorA can be translocated onto the cell surface without the T9SS translocation machinery. X-ray crystallography revealed that PorA has a domain similar to the mannose-binding domain of Escherichia coli FimH, the tip protein of Type 1 pilus. Mutations in the cytoplasmic domain of the sensor kinase PorY conferred phenotypic recovery on the ΔporA mutant. The SigP sigma factor, which is activated by the PorXY two-component system, markedly decreased in the ΔporA mutant. These results strongly support a potential role for PorA in relaying a signal from the cell surface to the PorXY-SigP signaling pathway.

Reassessment of minimal media reveals differences in growth among Porphyromonas gingivalis standard strains

OBJECTIVES

Porphyromonas gingivalis is one of the etiologic agents of chronic periodontitis. Our previous study showed that the use of minimal media for P. gingivalis allowed to isolate novel inhibitors of P. gingivalis growth. However, growth of P. gingivalis in minimal media was not always reproducible.

METHODS

To explain this phenomenon, we analyzed the growth of seven wild-type ATCC 33277 strains and two wild-type W83 strains in 10 minimal media and three complex media.

RESULTS

All nine strains grew in LF (Lactalbumin-Ferric chloride), GC (bovine γ-immunoglobulin G-Calcium chloride), and newly developed mC (milk-Casein) minimal media. Therefore, LF, GC, and mC could be used as minimal media for P. gingivalis. In contrast, other six minimal media containing bovine serum albumin (BSA) supported the growth of several less strains; among these, two media also showed lack of reproducibility in growth among ATCC 33277 strains. On the other hand, four ATCC 33277 strains grew similarly in all 13 media, but two W83 and other three ATCC 33277 strains grew differently in at least one medium.

CONCLUSIONS

These results suggest that the lack of reproducibility of P. gingivalis growth on minimal media is caused by the presence of BSA, and by differences among the standard strains of P. gingivalis.

Gingimaps: Protein Localization in the Oral Pathogen Porphyromonas gingivalis

Porphyromonas gingivalis is an oral pathogen involved in the widespread disease periodontitis. In recent years, however, this bacterium has been implicated in the etiology of another common disorder, the autoimmune disease rheumatoid arthritis. Periodontitis and rheumatoid arthritis were known to correlate for decades, but only recently a possible molecular connection underlying this association has been unveiled. P. gingivalis possesses an enzyme that citrullinates certain host proteins and, potentially, elicits autoimmune antibodies against such citrullinated proteins. These autoantibodies are highly specific for rheumatoid arthritis and have been purported both as a symptom and a potential cause of the disease. The citrullinating enzyme and other major virulence factors of P. gingivalis, including some that were implicated in the etiology of rheumatoid arthritis, are targeted to the host tissue as secreted or outer-membrane-bound proteins. These targeting events play pivotal roles in the interactions between the pathogen and its human host. Accordingly, the overall protein sorting and secretion events in P. gingivalis are of prime relevance for understanding its full disease-causing potential and for developing preventive and therapeutic approaches. The aim of this review is therefore to offer a comprehensive overview of the subcellular and extracellular localization of all proteins in three reference strains and four clinical isolates of P. gingivalis, as well as the mechanisms employed to reach these destinations.

A screening system using minimal media identifies a flavin-competing inhibitor of Porphyromonas gingivalis growth

Chronic periodontitis is caused by dysbiosis of human oral commensals and especially by increase in Porphyromonas gingivalis. Inhibitors of P. gingivalis growth are expected to serve as effective drugs for the periodontal therapy. In the present study, we isolated new growth inhibitors of P. gingivalis using minimal media for P. gingivalis. The minimal media included the previously reported Globulin-Albumin (GA) and the newly developed Lactalbumin-Ferric chloride (LF) and Globulin-Calcium chloride (GC); all supported growth of the wild-type strain of P. gingivalis but did not support the growth of a mutant defective for a type IX secretion system. GC contains CaCl2, indicating that P. gingivalis requires a calcium ion for growth. Using LF and GA, we screened about 100 000 compounds and identified 73 that strongly inhibited the growth of P. gingivalis. More than half of these candidates would not have been obtained if these minimal media had not been used in our screen. One of our candidate inhibitors was diphenyleneiodonium chloride (DPIC), which showed strong bactericidal activity against P. gingivalis. Excess amounts of flavin adenine dinucleotide or flavin mononucleotide suppressed the inhibitory activity of DPIC, suggesting that DPIC would be a novel potent growth inhibitor.

Comparative genome analysis provides novel insight into the interaction of Aquimarina sp. AD1, BL5 and AD10 with their macroalgal host

The Aquimarina genus is widely distributed throughout the marine environment, however little is understood regarding its ecological role, particularly when in association with eukaryotic hosts. Here, we examine the genomes of two opportunistic pathogens, Aquimarina sp. AD1 and BL5, and a non-pathogenic strain Aquimarina sp. AD10, that were isolated from diseased individuals of the red alga Delisea pulchra. Each strain encodes multiple genes for the degradation of marine carbohydrates and vitamin biosynthesis. These traits are hypothesised to promote nutrient exchange between the Aquimarina strains and their algal host, facilitating a close symbiotic relationship. Moreover, each strain harbours the necessary genes for the assembly of a Type 9 Secretion System (T9SS) and the associated gliding motility apparatus. In addition to these common features, pathogenic strains AD1 and BL5, encode genes for the production of flexirubin type pigments and a number of unique non-ribosomal peptide synthesis (NRPS) gene clusters, suggesting a role for these uncharacterised traits in virulence. This study provides valuable insight into the potential ecological role of Aquimarina in the marine environment and the complex factors driving pathogenesis and symbiosis in this genus.

Bacteroidetes Gliding Motility and the Type IX Secretion System

Members of the phylum Bacteroidetes have many unique features, including gliding motility and the type IX protein secretion system (T9SS). Bacteroidetes gliding and T9SSs are common in, but apparently confined to, this phylum. Most, but not all, members of the phylum secrete proteins using the T9SS, and most also exhibit gliding motility. T9SSs secrete cell surface components of the gliding motility machinery and also secrete many extracellular or cell surface enzymes, adhesins, and virulence factors. The components of the T9SS are novel and are unrelated to those of other bacterial secretion systems. Proteins secreted by the T9SS rely on the Sec system to cross the cytoplasmic membrane, and they use the T9SS for delivery across the outer membrane. Secreted proteins typically have conserved C-terminal domains that target them to the T9SS. Some of the T9SS components were initially identified as proteins required for gliding motility. Gliding does not involve flagella or pili and instead relies on the rapid movement of motility adhesins, such as SprB, along the cell surface by the gliding motor. Contact of the adhesins with the substratum provides the traction that results in cell movement. SprB and other motility adhesins are delivered to the cell surface by the T9SS. Gliding and the T9SS appear to be intertwined, and components of the T9SS that span the cytoplasmic membrane may energize both gliding and protein secretion. The functions of the individual proteins in each process are the subject of ongoing investigations.

PorV is an Outer Membrane Shuttle Protein for the Type IX Secretion System

Porphyromonas gingivalis is a keystone pathogen associated with chronic periodontitis. Major virulence factors named gingipains (cysteine proteinases, RgpA, RgpB and Kgp) are secreted via the Type IX Secretion System (T9SS). These, together with approximately 30 other proteins, are secreted to the cell surface and anchored to the outer membrane by covalent modification to anionic lipopolysaccharide (A-LPS) via the novel Gram negative sortase, PorU. PorU is localised on the cell surface and cleaves the C-terminal domain signal (CTD) of T9SS substrates and conjugates their new C-termini to A-LPS. A 440 kDa-attachment complex was identified in the wild-type (WT) comprising of PorU:PorV:PorQ:PorZ. In mutant strains, sub-complexes comprising PorU:PorV or PorQ:PorZ were also identified at smaller native sizes suggesting that PorU and PorZ are anchored to the cell surface via interaction with the PorV and PorQ outer membrane proteins, respectively. Analysis of porU mutants and a CTD cleavage mutant revealed accumulation of immature T9SS substrates in a PorV-bound form. Quantitative label-free proteomics of WT whole cell lysates estimated that the proportion of secretion channels:attachment complexes:free PorV:T9SS substrates was 1:6:110:2000 supporting a role for PorV as a shuttle protein delivering secreted proteins to the attachment complex for CTD signal cleavage and A-LPS modification.

Mechanisms by which Porphyromonas gingivalis evades innate immunity

The oral cavity is home to unique resident microbial communities whose interactions with host immunity are less frequently studied than those of the intestinal microbiome. We examined the stimulatory capacity and the interactions of two oral bacteria, Porphyromonas gingivalis (P. gingivalis) and Fusobacterium nucleatum (F. nucleatum), on Dendritic Cell (DC) activation, comparing them to the effects of the well-studied intestinal microbe Escherichia coli (E. coli). Unlike F. nucleatum and E. coli, P. gingivalis failed to activate DCs, and in fact silenced DC responses induced by F. nucleatum or E. coli. We identified a variant strain of P. gingivalis (W50) that lacked this immunomodulatory activity. Using biochemical approaches and whole genome sequencing to compare the two substrains, we found a point mutation in the hagA gene. This protein is though to be involved in the alteration of the PorSS/gingipain pathway, which regulates protein secretion into the extracellular environment. A proteomic comparison of the secreted products of the two substrains revealed enzymatic differences corresponding to this phenotype. We found that P. gingivalis secretes gingipain(s) that inactivate several key proinflammatory mediators made by DCs and/or T cells, but spare Interleukin-1 (IL-1) and GM-CSF, which can cause capillary leaks that serve as a source of the heme that P. gingivalis requires for its survival, and GM-CSF, which can cause epithelial-cell growth. Taken together, our results suggest that P. gingivalis has evolved potent mechanisms to modulate its virulence factors and dampen the innate immune response by selectively inactivating most proinflammatory cytokines.

The Type IX Secretion System (T9SS): Highlights and Recent Insights into Its Structure and Function

Protein secretion systems are vital for prokaryotic life, as they enable bacteria to acquire nutrients, communicate with other species, defend against biological and chemical agents, and facilitate disease through the delivery of virulence factors. In this review, we will focus on the recently discovered type IX secretion system (T9SS), a complex translocon found only in some species of the Bacteroidetes phylum. T9SS plays two roles, depending on the lifestyle of the bacteria. It provides either a means of movement (called gliding motility) for peace-loving environmental bacteria or a weapon for pathogens. The best-studied members of these two groups are Flavobacterium johnsoniae, a commensal microorganism often found in water and soil, and Porphyromonas gingivalis, a human oral pathogen that is a major causative agent of periodontitis. In P. gingivalis and some other periodontopathogens, T9SS translocates proteins, especially virulence factors, across the outer membrane (OM). Proteins destined for secretion bear a conserved C-terminal domain (CTD) that directs the cargo to the OM translocon. At least 18 proteins are involved in this still enigmatic process, with some engaged in the post-translational modification of T9SS cargo proteins. Upon translocation across the OM, the CTD is removed by a protease with sortase-like activity and an anionic LPS is attached to the newly formed C-terminus. As a result, a cargo protein could be secreted into the extracellular milieu or covalently attached to the bacterial surface. T9SS is regulated by a two-component system; however, the precise environmental signal that triggers it has not been identified. Exploring unknown systems contributing to bacterial virulence is exciting, as it may eventually lead to new therapeutic strategies. During the past decade, the major components of T9SS were identified, as well as hints suggesting the possible mechanism of action. In addition, the list of characterized cargo proteins is constantly growing. The actual structure of the translocon, situated in the OM of bacteria, remains the least explored area; however, new technical approaches and increasing scientific attention have resulted in a growing body of data. Therefore, we present a compact up-to-date review of this topic.

The Type IX Secretion System (T9SS): Highlights and Recent Insights into Its Structure and Function

Protein secretion systems are vital for prokaryotic life, as they enable bacteria to acquire nutrients, communicate with other species, defend against biological and chemical agents, and facilitate disease through the delivery of virulence factors. In this review, we will focus on the recently discovered type IX secretion system (T9SS), a complex translocon found only in some species of the Bacteroidetes phylum. T9SS plays two roles, depending on the lifestyle of the bacteria. It provides either a means of movement (called gliding motility) for peace-loving environmental bacteria or a weapon for pathogens. The best-studied members of these two groups are Flavobacterium johnsoniae, a commensal microorganism often found in water and soil, and Porphyromonas gingivalis, a human oral pathogen that is a major causative agent of periodontitis. In P. gingivalis and some other periodontopathogens, T9SS translocates proteins, especially virulence factors, across the outer membrane (OM). Proteins destined for secretion bear a conserved C-terminal domain (CTD) that directs the cargo to the OM translocon. At least 18 proteins are involved in this still enigmatic process, with some engaged in the post-translational modification of T9SS cargo proteins. Upon translocation across the OM, the CTD is removed by a protease with sortase-like activity and an anionic LPS is attached to the newly formed C-terminus. As a result, a cargo protein could be secreted into the extracellular milieu or covalently attached to the bacterial surface. T9SS is regulated by a two-component system; however, the precise environmental signal that triggers it has not been identified. Exploring unknown systems contributing to bacterial virulence is exciting, as it may eventually lead to new therapeutic strategies. During the past decade, the major components of T9SS were identified, as well as hints suggesting the possible mechanism of action. In addition, the list of characterized cargo proteins is constantly growing. The actual structure of the translocon, situated in the OM of bacteria, remains the least explored area; however, new technical approaches and increasing scientific attention have resulted in a growing body of data. Therefore, we present a compact up-to-date review of this topic.

PG1058 Is a Novel Multidomain Protein Component of the Bacterial Type IX Secretion System

Porphyromonas gingivalis utilises the Bacteroidetes-specific type IX secretion system (T9SS) to export proteins across the outer membrane (OM), including virulence factors such as the gingipains. The secreted proteins have a conserved carboxy-terminal domain essential for type IX secretion that is cleaved upon export. In P. gingivalis the T9SS substrates undergo glycosylation with anionic lipopolysaccharide (A-LPS) and are attached to the OM. In this study, comparative analyses of 24 Bacteroidetes genomes identified ten putative novel components of the T9SS in P. gingivalis, one of which was PG1058. Computer modelling of the PG1058 structure predicted a novel N- to C-terminal architecture comprising a tetratricopeptide repeat (TPR) domain, a β-propeller domain, a carboxypeptidase regulatory domain-like fold (CRD) and an OmpA_C-like putative peptidoglycan binding domain. Inactivation of pg1058 in P. gingivalis resulted in loss of both colonial pigmentation and surface-associated proteolytic activity; a phenotype common to T9SS mutants. Immunoblot and LC-MS/MS analyses of subcellular fractions revealed T9SS substrates accumulated within the pg1058 mutant periplasm whilst whole-cell ELISA showed the Kgp gingipain was absent from the cell surface, confirming perturbed T9SS function. Immunoblot, TEM and whole-cell ELISA analyses indicated A-LPS was produced and present on the pg1058 mutant cell surface although it was not linked to T9SS substrate proteins. This indicated that PG1058 is crucial for export of T9SS substrates but not for the translocation of A-LPS. PG1058 is a predicted lipoprotein and was localised to the periplasmic side of the OM using whole-cell ELISA, immunoblot and LC-MS/MS analyses of subcellular fractions. The structural prediction and localisation of PG1058 suggests that it may have a role as an essential scaffold linking the periplasmic and OM components of the T9SS.

The outer-membrane export signal of Porphyromonas gingivalis type IX secretion system (T9SS) is a conserved C-terminal β-sandwich domain

In the recently characterized Type IX Secretion System (T9SS), the conserved C-terminal domain (CTD) in secreted proteins functions as an outer membrane translocation signal for export of virulence factors to the cell surface in the Gram-negative Bacteroidetes phylum. In the periodontal pathogen Porphyromonas gingivalis, the CTD is cleaved off by PorU sortase in a sequence-independent manner, and anionic lipopolysaccharide (A-LPS) is attached to many translocated proteins, thus anchoring them to the bacterial surface. Here, we solved the atomic structure of the CTD of gingipain B (RgpB) from P. gingivalis, alone and together with a preceding immunoglobulin-superfamily domain (IgSF). The CTD was found to possess a typical Ig-like fold encompassing seven antiparallel β-strands organized in two β-sheets, packed into a β-sandwich structure that can spontaneously dimerise through C-terminal strand swapping. Small angle X-ray scattering (SAXS) revealed no fixed orientation of the CTD with respect to the IgSF. By introducing insertion or substitution of residues within the inter-domain linker in the native protein, we were able to show that despite the region being unstructured, it nevertheless is resistant to general proteolysis. These data suggest structural motifs located in the two adjacent Ig-like domains dictate the processing of CTDs by the T9SS secretion pathway.

Porphyromonas gingivalis: An Overview of Periodontopathic Pathogen below the Gum Line

Periodontal disease represents a group of oral inflammatory infections initiated by oral pathogens which exist as a complex biofilms on the tooth surface and cause destruction to tooth supporting tissues. The severity of this disease ranges from mild and reversible inflammation of the gingiva (gingivitis) to chronic destruction of connective tissues, the formation of periodontal pocket and ultimately result in loss of teeth. While human subgingival plaque harbors more than 500 bacterial species, considerable research has shown that Porphyromonas gingivalis, a Gram-negative anaerobic bacterium, is the major etiologic agent which contributes to chronic periodontitis. This black-pigmented bacterium produces a myriad of virulence factors that cause destruction to periodontal tissues either directly or indirectly by modulating the host inflammatory response. Here, this review provides an overview of P. gingivalis and how its virulence factors contribute to the pathogenesis with other microbiome consortium in oral cavity.

Porphyromonas gingivalis and related bacteria: from colonial pigmentation to the type IX secretion system and gliding motility

Porphyromonas gingivalis is a gram-negative, non-motile, anaerobic bacterium implicated as a major pathogen in periodontal disease. P. gingivalis grows as black-pigmented colonies on blood agar, and many bacteriologists have shown interest in this property. Studies of colonial pigmentation have revealed a number of important findings, including an association with the highly active extracellular and surface proteinases called gingipains that are found in P. gingivalis. The Por secretion system, a novel type IX secretion system (T9SS), has been implicated in gingipain secretion in studies using non-pigmented mutants. In addition, many potent virulence proteins, including the metallocarboxypeptidase CPG70, 35 kDa hemin-binding protein HBP35, peptidylarginine deiminase PAD and Lys-specific serine endopeptidase PepK, are secreted through the T9SS. These findings have not been limited to P. gingivalis but have been extended to other bacteria belonging to the phylum Bacteroidetes. Many Bacteroidetes species possess the T9SS, which is associated with gliding motility for some of these bacteria.

Physiological adaptations of key oral bacteria

Oral colonising bacteria are highly adapted to the various environmental niches harboured within the mouth, whether that means while contributing to one of the major oral diseases of caries, pulp infections, or gingival/periodontal disease or as part of a commensal lifestyle. Key to these infections is the ability to adhere to surfaces via a range of specialised adhesins targeted at both salivary and epithelial proteins, their glycans and to form biofilm. They must also resist the various physical stressors they are subjected to, including pH and oxidative stress. Possibly most strikingly, they have developed the ability to harvest both nutrient sources provided by the diet and those derived from the host, such as protein and surface glycans. We have attempted to review recent developments that have revealed much about the molecular mechanisms at work in shaping the physiology of oral bacteria and how we might use this information to design and implement new treatment strategies.

The S-layer proteins of Tannerella forsythia are secreted via a type IX secretion system that is decoupled from protein O-glycosylation

Conserved C-terminal domains (CTD) have been shown to act as a signal for the translocation of certain proteins across the outer membrane of Bacteroidetes via a type IX secretion system (T9SS). The genome sequence of the periodontal pathogen Tannerella forsythia predicts the presence of the components for a T9SS in conjunction with a suite of CTD proteins. T. forsythia is covered with a two-dimensional crystalline surface (S-) layer composed of the glycosylated CTD proteins TfsA and TfsB. To investigate, if T9SS is functional in T. forsythia, T9SS-deficient mutants were generated by targeting either TF0955 (putative C-terminal signal peptidase) or TF2327 (PorK ortholog), and the mutants were analyzed with respect to secretion, assembly and glycosylation of the S-layer proteins as well as proteolytic processing of the CTD and biofilm formation. In either mutant, TfsA and TfsB were incapable of translocation, as evidenced by the absence of the S-layer in transmission electron microscopy of ultrathin-sectioned bacterial cells. Despite being entrapped within the periplasm, mass spectrometry analysis revealed that the S-layer proteins were modified with the complete, mature glycan found on the secreted proteins, indicating that protein translocation and glycosylation are two independent processes. Further, the T9SS mutants showed a denser biofilm with fewer voids compared with the wild-type. This study demonstrates the functionality of T9SS and the requirement of CTD for the outer membrane passage of extracellular proteins in T. forsythia, exemplified by the two S-layer proteins. In addition, T9SS protein translocation is decoupled from O-glycan attachment in T. forsythia.

FLP-FRT-based method to obtain unmarked deletions of CHU_3237 (porU) and large genomic fragments of Cytophaga hutchinsonii

Cytophaga hutchinsonii is a widely distributed cellulolytic bacterium in the phylum Bacteroidetes. It can digest crystalline cellulose rapidly without free cellulases or cellulosomes. The mechanism of its cellulose utilization remains a mystery. We developed an efficient method based on a linear DNA double-crossover and FLP-FRT recombination system to obtain unmarked deletions of both single genes and large genomic fragments in C. hutchinsonii. Unmarked deletion of CHU_3237 (porU), an ortholog of the C-terminal signal peptidase of a type IX secretion system (T9SS), resulted in defects in colony spreading, cellulose degradation, and protein secretion, indicating that it is a component of the T9SS and that T9SS plays an important role in cellulose degradation by C. hutchinsonii. Furthermore, deletions of four large genomic fragments were obtained using our method, and the sizes of the excised fragments varied from 9 to 19 kb, spanning from 6 to 22 genes. The customized FLP-FRT method provides an efficient tool for more rapid progress in the cellulose degradation mechanism and other physiological aspects of C. hutchinsonii.