Disruption of heterotypic community development by Porphyromonas gingivalis with small molecule inhibitors

The Application of Small Molecules to the Control of Typical Species Associated With Oral Infectious Diseases

Oral microbial dysbiosis is the major causative factor for common oral infectious diseases including dental caries and periodontal diseases. Interventions that can lessen the microbial virulence and reconstitute microbial ecology have drawn increasing attention in the development of novel therapeutics for oral diseases. Antimicrobial small molecules are a series of natural or synthetic bioactive compounds that have shown inhibitory effect on oral microbiota associated with oral infectious diseases. Novel small molecules, which can either selectively inhibit keystone microbes that drive dysbiosis of oral microbiota or inhibit the key virulence of the microbial community without necessarily killing the microbes, are promising for the ecological management of oral diseases. Here we discussed the research progress in the development of antimicrobial small molecules and delivery systems, with a particular focus on their antimicrobial activity against typical species associated with oral infectious diseases and the underlying mechanisms.

Porphyromonas gingivalis: where do we stand in our battle against this oral pathogen?

Periodontal diseases, such as gingivitis and periodontitis, are inflammatory diseases triggered by pathogenic bacteria that lead to damage of the soft tissue and bone supporting the teeth. Amongst the identified oral periodontopathogenic bacteria, Porphyromonas gingivalis is able to enhance oral dysbiosis, which is an imbalance in the beneficial commensal and periodontal pathogenic bacteria that induces chronic inflammation. Given the critical role of oral pathogenic bacteria like P. gingivalis in the pathogenesis of periodontitis, local and/or systemic antibacterial therapy has been suggested to treat this disease, especially in its severe or refractory forms. Nevertheless, the majority of the antibacterial agents currently used for the treatment of periodontal diseases are broad-spectrum, which harms beneficial bacterial species that are critical in health, inhibit the growth of pathogenic bacteria, contribute in protecting the periodontal tissues to damage and aid in its healing. Thus, the development of more effective and specific antibacterial agents is needed to control oral pathogens in a polymicrobial environment. The strategies for the development of novel antibacterial agents include natural product isolation as well as synthetic and semi-synthetic methodologies. This review presents an overview of the periodontal diseases gingivitis and periodontitis along with current antibacterial treatment options (i.e., classes of antibacterial agents and the mechanism(s) of resistance that hinder their usage) used in periodontal diseases that specifically target oral pathogens such as P. gingivalis. In addition, to help medicinal chemists gain a better understanding of potentially promising scaffolds, this review provides an in-depth coverage of the various families of small molecules that have been investigated as potential anti-P. gingivalis agents, including novel families of compounds, repositioned drugs, as well as natural products.

Porphyromonas gingivalis fimbrial protein Mfa5 contains a von Willebrand factor domain and an intramolecular isopeptide

The Gram-negative bacterium Porphyromonas gingivalis is a secondary colonizer of the oral biofilm and is involved in the onset and progression of periodontitis. Its fimbriae, of type-V, are important for attachment to other microorganisms in the biofilm and for adhesion to host cells. The fimbriae are assembled from five proteins encoded by the mfa1 operon, of which Mfa5 is one of the ancillary tip proteins. Here we report the X-ray structure of the N-terminal half of Mfa5, which reveals a von Willebrand factor domain and two IgG-like domains. One of the IgG-like domains is stabilized by an intramolecular isopeptide bond, which is the first such bond observed in a Gram-negative bacterium. These features make Mfa5 structurally more related to streptococcal adhesins than to the other P. gingivalis Mfa proteins. The structure reported here indicates that horizontal gene transfer has occurred among the bacteria within the oral biofilm.

Natural products as inspiration for the development of bacterial antibiofilm agents

Natural products have historically been a rich source of diverse chemical matter with numerous biological activities, and have played an important role in drug discovery in many areas including infectious disease. Synthetic and medicinal chemistry have been, and continue to be, important tools to realize the potential of natural products as therapeutics and as chemical probes. The formation of biofilms by bacteria in an infection setting is a significant factor in the recalcitrance of many bacterial infections, conferring increased tolerance to many antibiotics and to the host immune response, and as yet there are no approved therapeutics for combatting biofilm-based bacterial infections. Small molecules that interfere with the ability of bacteria to form and maintain biofilms can overcome antibiotic tolerance conferred by the biofilm phenotype, and have the potential to form combination therapies with conventional antibiotics. Many natural products with anti-biofilm activity have been identified from plants, microbes, and marine life, including: elligic acid glycosides, hamamelitannin, carolacton, skyllamycins, promysalin, phenazines, bromoageliferin, flustramine C, meridianin D, and brominated furanones. Total synthesis and medicinal chemistry programs have facilitated structure confirmation, identification of critical structural motifs, better understanding of mechanistic pathways, and the development of more potent, more accessible, or more pharmacologically favorable derivatives of anti-biofilm natural products.

In Vitro and In Vivo Assessment of the Efficacy of Bromoageliferin, an Alkaloid Isolated from the Sponge Agelas dilatata, against Pseudomonas aeruginosa

The pyrrole-imidazoles, a group of alkaloids commonly found in marine sponges belonging to the genus Agelas, display a wide range of biological activities. Herein, we report the first chemical study of the secondary metabolites of the sponge A. dilatata from the coastal area of the Yucatan Peninsula (Mexico). In this study, we isolated eight known alkaloids from an organic extract of the sponge. We used NMR and MS analysis and comparison with existing databases to characterize the alkaloids: ageliferin (1), bromoageliferin (2), dibromoageliferin (3), sceptrin (4), nakamuric acid (5), 4-bromo-1H-pyrrole-2-carboxylic acid (6), 4,5-dibromopyrrole-2-carboxylic acid (7) and 3,7-dimethylisoguanine (8). We also evaluated, for the first time, the activity of these alkaloids against the most problematic multidrug-resistant (MDR) pathogens, i.e., the Gram-negative bacteria Pseudomonas aeruginosa, Klebsiella pneumoniae and Acinetobacter baumannii. Bromoageliferin (2) displayed significant activity against P. aeruginosa. Comparison of the antibacterial activity of ageliferins 1–3 (of similar structure) against P. aeruginosa revealed some relationship between structure and activity. Furthermore, in in vitro assays, 2 inhibited growth and biofilm production in clinical strains of P. aeruginosa. Moreover, 2 increased the survival time in an in vivo Galleria mellonella model of infection. The findings confirm bromoageliferin (2) as a potential lead for designing new antibacterial drugs.

Peptide and non-peptide mimetics as potential therapeutics targeting oral bacteria and oral biofilms

The development of the oral biofilm requires a complex series of interactions between host tissues and the colonizing bacteria as well as numerous interspecies interactions between the organisms themselves. Disruption of normal host-microbe homoeostasis in the oral cavity can lead to a dysbiotic microbial community that contributes to caries or periodontal disease. A variety of approaches have been pursued to develop novel potential therapeutics that are active against the oral biofilm and/or target specific oral bacteria. The structure and function of naturally occurring antimicrobial peptides from oral tissues and secretions as well as external sources such as frog skin secretions have been exploited to develop numerous peptide mimetics and small molecule peptidomimetics that show improved antimicrobial activity, increased stability and other desirable characteristics relative to the parent peptides. In addition, a rational and minimalist approach has been developed to design small artificial peptides with amphipathic α-helical properties that exhibit potent antibacterial activity. Furthermore, with an increased understanding of the molecular mechanisms of beneficial and/or antagonistic interspecies interactions that contribute to the formation of the oral biofilm, new potential targets for therapeutic intervention have been identified and both peptide-based and small molecule mimetics have been developed that target these key components. Many of these mimetics have shown promising results in in vitro and pre-clinical testing and the initial clinical evaluation of several novel compounds has demonstrated their utility in humans.

Streptococcus gordonii programs epithelial cells to resist ZEB2 induction by Porphyromonas gingivalis

The polymicrobial microbiome of the oral cavity is a direct precursor of periodontal diseases, and changes in microhabitat or shifts in microbial composition may also be linked to oral squamous cell carcinoma. Dysbiotic oral epithelial responses provoked by individual organisms, and which underlie these diseases, are widely studied. However, organisms may influence community partner species through manipulation of epithelial cell responses, an aspect of the host microbiome interaction that is poorly understood. We report here that Porphyromonas gingivalis, a keystone periodontal pathogen, can up-regulate expression of ZEB2, a transcription factor which controls epithelial-mesenchymal transition and inflammatory responses. ZEB2 regulation by P. gingivalis was mediated through pathways involving β-catenin and FOXO1. Among the community partners of P. gingivalis, Streptococcus gordonii was capable of antagonizing ZEB2 expression. Mechanistically, S. gordonii suppressed FOXO1 by activating the TAK1-NLK negative regulatory pathway, even in the presence of P. gingivalis Collectively, these results establish S. gordonii as homeostatic commensal, capable of mitigating the activity of a more pathogenic organism through modulation of host signaling.

Peptide-Based Inhibitors of Fimbrial Biogenesis in Porphyromonas gingivalis

Periodontitis is a progressive inflammatory disease that affects roughly half of American adults. Colonization of the oral cavity by the Gram-negative bacterial pathogen Porphyromonas gingivalis is a key event in the initiation and development of periodontal disease. Adhesive surface structures termed fimbriae (pili) mediate interactions of P. gingivalis with other bacteria and with host cells throughout the course of disease. The P. gingivalis fimbriae are assembled via a novel mechanism that involves proteolytic processing of lipidated precursor subunits and their subsequent polymerization on the bacterial surface. Given their extracellular assembly mechanism and central roles in pathogenesis, the P. gingivalis fimbriae are attractive targets for anti-infective therapeutics to prevent or treat periodontal disease. Here we confirm that conserved sequences in the N and C termini of the Mfa1 fimbrial subunit protein perform critical roles in subunit polymerization. We show that treatment of P. gingivalis with peptides corresponding to the conserved C-terminal region inhibits the extracellular assembly of Mfa fimbriae on the bacterial surface. We also show that peptide treatment interferes with the function of Mfa fimbriae by reducing P. gingivalis adhesion to Streptococcus gordonii in a dual-species biofilm model. Finally, we show that treatment of bacteria with similar peptides inhibits extracellular polymerization of the Fim fimbriae, which are also expressed by P. gingivalis These results support a donor strand-based assembly mechanism for the P. gingivalis fimbriae and demonstrate the feasibility of using extracellular peptides to disrupt the biogenesis and function of these critical periodontal disease virulence factors.

Antimicrobial Photodynamic Therapy to Control Clinically Relevant Biofilm Infections

Biofilm describes a microbially-derived sessile community in which microbial cells are firmly attached to the substratum and embedded in extracellular polymeric matrix. Microbial biofilms account for up to 80% of all bacterial and fungal infections in humans. Biofilm-associated pathogens are particularly resistant to antibiotic treatment, and thus novel antibiofilm approaches needed to be developed. Antimicrobial Photodynamic therapy (aPDT) had been recently proposed to combat clinically relevant biofilms such as dental biofilms, ventilator associated pneumonia, chronic wound infections, oral candidiasis, and chronic rhinosinusitis. aPDT uses non-toxic dyes called photosensitizers (PS), which can be excited by harmless visible light to produce reactive oxygen species (ROS). aPDT is a multi-stage process including topical PS administration, light irradiation, and interaction of the excited state with ambient oxygen. Numerous in vitro and in vivo aPDT studies have demonstrated biofilm-eradication or substantial reduction. ROS are produced upon photo-activation and attack adjacent targets, including proteins, lipids, and nucleic acids present within the biofilm matrix, on the cell surface and inside the microbial cells. Damage to non-specific targets leads to the destruction of both planktonic cells and biofilms. The review aims to summarize the progress of aPDT in destroying biofilms and the mechanisms mediated by ROS. Finally, a brief section provides suggestions for future research.

A novel peptidic inhibitor derived from Streptococcus cristatus ArcA attenuates virulence potential of Porphyromonas gingivalis

Periodontitis is a global health problem and the 6th most common infectious disease worldwide. Porphyromonas gingivalis is considered a keystone pathogen in the disease and is capable of elevating the virulence potential of the periodontal microbial community. Strategies that interfere with P. gingivalis colonization and expression of virulence factor are therefore attractive approaches for preventing and treating periodontitis. We have previously reported that an 11-mer peptide (SAPP) derived from Streptococcus cristatus arginine deiminase (ArcA) was able to repress the expression and production of several well-known P. gingivalis virulence factors including fimbrial proteins and gingipains. Herein we expand and develop these studies to ascertain the impact of this peptide on phenotypic properties of P. gingivalis related to virulence potential. We found that growth rate was not altered by exposure of P. gingivalis to SAPP, while monospecies and heterotypic biofilm formation, and invasion of oral epithelial cells were inhibited. Additionally, SAPP was able to impinge the ability of P. gingivalis to dysregulate innate immunity by repressing gingipain-associated degradation of interleukin-8 (IL8). Hence, SAPP has characteristics that could be exploited for the manipulation of P. gingivalis levels in oral communities and preventing realization of virulence potential.

New approaches to combat Porphyromonas gingivalis biofilms

In nature, bacteria predominantly reside in structured, surface-attached communities embedded in a self-produced, extracellular matrix. These so-called biofilms play an important role in the development and pathogenesis of many infections, as they are difficult to eradicate due to their resistance to antimicrobials and host defense mechanisms. This review focusses on the biofilm-forming periodontal bacterium Porphyromonas gingivalis. Current knowledge on the virulence mechanisms underlying P. gingivalis biofilm formation is presented. In addition, oral infectious diseases in which P. gingivalis plays a key role are described, and an overview of conventional and new therapies for combating P. gingivalis biofilms is given. More insight into this intriguing pathogen might direct the development of better strategies to combat oral infections.

Natural antigenic differences in the functionally equivalent extracellular DNABII proteins of bacterial biofilms provide a means for targeted biofilm therapeutics

Bacteria that persist in the oral cavity exist within complex biofilm communities. A hallmark of biofilms is the presence of an extracellular polymeric substance (EPS), which consists of polysaccharides, extracellular DNA (eDNA), and proteins, including the DNABII family of proteins. The removal of DNABII proteins from a biofilm results in the loss of structural integrity of the eDNA and the collapse of the biofilm structure. We examined the role of DNABII proteins in the biofilm structure of the periodontal pathogen Porphyromonas gingivalis and the oral commensal Streptococcus gordonii. Co-aggregation with oral streptococci is thought to facilitate the establishment of P. gingivalis within the biofilm community. We demonstrate that DNABII proteins are present in the EPS of both S. gordonii and P. gingivalis biofilms, and that these biofilms can be disrupted through the addition of antisera derived against their respective DNABII proteins. We provide evidence that both eDNA and DNABII proteins are limiting in S. gordonii but not in P. gingivalis biofilms. In addition, these proteins are capable of complementing one another functionally. We also found that whereas antisera derived against most DNABII proteins are capable of binding a wide variety of DNABII proteins, the P. gingivalis DNABII proteins are antigenically distinct. The presence of DNABII proteins in the EPS of these biofilms and the antigenic uniqueness of the P. gingivalis proteins provide an opportunity to develop therapies that are targeted to remove P. gingivalis and biofilms that contain P. gingivalis from the oral cavity.

The enduring importance of animal models in understanding periodontal disease

Whereas no single animal model can reproduce the complexity of periodontitis, different aspects of the disease can be addressed by distinct models. Despite their limitations, animal models are essential for testing the biological significance of in vitro findings and for establishing cause-and-effect relationships relevant to clinical observations, which are typically correlative. We provide evidence that animal-based studies have generated a durable framework for dissecting the mechanistic basis of periodontitis. These studies have solidified the etiologic role of bacteria in initiating the inflammatory response that leads to periodontal bone loss and have identified key mediators (IL-1, TNF, prostaglandins, complement, RANKL) that induce inflammatory breakdown. Moreover, animal studies suggest that dysbiosis, rather than individual bacterial species, are important in initiating periodontal bone loss and have introduced the concept that organisms previously considered commensals can play important roles as accessory pathogens or pathobionts. These studies have also provided insight as to how systemic conditions, such as diabetes or leukocyte adhesion deficiency, contribute to tissue destruction. In addition, animal studies have identified and been useful in testing therapeutic targets.

Going beyond the Control of Quorum-Sensing to Combat Biofilm Infections

Most bacteria attach to surfaces where they form a biofilm, cells embedded in a complex matrix of polymers. Cells in biofilms are much better protected against noxious agents than free-living cells. As a consequence it is very difficult to control pathogens with antibiotics in biofilm infections and novel targets are urgently needed. One approach aims at the communication between cells to form and to maintain a biofilm, a process called quorum-sensing. Water soluble small-sized molecules mediate this process and a number of antagonists of these compounds have been found. In this review natural compounds and synthetic drugs which do not interfere with the classical quorum-sensing compounds are discussed. For some of these compounds the targets are still not known, but others interfere with the formation of exopolysaccharides, virulence factors, or cell wall synthesis or they start an internal program of biofilm dispersal. Some of their targets are more conserved among pathogens than the receptors for quorum sensing autoinducers mediating quorum-sensing, enabling a broader application of the drug. The broad spectrum of mechanisms, the diversity of bioactive compounds, their activity against several targets, and the conservation of some targets among bacterial pathogens are promising aspects for several clinical applications of this type of biofilm-controlling compound in the future.

FOXO responses to Porphyromonas gingivalis in epithelial cells

Porphyromonas gingivalis is a prominent periodontal, and emerging systemic, pathogen that redirects host cell signalling pathways and modulates innate immune responses. In this study, we show that P. gingivalis infection induces the dephosphorylation and activation of forkhead box-O (FOXO)1, 3 and 4 in gingival epithelial cells. In addition, immunofluorescence showed that FOXO1 accumulated in the nucleus of P. gingivalis-infected cells. Quantitative reverse transcription PCR demonstrated that transcription of genes involved in protection against oxidative stress (Cat, Sod2, Prdx3), inflammatory responses (IL1β) and anti-apoptosis (Bcl-6) was induced by P. gingivalis, while small-interfering RNA (siRNA)-mediated knockdown of FOXO1 suppressed the transcriptional activation of these genes. P. gingivalis-induced secretion of interleukin (IL)-1β and inhibition of apoptosis were also impeded by FOXO1 knockdown. Neutralization of reactive oxygen species (ROS) by N-acetyl-l-cysteine blocked the activation of FOXO1 by P. gingivalis and concomitantly suppressed the activation of oxidative stress responses, anti-apoptosis programmes and IL-β production. Inhibition of c-Jun-N-terminal kinase (JNK) either pharmacologically or by siRNA, reduced FOXO1 activation and downstream FOXO1-dependent gene regulation in response to P. gingivalis. The results indicate that P. gingivalis-induced ROS activate FOXO transcription factors through JNK signalling, and that FOXO1 controls oxidative stress responses, inflammatory cytokine production and cell survival. These data position FOXO as an important signalling node in the epithelial cell-P. gingivalis interaction, with particular relevance to cell fate and dysbiotic host responses.

Solving the etiology of dental caries

For decades, the sugar-fermenting, acidogenic species Streptococcus mutans has been considered the main causative agent of dental caries and most diagnostic and therapeutic strategies have been targeted toward this microorganism. However, recent DNA- and RNA-based studies from carious lesions have uncovered an extraordinarily diverse ecosystem where S. mutans accounts only a tiny fraction of the bacterial community. This supports the concept that consortia formed by multiple microorganisms act collectively, probably synergistically, to initiate and expand the cavity. Thus, antimicrobial therapies are not expected to be effective in the treatment of caries and other polymicrobial diseases that do not follow classical Koch's postulates.

Polymicrobial synergy and dysbiosis in inflammatory disease

Uncontrolled inflammation of the periodontal area may arise when complex microbial communities transition from a commensal to a pathogenic entity. Communication among constituent species leads to polymicrobial synergy between metabolically compatible organisms that acquire functional specialization within the developing community. Keystone pathogens, even at low abundance, elevate community virulence, and the resulting dysbiotic community targets specific aspects of host immunity to further disable immune surveillance while promoting an overall inflammatory response. Inflammophilic organisms benefit from proteinaceous substrates derived from inflammatory tissue breakdown. Inflammation and dysbiosis reinforce each other, and the escalating environmental changes further select for a pathobiotic community. We have synthesized the polymicrobial synergy and dysbiotic components of the process into a new model for inflammatory diseases.