HmuY is an important virulence factor for Porphyromonas gingivalis growth in the heme-limited host environment and infection of macrophages

HemN2 Regulates the Virulence of Pseudomonas donghuensis HYS through 7-Hydroxytropolone Synthesis and Oxidative Stress

Compared to pathogens Pseudomonas aeruginosa and P. putida, P. donghuensis HYS has stronger virulence towards Caenorhabditis elegans. However, the underlying mechanisms haven't been fully understood. The heme synthesis system is essential for Pseudomonas virulence, and former studies of HemN have focused on the synthesis of heme, while the relationship between HemN and Pseudomonas virulence were barely pursued. In this study, we hypothesized that hemN2 deficiency affected 7-hydroxytropolone (7-HT) biosynthesis and redox levels, thereby reducing bacterial virulence. There are four hemN genes in P. donghuensis HYS, and we reported for the first time that deletion of hemN2 significantly reduced the virulence of HYS towards C. elegans, whereas the reduction in virulence by the other three genes was not significant. Interestingly, hemN2 deletion significantly reduced colonization of P. donghuensis HYS in the gut of C. elegans. Further studies showed that HemN2 was regulated by GacS and participated in the virulence of P. donghuensis HYS towards C. elegans by mediating the synthesis of the virulence factor 7-HT. In addition, HemN2 and GacS regulated the virulence of P. donghuensis HYS by affecting antioxidant capacity and nitrative stress. In short, the findings that HemN2 was regulated by the Gac system and that it was involved in bacterial virulence via regulating 7-HT synthesis and redox levels were reported for the first time. These insights may enlighten further understanding of HemN-based virulence in the genus Pseudomonas.

Hemophore-like proteins of the HmuY family in the oral and gut microbiome: unraveling the mystery of their evolution

SUMMARY

Heme (iron protoporphyrin IX, FePPIX) is the main source of iron and PPIX for host-associated pathogenic bacteria, including members of the Bacteroidota (formerly Bacteroidetes) phylum. Porphyromonas gingivalis, a keystone oral pathogen, uses a unique heme uptake (Hmu) system, comprising a hemophore-like protein, designated as the first member of the novel HmuY family. Compared to classical, secreted hemophores utilized by Gram-negative bacteria or near-iron transporter domain-based hemophores utilized by Gram-positive bacteria, the HmuY family comprises structurally similar proteins that have undergone diversification during evolution. The best characterized are P. gingivalis HmuY and its homologs from Tannerella forsythia (Tfo), Prevotella intermedia (PinO and PinA), Bacteroides vulgatus (Bvu), and Bacteroides fragilis (BfrA, BfrB, and BfrC). In contrast to the two histidine residues coordinating heme iron in P. gingivalis HmuY, Tfo, PinO, PinA, Bvu, and BfrA preferentially use two methionine residues. Interestingly, BfrB, despite conserved methionine residue, binds the PPIX ring without iron coordination. BfrC binds neither heme nor PPIX in keeping with the lack of conserved histidine or methionine residues used by other members of the HmuY family. HmuY competes for heme binding and heme sequestration from host hemoproteins with other members of the HmuY family to increase P. gingivalis competitiveness. The participation of HmuY in the host immune response confirms its relevance in relation to the survival of P. gingivalis and its ability to induce dysbiosis not only in the oral microbiome but also in the gut microbiome or other host niches, leading to local injuries and involvement in comorbidities.

Comparative analysis of Porphyromonas gingivalis A7436 and ATCC 33277 strains reveals differences in the expression of heme acquisition systems

Porphyromonas gingivalis strains exhibit different phenotypes in vitro, different virulence potential in animal models, and different associations with human diseases, with strains classified as virulent/more virulent (e.g., A7436 and W83) or as less virulent/avirulent (e.g., ATCC 33277). In this study, we comparatively analyzed the A7436 and ATCC 33277 strains to better understand their variability. Global gene expression analysis in response to heme and iron limitation revealed more pronounced differences in the A7436 than in the ATCC 33277 strain; however, in both strains, the largest changes were observed in genes encoding hypothetical proteins, genes whose products participate in energy metabolism, and in genes encoding proteins engaged in transport and binding proteins. Our results confirmed that variability between P. gingivalis strains is due to differences in the arrangement of their genomes. Analysis of gene expression of heme acquisition systems demonstrated that not only the availability of iron and heme in the external environment but also the ability to store iron intracellularly can influence the P. gingivalis phenotype. Therefore, we assume that differences in virulence potential may also be due to differences in the production of systems involved in iron and heme acquisition, mainly the Hmu system. In addition, our study showed that hemoglobin, in a concentration-dependent manner, differentially influences the virulence potential of P. gingivalis strains. We conclude that iron and heme homeostasis may add to the variability observed between P. gingivalis strains.

IMPORTANCE

Periodontitis belongs to a group of multifactorial diseases, characterized by inflammation and destruction of tooth-supporting tissues. P. gingivalis is one of the most important microbial factors involved in the initiation and progression of periodontitis. To survive in the host, the bacterium must acquire heme as a source of iron and protoporphyrin IX. P. gingivalis strains respond differently to changing iron and heme concentrations, which may be due to differences in the expression of systems involved in iron and heme acquisition. The ability to accumulate iron intracellularly, being different in more and less virulent P. gingivalis strains, may influence their phenotypes, production of virulence factors (including proteins engaged in heme acquisition), and virulence potential of this bacterium.

In vitro phenotypic evidence for the utilization of iron from different sources and siderophores production in the fish pathogen Tenacibaculum dicentrarchi

Iron uptake during infection is an essential pathogenicity factor of several bacteria, including Tenacibaculum dicentrarchi, an emerging pathogen for salmonid and red conger eel (Genypterus chilensis) farms in Chile. Iron-related protein families were recently found in eight T. dicentrarchi genomes, but biological studies have not yet confirmed functions. The investigation reported herein clearly demonstrated for the first time that T. dicentrarchi possesses different systems for iron acquisition-one involving the synthesis of siderophores and another allowing for the utilization of heme groups. Using 38 isolates of T. dicentrarchi and the type strain CECT 7612T , all strains grew in the presence of the chelating agent 2.2'-dipyridyl (from 50 to 150 μM) and produced siderophores on chrome azurol S plates. Furthermore, 37 of the 38 T. dicentrarchi isolates used at least four of the five iron sources (i.e. ammonium iron citrate, ferrous sulfate, iron chloride hexahydrate, haemoglobin and/or hemin) when added to iron-deficient media, although the cell yield was less when using hemin. Twelve isolates grew in the presence of hemin, and 10 of them used only 100 μM. Under iron-supplemented or iron-restricted conditions, whole cells of three isolates and the type strain showed at least one membrane protein induced in iron-limiting conditions (c.a. 37.9 kDa), regardless of the isolation host. All phenotypic results were confirmed by in-silico genomic T. dicentrarchi analysis. Future studies will aim to establish a relationship between iron uptake ability and virulence in T. dicentrarchi through in vivo assays.

Hemophore-like proteins produced by periodontopathogens are recognized by the host immune system and react differentially with IgG antibodies

Aims

Hemophore-like proteins sequester heme from host hemoproteins. We aimed to determine whether the host immune system can recognize not only Porphyromonas gingivalis HmuY but also its homologs expressed by other periodontopathogens, and how periodontitis influences the production of respective antibodies.

Methods

The reactivity of total bacterial antigens and purified proteins with serum IgG antibodies of 18 individuals with periodontitis and 17 individuals without periodontitis was examined by enzyme-linked immunosorbent assay (ELISA). To compare IgG reactivity between groups with and without periodontitis and within the various dilutions of sera, statistical analysis was performed using the Mann-Whitney U-test and two-way ANOVA test with the post-hoc Bonferroni test.

Results

Individuals with periodontitis produced IgG antibodies reacting more strongly not only with total P. gingivalis antigens (P = 0.0002; 1:400) and P. gingivalis HmuY (P = 0.0016; 1:100) but also with Prevotella intermedia PinA (P = 0.0059; 1:100), and with low efficiency with P. intermedia PinO (P = 0.0021; 1:100). No increase in the reactivity of IgG antibodies with Tannerella forsythia Tfo and P. gingivalis HusA was found in individuals with periodontitis.

Conclusions

Although hemophore-like proteins are structurally related, they are differentially recognized by the host immune system. Our findings point to specific antigens, mainly P. gingivalis HmuY and P. intermedia PinA, whose immunoreactivity could be further investigated to develop markers of periodontitis.

Interplay between Porphyromonas gingivalis Hemophore-Like Protein HmuY and Kgp/RgpA Gingipains Plays a Superior Role in Heme Supply

To acquire heme as a source of iron and protoporphyrin IX, Porphyromonas gingivalis uses gingipains, Hmu, and Hus systems. The aim of this study was to assess the correlation between the production and function of the most important virulence factors of P. gingivalis involved in heme supply, namely, hemophore-like proteins (HmuY and HusA) and gingipains. Respective mutant strains were used, and the expression of genes at the transcript and protein levels, as well as the importance of these genes' products for virulence potential, was examined. We found that HmuY and Kgp/RgpA gingipains are among the main P. gingivalis virulence factors synergistically engaged in heme supply. Their expression is related mainly when P. gingivalis grows in conditions rich in iron and heme sources, resembling those found in severe periodontitis. We confirmed that HmuY production is strictly dependent on the availability of heme and iron in the external environment, whereas we did not observe such dependence in the production of HusA. Moreover, we found that the HmuY protein can easily sequester heme from the HusA protein. The only correlation in the production of HmuY and HusA hemophore-like proteins could occur in P. gingivalis grown in conditions rich in iron and heme sources, mimicking an environment typical for severe periodontitis. Based on our observations, we suggest that HmuY is the major heme-binding protein produced by P. gingivalis, especially in iron- and heme-depleted conditions, typical for healthy periodontium and the initial stages of infection. The HusA protein could play a supporting role in P. gingivalis heme uptake. IMPORTANCE Altered or disturbed mutualism between oral microbiome members results in dysbiosis with local injuries and subsequently in systemic diseases. Periodontitis belongs to a group of multifactorial infectious diseases, characterized by inflammation and destruction of tooth-supporting tissues. Porphyromonas gingivalis is considered the main etiologic agent and keystone pathogen responsible for developing advanced periodontitis. As part of the infective process, P. gingivalis must acquire heme to survive and multiply at the infection site. Analysis of the mutual relationship between its main virulence factors showed that heme acquisition in P. gingivalis is a complex process in which mainly the Hmu system, with the leading role played by the HmuY hemophore-like protein, and Kgp and RgpA gingipains prefer cooperative interplay. It seems that the Hus system, including HusA hemophore-like protein, could be involved in another, so far uncharacterized, stage of iron and heme supply.

Growth of Porphyromonas gingivalis on human serum albumin triggers programmed cell death

Aims

Gingival crevicular fluid (GCF) constitutes the primary growth substrate for Porphyromonas gingivalis in vivo. The goal of this work was to evaluate the growth of different strains of P. gingivalis on human serum albumin (HSA), a major constituent of GCF.

Methods

Growth of five different strains of P. gingivalis in the HSA medium was examined and, surprisingly, three of the strains underwent autolysis within 24 h. Comparative transcriptomic analysis was used to identify genes involved in autolysis.

Results

Two highly related reference strains (W50 and W83) differed dramatically in their survival when grown on HSA. Strain W83 grew fast and lysed within 24 h, while W50 survived for an additional 20 h. Differential gene expression analysis led us to a gene cluster containing enzymes involved in arginine metabolism and a gene predicted to be lytic murein transglycosylase, which are known to play a role in autolysis. Deletion of this gene (PG0139) resulted in a mutant that did not lyse, and complementation restored the HSA lysis phenotype, indicating that this enzyme plays a central role in the autolysis of P. gingivalis.

Conclusions

P. gingivalis undergoes autolysis when provided with HSA as a substrate for growth.

Modulatory Mechanisms of Pathogenicity in Porphyromonas gingivalis and Other Periodontal Pathobionts

The pathogenesis of periodontitis depends on a sustained feedback loop where bacterial virulence factors and immune responses both contribute to inflammation and tissue degradation. Periodontitis is a multifactorial disease that is associated with a pathogenic shift in the oral microbiome. Within this shift, low-abundance Gram-negative anaerobic pathobionts transition from harmless colonisers of the subgingival environment to a virulent state that drives evasion and subversion of innate and adaptive immune responses. This, in turn, drives the progression of inflammatory disease and the destruction of tooth-supporting structures. From an evolutionary perspective, bacteria have developed this phenotypic plasticity in order to respond and adapt to environmental stimuli or external stressors. This review summarises the available knowledge of genetic, transcriptional, and post-translational mechanisms which mediate the commensal-pathogen transition of periodontal bacteria. The review will focus primarily on Porphyromonas gingivalis.

Flavobacterium columnare ferric iron uptake systems are required for virulence

Flavobacterium columnare, which causes columnaris disease, is one of the costliest pathogens in the freshwater fish-farming industry. The virulence mechanisms of F. columnare are not well understood and current methods to control columnaris outbreaks are inadequate. Iron is an essential nutrient needed for metabolic processes and is often required for bacterial virulence. F. columnare produces siderophores that bind ferric iron for transport into the cell. The genes needed for siderophore production have been identified, but other components involved in F. columnare iron uptake have not been studied in detail. We identified the genes encoding the predicted secreted heme-binding protein HmuY, the outer membrane iron receptors FhuA, FhuE, and FecA, and components of an ATP binding cassette (ABC) transporter predicted to transport ferric iron across the cytoplasmic membrane. Deletion mutants were constructed and examined for growth defects under iron-limited conditions and for virulence against zebrafish and rainbow trout. Mutants with deletions in genes encoding outer membrane receptors, and ABC transporter components exhibited growth defects under iron-limited conditions. Mutants lacking multiple outer membrane receptors, the ABC transporter, or HmuY retained virulence against zebrafish and rainbow trout mirroring that exhibited by the wild type. Some mutants predicted to be deficient in multiple steps of iron uptake exhibited decreased virulence. Survivors of exposure to such mutants were partially protected against later infection by wild-type F. columnare.

Immunoassay standardization for the detection of immunoglobulin A (IgA) against Porphyromonas gingivalis antigens in saliva of individuals with and without leprosy

Leprosy reactions are immune processes that cause neural damage in individuals with leprosy. As periodontitis is an infectious disease related to its development, specific antibodies to periodontal pathogens must be evaluated to better understand the humoral mechanisms underlying this relationship. Therefore, the objective of this study was to standardize an immunoassay to measure IgA specific to P. gingivalis antigens in the saliva of individuals with leprosy. An ELISA checkerboard titration was performed. A validation test involving 53 individuals with leprosy, 24 with and 19 without periodontitis, was conducted and a ROC curve constructed to calculate sensitivity and specificity. The coefficient of the optical densities was 2.21 and 2.66 for P. gingivalis crude extract and the recombinant protein HmuY, respectively. Sensitivity and specificity for the P. gingivalis crude extract were 66.7% and 73.7%, respectively, and for HmuY, were 62.5% and 52.6%, respectively. Specific recognition of P. gingivalis occurred predominantly in individuals with periodontitis, which validates the use of this test for studying periodontitis in individuals with leprosy.

Trial registration CAEE 64476117.3.0000.0049, 21/07/2017, retrospectively registered

Glycation of Host Proteins Increases Pathogenic Potential of Porphyromonas gingivalis

The non-enzymatic addition of glucose (glycation) to circulatory and tissue proteins is a ubiquitous pathophysiological consequence of hyperglycemia in diabetes. Given the high incidence of periodontitis and diabetes and the emerging link between these conditions, it is of crucial importance to define the basic virulence mechanisms employed by periodontopathogens such as Porphyromonas gingivalis in mediating the disease process. The aim of this study was to determine whether glycated proteins are more easily utilized by P. gingivalis to stimulate growth and promote the pathogenic potential of this bacterium. We analyzed the properties of three commonly encountered proteins in the periodontal environment that are known to become glycated and that may serve as either protein substrates or easily accessible heme sources. In vitro glycated proteins were characterized using colorimetric assays, mass spectrometry, far- and near-UV circular dichroism and UV–visible spectroscopic analyses and SDS-PAGE. The interaction of glycated hemoglobin, serum albumin and type one collagen with P. gingivalis cells or HmuY protein was examined using spectroscopic methods, SDS-PAGE and co-culturing P. gingivalis with human keratinocytes. We found that glycation increases the ability of P. gingivalis to acquire heme from hemoglobin, mostly due to heme sequestration by the HmuY hemophore-like protein. We also found an increase in biofilm formation on glycated collagen-coated abiotic surfaces. We conclude that glycation might promote the virulence of P. gingivalis by making heme more available from hemoglobin and facilitating bacterial biofilm formation, thus increasing P. gingivalis pathogenic potential in vivo.

Porphyromonas gingivalis HmuY and Bacteroides vulgatus Bvu-A Novel Competitive Heme Acquisition Strategy

Human oral and gut microbiomes are crucial for maintenance of homeostasis in the human body. Porphyromonas gingivalis, the key etiologic agent of chronic periodontitis, can cause dysbiosis in the mouth and gut, which results in local and systemic infectious inflammatory diseases. Our previous work resulted in extensive biochemical and functional characterization of one of the major P. gingivalis heme acquisition systems (Hmu), with the leading role played by the HmuY hemophore-like protein. We continued our studies on the homologous heme acquisition protein (Bvu) expressed by Bacteroides vulgatus, the dominant species of the gut microbiome. Results from spectrophotometric experiments showed that Bvu binds heme preferentially under reducing conditions using Met145 and Met172 as heme iron-coordinating ligands. Bvu captures heme bound to human serum albumin and only under reducing conditions. Importantly, HmuY is able to sequester heme complexed to Bvu. This is the first study demonstrating that B. vulgatus expresses a heme-binding hemophore-like protein, thus increasing the number of members of a novel HmuY-like family. Data gained in this study confirm the importance of HmuY in the context of P. gingivalis survival in regard to its ability to cause dysbiosis also in the gut microbiome.

Prevotella intermedia produces two proteins homologous to Porphyromonas gingivalis HmuY but with different heme coordination mode

As part of the infective process, Porphyromonas gingivalis must acquire heme which is indispensable for life and enables the microorganism to survive and multiply at the infection site. This oral pathogenic bacterium uses a newly discovered novel hmu heme uptake system with a leading role played by the HmuY hemophore-like protein, responsible for acquiring heme and increasing virulence of this periodontopathogen. We demonstrated that Prevotella intermedia produces two HmuY homologs, termed PinO and PinA. Both proteins were produced at higher mRNA and protein levels when the bacterium grew under low-iron/heme conditions. PinO and PinA bound heme, but preferentially under reducing conditions, and in a manner different from that of the P. gingivalis HmuY. The analysis of the three-dimensional structures confirmed differences between apo-PinO and apo-HmuY, mainly in the fold forming the heme-binding pocket. Instead of two histidine residues coordinating heme iron in P. gingivalis HmuY, PinO and PinA could use one methionine residue to fulfill this function, with potential support of additional methionine residue/s. The P. intermedia proteins sequestered heme only from the host albumin-heme complex under reducing conditions. Our findings suggest that HmuY-like family might comprise proteins subjected during evolution to significant diversification, resulting in different heme coordination modes. The newer data presented in this manuscript on HmuY homologs produced by P. intermedia sheds more light on the novel mechanism of heme uptake, could be helpful in discovering their biological function, and in developing novel therapeutic approaches.

PgRsp Is a Novel Redox-Sensing Transcription Regulator Essential for Porphyromonas gingivalis Virulence

Porphyromonas gingivalis is one of the etiological agents of chronic periodontitis. Both heme and oxidative stress impact expression of genes responsible for its survival and virulence. Previously we showed that P. gingivalis ferric uptake regulator homolog affects expression of a gene encoding a putative Crp/Fnr superfamily member, termed P. gingivalis redox-sensing protein (PgRsp). Although PgRsp binds heme and shows the highest similarity to proteins assigned to the CooA family, it could be a member of a novel, separate family of proteins with unknown function. Expression of the pgrsp gene is autoregulated and iron/heme dependent. Genes encoding proteins engaged in the oxidative stress response were upregulated in the pgrsp mutant (TO11) strain compared with the wild-type strain. The TO11 strain showed higher biomass production, biofilm formation, and coaggregation ability with Tannerella forsythia and Prevotella intermedia. We suggest that PgRsp may regulate production of virulence factors, proteases, Hmu heme acquisition system, and FimA protein. Moreover, we observed growth retardation of the TO11 strain under oxidative conditions and decreased survival ability of the mutant cells inside macrophages. We conclude that PgRsp protein may play a role in the oxidative stress response using heme as a ligand for sensing changes in redox status, thus regulating the alternative pathway of the oxidative stress response alongside OxyR.

Porphyromonas gingivalis PgFur Is a Member of a Novel Fur Subfamily With Non-canonical Function

Porphyromonas gingivalis, a keystone pathogen of chronic periodontitis, uses ferric uptake regulator homolog (PgFur) to regulate production of virulence factors. This study aimed to characterize PgFur protein in regard to its structure-function relationship. We experimentally identified the 5′ mRNA sequence encoding the 171-amino-acid-long PgFur protein in the A7436 strain and examined this PgFur version as a full-length protein. PgFur protein did not bind to the canonical Escherichia coli Fur box, but the wild-type phenotype of the mutant Δpgfur strain was restored partially when expression of the ecfur gene was induced from the native pgfur promoter. The full-length PgFur protein contained one zinc atom per protein monomer, but did not bind iron, manganese, or heme. Single cysteine substitutions of CXXC motifs resulted in phenotypes similar to the mutant Δpgfur strain. The modified proteins were produced in E. coli at significantly lower levels, were highly unstable, and did not bind zinc. The pgfur gene was expressed at the highest levels in bacteria cultured for 24 h in the absence of iron and heme or at higher levels in bacteria cultured for 10 h in the presence of protoporphyrin IX source. No influence of high availability of Fe²⁺, Zn²⁺, or Mn²⁺ on pgfur gene expression was observed. Two chromosomal mutant strains producing protein lacking 4 (pgfurΔ4aa) or 13 (pgfurΔ13aa) C-terminal amino acid residues were examined in regard to importance of the C-terminal lysine-rich region. The pgfurΔ13aa strain showed a phenotype typical for the mutant Δpgfur strain, but both the wild-type PgFur protein and its truncated version bound zinc with similar ability. The Δpgfur mutant strain produced higher amounts of HmuY protein compared with the wild-type strain, suggesting compromised regulation of its expression. Potential PgFur ligands, Fe²⁺, Mn²⁺, Zn²⁺, PPIX, or serum components, did not influence HmuY production in the Δpgfur mutant strain. The mutant pgfurΔ4aa and pgfurΔ13aa strains exhibited affected HmuY protein production. PgFur, regardless of the presence of the C-terminal lysine-rich region, bound to the hmu operon promoter. Our data suggest that cooperation of PgFur with partners/cofactors and/or protein/DNA modifications would be required to accomplish its role played in an in vivo multilayer regulatory network.

PgFur participates differentially in expression of virulence factors in more virulent A7436 and less virulent ATCC 33277 Porphyromonas gingivalis strains

Background

Porphyromonas gingivalis is considered a keystone pathogen responsible for chronic periodontitis. Although several virulence factors produced by this bacterium are quite well characterized, very little is known about regulatory mechanisms that allow different strains of P. gingivalis to efficiently survive in the hostile environment of the oral cavity, a typical habitat characterized by low iron and heme concentrations. The aim of this study was to characterize P. gingivalis Fur homolog (PgFur) in terms of its role in production of virulence factors in more (A7436) and less (ATCC 33277) virulent strains.

Results

Expression of a pgfur depends on the growth phase and iron/heme concentration. To better understand the role played by the PgFur protein in P. gingivalis virulence under low- and high-iron/heme conditions, a pgfur-deficient ATCC 33277 strain (TO16) was constructed and its phenotype compared with that of a pgfur A7436-derived mutant strain (TO6). In contrast to the TO6 strain, the TO16 strain did not differ in the growth rate and hemolytic activity compared with the ATCC 33277 strain. However, both mutant strains were more sensitive to oxidative stress and they demonstrated changes in the production of lysine- (Kgp) and arginine-specific (Rgp) gingipains. In contrast to the wild-type strains, TO6 and TO16 mutant strains produced larger amounts of HmuY protein under high iron/heme conditions. We also demonstrated differences in production of glycoconjugates between the A7436 and ATCC 33277 strains and we found evidence that PgFur protein might regulate glycosylation process. Moreover, we revealed that PgFur protein plays a role in interactions with other periodontopathogens and is important for P. gingivalis infection of THP-1-derived macrophages and survival inside the cells. Deletion of the pgfur gene influences expression of many transcription factors, including two not yet characterized transcription factors from the Crp/Fnr family. We also observed lower expression of the CRISPR/Cas genes.

Conclusions

We show here for the first time that inactivation of the pgfur gene exerts a different influence on the phenotype of the A7436 and ATCC 33277 strains. Our findings further support the hypothesis that PgFur regulates expression of genes encoding surface virulence factors and/or genes involved in their maturation.

Electronic supplementary material

The online version of this article (10.1186/s12866-019-1511-x) contains supplementary material, which is available to authorized users.

Heme Uptake and Utilization by Gram-Negative Bacterial Pathogens

Iron is a transition metal utilized by nearly all forms of life for essential cellular processes, such as DNA synthesis and cellular respiration. During infection by bacterial pathogens, the host utilizes various strategies to sequester iron in a process termed, nutritional immunity. To circumvent these defenses, Gram-negative pathogens have evolved numerous mechanisms to obtain iron from heme. In this review we outline the systems that exist in several Gram-negative pathogens that are associated with heme transport and utilization, beginning with hemolysis and concluding with heme degradation. In addition, Gram-negative pathogens must also closely regulate the intracellular concentrations of iron and heme, since high levels of iron can lead to the generation of toxic reactive oxygen species. As such, we also provide several examples of regulatory pathways that control heme utilization, showing that co-regulation with other cellular processes is complex and often not completely understood.

Iron Acquisition in Mycobacterium tuberculosis

The highly contagious disease tuberculosis (TB) is caused by the bacterium Mycobacterium tuberculosis (Mtb), which has been evolving drug resistance at an alarming rate. Like all human pathogens, Mtb requires iron for growth and virulence. Consequently, Mtb iron transport is an emerging drug target. However, the development of anti-TB drugs aimed at these metabolic pathways has been restricted by the dearth of information on Mtb iron acquisition. In this Review, we describe the multiple strategies utilized by Mtb to acquire ferric iron and heme iron. Mtb iron uptake is a complex process, requiring biosynthesis and subsequent export of Mtb siderophores, followed by ferric iron scavenging and ferric-siderophore import into Mtb. Additionally, Mtb possesses two possible heme uptake pathways and an Mtb-specific mechanism of heme degradation that yields iron and novel heme-degradation products. We conclude with perspectives for potential therapeutics that could directly target Mtb heme and iron uptake machineries. We also highlight how hijacking Mtb heme and iron acquisition pathways for drug import may facilitate drug transport through the notoriously impregnable Mtb cell wall.

Tannerella forsythia Tfo belongs to Porphyromonas gingivalis HmuY-like family of proteins but differs in heme-binding properties

Porphyromonas gingivalis is considered the principal etiologic agent and keystone pathogen of chronic periodontitis. As an auxotrophic bacterium, it must acquire heme to survive and multiply at the infection site. P. gingivalis HmuY is the first member of a novel family of hemophore-like proteins. Bacterial heme-binding proteins usually use histidine-methionine or histidine-tyrosine residues to ligate heme-iron, whereas P. gingivalis HmuY uses two histidine residues. We hypothesized that other 'red complex' members, i.e. Tannerella forsythia and Treponema denticola might utilize similar heme uptake mechanisms to the P. gingivalis HmuY. Comparative and phylogenetic analyses suggested differentiation of HmuY homologs and low conservation of heme-coordinating histidine residues present in HmuY. The homologs were subjected to duplication before divergence of Bacteroidetes lineages, which could facilitate evolution of functional diversification. We found that T. denticola does not code an HmuY homolog. T. forsythia protein, termed as Tfo, binds heme, but preferentially in the ferrous form, and sequesters heme from the albumin-heme complex under reducing conditions. In agreement with that, the 3D structure of Tfo differs from that of HmuY in the folding of heme-binding pocket, containing two methionine residues instead of two histidine residues coordinating heme in HmuY. Heme binding to apo-HmuY is accompanied by movement of the loop carrying the His¹⁶⁶ residue, closing the heme-binding pocket. Molecular dynamics simulations (MD) demonstrated that this conformational change also occurs in Tfo. In conclusion, our findings suggest that HmuY-like family might comprise proteins subjected during evolution to significant diversification, resulting in different heme-binding properties.

Identification and Characterization of MicroRNA Differentially Expressed in Macrophages Exposed to Porphyromonas gingivalis Infection

MicroRNAs (miRNAs) are short, noncoding RNAs involved in the regulation of several processes associated with inflammatory diseases and infection. Bacterial infection modulates miRNA expression to subvert any innate immune response. In this study we analyzed, using microarray analysis, the bacterial modulation of miRNAs in bone marrow-derived macrophages (BMMs) in which activity was induced by infection with Porphyromonas gingivalis The expression of several miRNAs was modulated 3 h postinfection (at a multiplicity of infection of 25). A bioinformatic analysis was performed to further identify pathways related to the innate immune host response under the influence of selected miRNAs. To assess the effects of the miRNAs identified on cytokine secretion (tumor necrosis factor alpha [TNF-α] and interleukin-10 [IL-10]), BMMs were transfected with selected miRNA mimics and inhibitors. Transfection with mmu-miR-155 and mmu-miR-2137 did not modify TNF-α secretion, while their inhibitors increased it. Inhibitors of mmu-miR-2137 and mmu-miR-7674 increased the secretion of the anti-inflammatory factor IL-10. In P. gingivalis-infected BMMs, mmu-miR-155-5p significantly decreased TNF-α secretion while inhibitor of mmu-miR-2137 increased IL-10 secretion. In vivo, in a mouse model of P. gingivalis-induced calvarial bone resorption, injection of mmu-miR-155-5p or anti-mmu-miR-2137 reduced the size of the lesion significantly. Furthermore, anti-mmu-miR-2137 significantly reduced inflammatory cell infiltration, osteoclast activity, and bone loss. Bioinformatic analysis demonstrated that pathways related to cytokine- and chemokine-related pathways but also osteoclast differentiation may be involved in the effects observed. This study contributes further to our understanding of P. gingivalis-induced modulation of miRNAs and their physiological effects. It highlights the potential therapeutic merits of targeting mmu-miR-155-5p and mmu-miR-2137 to control inflammation induced by P. gingivalis infection.

Antimicrobial activity of stable hemiaminals against Porphyromonas gingivalis

Porphyromonas gingivalis is a major etiologic agent and a key pathogen responsible for the development and progression of chronic periodontitis. Controlling the number of periodontal pathogens is one of the primary actions for maintaining oral health; therefore, active compounds with a capacity to exert antimicrobial activity have received considerable attention as they may represent potential new therapeutic agents for the treatment of chronic periodontitis. Heterocyclic compounds possessing 1,2,4- or 1,2,3-triazoles are known for several biological activities, including antibacterial properties. Among them are stable hemiaminals which can be obtained in reaction between nitrobenzaldehyde derivatives and 4-amino-1,2,4-triazole or 4-amino-3,5-dimethyl-1,2,4-triazole. In this study, we selected two relatively stable hemiaminals: (2,4-dinitrophenyl)(4H-1,2,4-triazole-4-ylamino)methanol (24DNTAM) and (2,4-dinitrophenyl)(4H-3,5-dimethyl-1,2,4-triazole-4-ylamino)methanol (24DNDMTAM). Both compounds showed promising anti-P. gingivalis activity, higher against ATCC 33277 strain as compared to A7436 strain. The lowest hemiaminal concentration inhibiting visible planktonic bacterial growth under high-iron/heme conditions was ∼0.06 mg/ml, and the lowest hemiaminal concentration showing killing of bacteria was ∼0.25 mg/ml. Antimicrobial activity was also observed against P. gingivalis grown on blood agar plates. Slightly higher antimicrobial activity of both compounds was observed when P. gingivalis was grown in co-cultures with epithelial HeLa cells under low-iron/heme conditions, which mimic those occurring in vivo. 24DNTAM was more effective against P. gingivalis, but exhibited higher cytotoxic activity against epithelial and red blood cells, as compared with 24DNDMTAM. We conclude that both hemiaminals might originate a novel group of biologically important molecules.

SigCH, an extracytoplasmic function sigma factor of Porphyromonas gingivalis regulates the expression of cdhR and hmuYR

Extracytoplasmic function (ECF) sigma factors play an important role in the bacterial response to various environmental stresses. Porphyromonas gingivalis, a prominent etiological agent in human periodontitis, possesses six putative ECF sigma factors. So far, information is limited on the ECF sigma factor, PGN_0319. The aim of this study was to investigate the role of PGN_0319 (SigCH) of P. gingivalis, focusing on the regulation of hmuY and hmuR, which encode outer-membrane proteins involved in hemin utilization, and cdhR, a transcriptional regulator of hmuYR. First, we evaluated the gene expression profile of the sigCH mutant by DNA microarray. Among the genes with altered expression levels, those involved in hemin utilization were downregulated in the sigCH mutant. To verify the microarray data, quantitative reverse transcription PCR analysis was performed. The RNA samples used were obtained from bacterial cells grown to early-log phase, in which sigCH expression in the wild type was significantly higher than that in mid-log and late-log phases. The expression levels of hmuY, hmuR, and cdhR were significantly decreased in the sigCH mutant compared to wild type. Transcription of these genes was restored in a sigCH complemented strain. Compared to the wild type, the sigCH mutant showed reduced growth in log phase under hemin-limiting conditions. Electrophoretic mobility shift assays showed that recombinant SigCH protein bound to the promoter region of hmuY and cdhR. These results suggest that SigCH plays an important role in the early growth of P. gingivalis, and directly regulates cdhR and hmuYR, thereby playing a potential role in the mechanisms of hemin utilization by P. gingivalis.

Role of Porphyromonas gingivalis HmuY in Immunopathogenesis of Chronic Periodontitis

Periodontitis is a multifactorial disease, with participation of bacterial, environmental, and host factors. It results from synergistic and dysbiotic multispecies microorganisms, critical "keystone pathogens," affecting the whole bacterial community. The purpose of this study was to review the role of Porphyromonas gingivalis in the immunopathogenesis of chronic periodontitis, with special attention paid to HmuY. The host response during periodontitis involves the innate and adaptive immune system, leading to chronic inflammation and progressive destruction of tooth-supporting tissues. In this proinflammatory process, the ability of P. gingivalis to evade the host immune response and access nutrients in the microenvironment is directly related to its survival, proliferation, and infection. Furthermore, heme is an essential nutrient for development of these bacteria, and HmuY is responsible for its capture from host heme-binding proteins. The inflammatory potential of P. gingivalis HmuY has been shown, including induction of high levels of proinflammatory cytokines and CCL2, decreased levels of IL-8, and increased levels of anti-HmuY IgG and IgG1 antibodies in individuals with chronic periodontitis. Therefore, the HmuY protein might be a promising target for therapeutic strategies and for development of diagnostic methods in chronic periodontitis, especially in the case of patients with chronic periodontitis not responding to treatment, monitoring, and maintenance therapy.

Heme acquisition mechanisms of Porphyromonas gingivalis - strategies used in a polymicrobial community in a heme-limited host environment

Porphyromonas gingivalis, a main etiologic agent and key pathogen responsible for initiation and progression of chronic periodontitis requires heme as a source of iron and protoporphyrin IX for its survival and the ability to establish an infection. Porphyromonas gingivalis is able to accumulate a defensive cell-surface heme-containing pigment in the form of μ-oxo bisheme. The main sources of heme for P. gingivalis in vivo are hemoproteins present in saliva, gingival crevicular fluid, and erythrocytes. To acquire heme, P. gingivalis uses several mechanisms. Among them, the best characterized are those employing hemagglutinins, hemolysins, and gingipains (Kgp, RgpA, RgpB), TonB-dependent outer-membrane receptors (HmuR, HusB, IhtA), and hemophore-like proteins (HmuY, HusA). Proteins involved in intracellular heme transport, storage, and processing are less well characterized (e.g. PgDps). Importantly, P. gingivalis may also use the heme acquisition systems of other bacteria to fulfill its own heme requirements. Porphyromonas gingivalis displays a novel paradigm for heme acquisition from hemoglobin, whereby the Fe(II)-containing oxyhemoglobin molecule must first be oxidized to methemoglobin to facilitate heme release. This process not only involves P. gingivalis arginine- and lysine-specific gingipains, but other proteases (e.g. interpain A from Prevotella intermedia) or pyocyanin produced by Pseudomonas aeruginosa. Porphyromonas gingivalis is then able to fully proteolyze the more susceptible methemoglobin substrate to release free heme or to wrest heme from it directly through the use of the HmuY hemophore.