Characterization and identification of a novel candidate vaccine protein through systematic analysis of extracellular proteins of Erysipelothrix rhusiopathiae

Population Structure and Genomic Characteristics of Australian Erysipelothrix rhusiopathiae Reveals Unobserved Diversity in the Australian Pig Industry

Erysipelothrix rhusiopathiae is a bacterial pathogen that is the causative agent of erysipelas in a variety of animals, including swine, emus, turkeys, muskox, caribou, moose, and humans. This study aims to investigate the population structure and genomic features of Australian isolates of E. rhusiopathiae in the Australian pig industry and compare them to the broader scope of isolates worldwide. A total of 178 isolates (154 Australian, seven vaccine isolates, six international isolates, and 11 of unknown origin) in this study were screened against an MLST scheme and publicly available reference isolates, identifying 59 new alleles, with isolates separating into two main single locus variant groups. Investigation with BLASTn revealed the presence of the spaA gene in 171 (96%) of the isolates, with three main groups of SpaA protein sequences observed amongst the isolates. Novel SpaA protein sequences, categorised here as group 3 sequences, consisted of two sequence types forming separate clades to groups 1 and 2, with amino acid variants at positions 195 (D/A), 303 (G/E) and 323(P/L). In addition to the newly identified groups, five new variant positions were identified, 124 (S/N), 307 (Q/R), 323 (P/L), 379 (M/I), and 400 (V/I). Resistance screening identified genes related to lincomycin, streptomycin, erythromycin, and tetracycline resistance. Of the 29 isolates carrying these resistance genes, 82% belonged to SpaA group 2-N101S (n = 22) or 2-N101S-I257L (n = 2). In addition, 79% (n = 23) of these 29 isolates belonged to MLST group ST 5. Our results illustrate that Australia appears to have a unique diversity of E. rhusiopathiae isolates in pig production industries within the wider global context of isolates.

The C-Terminal Repeat Units of SpaA Mediate Adhesion of Erysipelothrix rhusiopathiae to Host Cells and Regulate Its Virulence

Simple Summary

Erysipelothrix rhusiopathiae is an important zoonotic pathogen, which poses a serious harm to the pig industry. We aimed to evaluate the genomic differences between virulent and avirulent strains to study the pathogenic mechanism of Erysipelothrix rhusiopathiae. The results showed that the spaA gene of avirulent strain lacked 120bp, encoding repeat units at the C-terminal of SpaA, the virulence of the virulent strain with this 120 bp deletion was attenuated, and the mutant strain decreased adhesion to porcine iliac artery endothelial cells.

Abstract

Erysipelothrix rhusiopathiae is a causative agent of erysipelas in animals and erysipeloid in humans. However, current information regarding E. rhusiopathiae pathogenesis remains limited. Previously, we identified two E. rhusiopathiae strains, SE38 and G4T10, which were virulent and avirulent in pigs, respectively. Here, to further study the pathogenic mechanism of E. rhusiopathiae, we sequenced and assembled the genomes of strains SE38 and G4T10, and performed a comparative genomic analysis to identify differences or mutations in virulence-associated genes. Next, we comparatively analyzed 25 E. rhusiopathiae virulence-associated genes in SE38 and G4T10. Compared with that of SE38, the spaA gene of the G4T10 strain lacked 120 bp, encoding repeat units at the C-terminal of SpaA. To examine whether these deletions or splits influence E. rhusiopathiae virulence, these 120 bp were successfully deleted from the spaA gene in strain SE38 by homologous recombination. The mutant strain ΔspaA displayed attenuated virulence in mice and decreased adhesion to porcine iliac artery endothelial cells, which was also observed using the corresponding mutant protein SpaA’. Our results demonstrate that SpaA-mediated adhesion between E. rhusiopathiae and host cells is dependent on its C-terminal repeat units.

Designing of a Recombinant Multi-Epitopes Based Vaccine against Enterococcus mundtii Using Bioinformatics and Immunoinformatics Approaches

Enterococcus species are an emerging group of bacterial pathogens that have a significant role in hospital-associated infections and are associated with higher mortality and morbidity rates. Among these pathogens, Enterococcus mundtii is one of the causative agents of multiple hospital associated infections. Currently, no commercially available licensed vaccine is present, and multi-drug resistant strains of the pathogen are prominent. Due to several limitations of experimental vaccinology, computational vaccine designing proved to be helpful in vaccine designing against several bacterial pathogens. Herein, we designed a multi-epitope-based vaccine against E. mundtii using in silico approaches. After an in-depth analysis of the core genome, three probable antigenic proteins (lytic polysaccharide monooxygenase, siderophore ABC transporter substrate-binding protein, and lytic polysaccharide monooxygenase) were shortlisted for epitope prediction. Among predicted epitopes, ten epitopes-GPADGRIAS, TTINHGGAQA, SERTALSVTT, GDGGNGGGEV, GIKEPDLEK, KQADDRIEA, QAIGGDTSN, EPLDEQTASR, AQWEPQSIEA, QPLKFSDFEL-were selected for multi-epitope vaccine construct designing. The screened B- and T-cell epitopes were joined with each other via specific linkers and linked to the cholera toxin B subunit as an adjuvant to enhance vaccine immune protection efficacy. The designed vaccine construct induced cellular and humoral immune responses. Blind docking with immune cell receptors, followed by molecular dynamic simulation results confirms the good binding potency and stability of the vaccine in providing protection against the pathogen.

Evaluation of immunogenicity and protection mediated by Lawsonia intracellularis subunit vaccines

Lawsonia intracellularis is an economically important bacterium that causes ileitis in pigs. Current vaccines for L. intracellularis do not allow for differentiation between infected and vaccinated animals (DIVA), which is beneficial for disease tracking and surveillance. Previously, we identified five putative surface L. intracellularis proteins that were targeted by antibodies from pigs infected with L. intracellularis which could serve as antigens in a subunit vaccine. We conducted two trials to determine whether these antigens were immunogenic and provided protection against infectious challenge and whether truncated glycoprotein D could be used as a DIVA antigen. For Trial 1, 5 week-old piglets were administered intramuscular monovalent vaccines comprised of a recombinant (r) flagella subunit protein (rFliC,) and DIVA antigen (truncated glycoprotein D (TgD), a herpes virus antigen) both formulated with a combination adjuvant consisting of polyinosinic:polycytidylic acid(poly I:C), host defense peptide 1002 and polyphosphazene, referred to as Triple Adjuvant (TriAdj). Relative to control animals, animals vaccinated with rFliC and rTgD had significantly elevated antigen-specific humoral immunity in sera suggesting that rFliC and TgD are immunogenic. Control animals had negligible anti-TgD titres suggesting that TgD may be a suitable DIVA antigen for pigs. For Trial 2, piglets were immunized with a trivalent vaccine (FOG vaccine consisting of rFLiC, rOppA protein (a ABC Type dipeptide transport system) and rGroEL (a stress response protein)) and a divalent vaccine (CM vaccine consisting of rClpP (an ATP-dependent Clp protease proteolytic subunit) and rMetK (a S-adenosyl methionine synthase)) formulated with Emulsigen®. Relative to the control pigs, pigs immunized with the FOG vaccine produced robust and significantly higher serum IgG antibodies against rFliC and rGroEL, and significantly higher anti-FliC and anti-GroEL IgA antibodies in jejunal (GroEL only) and ileal intestinal mucosa. Pigs immunized with CM vaccine produced significantly higher serum antibodies against rClpP and rMetK and significantly higher anti-rClpP IgA antibodies in the ileum relative to the control pigs. Quantitative polymerase chain reaction (qPCR) analysis showed that 18 days after challenge with infectious L. intracellularis, challenged/control pigs and pigs that received the CM vaccine, but not the pigs vaccinated with the FOG vaccine, shed significantly more bacteria in feces than the unchallenged controls pigs. These data suggest that the FOG vaccinated pigs showed limited protection. While promising, more work is needed to enhance the efficiency of the intramuscular vaccine to show significant disease protection.

Erysipelothrix Spp.: Past, Present, and Future Directions in Vaccine Research

Erysipelothrix spp. comprise a group of small Gram-positive bacteria that can infect a variety of hosts including mammals, fish, birds, reptiles and insects. Among the eight Erysipelothrix species that have been described to date, only Erysipelothrix rhusiopathiae plays a major role in farmed livestock where it is the causative agent of erysipelas. E. rhusiopathiae also has zoonotic potential and can cause erysipeloid in humans with a clear occupational link to meat and fish industries. While there are 28 known Erysipelothrix serovars, over 80% of identified isolates belong to serovars 1 or 2. Vaccines to protect pigs against E. rhusiopathiae first became available in 1883 as a response to an epizootic of swine erysipelas in southern France. The overall vaccine repertoire was notably enlarged between the 1940s and 1960s following major outbreaks of swine erysipelas in the Midwest USA and has changed little since. Traditionally, E. rhusiopathiae serovar 1a or 2 isolates were inactivated (bacterins) or attenuated and these types of vaccines are still used today on a global basis. E. rhusiopathiae vaccines are most commonly used in pigs, poultry, and sheep where the bacterium can cause considerable economic losses. In addition, erysipelas vaccination is also utilized in selected vulnerable susceptible populations, such as marine mammals in aquariums, which are commonly vaccinated at regular intervals. While commercially produced erysipelas vaccines appear to provide good protection against clinical disease, in recent years there has been an increase in perceived vaccine failures in farmed animals, especially in organic outdoor operations. Moreover, clinical erysipelas outbreaks have been reported in animal populations not previously considered at risk. This has raised concerns over a possible lack of vaccine protection across various production species. This review focuses on summarizing the history and the present status of E. rhusiopathiae vaccines, the current knowledge on protection including surface antigens, and also provides an outlook into future directions for vaccine development.

Genomic and Immunogenic Protein Diversity of Erysipelothrix rhusiopathiae Isolated From Pigs in Great Britain: Implications for Vaccine Protection

Erysipelas, caused by the bacterium Erysipelothrix rhusiopathiae, is re-emerging in swine and poultry production systems worldwide. While the global genomic diversity of this species has been characterized, how much of this genomic and functional diversity is maintained at smaller scales is unclear. Specifically, while several key immunogenic surface proteins have been identified for E. rhusiopathiae, little is known about their presence among field strains and their divergence from vaccines, which could result in vaccine failure. Here, a comparative genomics approach was taken to determine the diversity of E. rhusiopathiae strains in pigs in Great Britain over nearly three decades, as well as to assess the field strains’ divergence from the vaccine strain most commonly used in British pigs. In addition, the presence/absence and variability of 13 previously described immunogenic surface proteins was determined, including SpaA which is considered a key immunogen. We found a high diversity of E. rhusiopathiae strains in British pigs, similar to the situation described in European poultry but in contrast to swine production systems in Asia. Of the four clades of E. rhusiopathiae found globally, three were represented among British pig isolates, with Clade 2 being the most common. All British pig isolates had one amino acid difference in the immunoprotective domain of the SpaA protein compared to the vaccine strain. However, we were able to confirm using in silico structural protein analyses that this difference is unlikely to compromise vaccine protection. Of 12 other known immunogenic surface proteins of E. rhusiopathiae examined, 11 were found to be present in all British pig isolates and the vaccine strain, but with highly variable degrees of conservation at the amino acid sequence level, ranging from 0.3 to 27% variant positions. Moreover, the phylogenetic incongruence of these proteins suggests that horizontal transfer of genes encoding for antigens is commonplace for this bacterium. We hypothesize that the sequence variants in these proteins could be responsible for differences in the efficacy of the immune response. Our results provide the necessary basis for testing this hypothesis through in vitro and in vivo studies.

The Formation, Persistence, and Resistance to Disinfectant of the Erysipelothrix piscisicarius Biofilm

Erysipelothrix piscisicarius is an emergent pathogen in fish aquaculture, particularly in the ornamental fish trade. Very little is known on the biology of this pathogen; however, the recurrence of infection and disease outbreaks after removing the fish from a system and disinfecting the tank suggest its environmental persistence. Moreover, biofilm lifestyle in E. piscisicarius has been suspected but not previously shown. The purpose of this study was to investigate the formation of biofilms on an abiotic surface in Erysipelothrix spp. We used hydroxyapatite-coated plastic pegs to demonstrate the attachment, growth, and persistence of E. piscisicarius on abiotic surfaces in both fresh and marine environments and to investigate the susceptibility of this pathogen to different disinfectants that are used in the aquaculture industry. E. piscisicarius formed biofilms that persisted significantly longer than planktonic cells did in both freshwater and saltwater over a period of 120 h (P = 0.004). The biofilms were also more resistant to disinfectants than the planktonic cells were. Hydrogen peroxide was the most effective disinfectant against E. piscisicarius, and it eradicated the biofilms and planktonic cells at the recommended concentrations. In contrast, Virkon and bleach were able to eradicate only the planktonic cells. This information should be taken into consideration when developing biosecurity protocols in aquaculture systems, aquariums, and private collections.

Characterization of protective antigen CbpB as an adhesin and a plasminogen-binding protein of Erysipelothrix rhusiopathiae

Erysipelothrix rhusiopathiae is the causative agent of animal erysipelas and human erysipeloid. E. rhusiopathiae CbpB has been reported to be a protective antigen, but its pathogenic roles are not known. The aim of this study was to evaluate the ability of CbpB to act as an adhesin in E. rhusiopathiae adhesion to porcine endothelial cells as well as a host plasminogen- and fibronectin- binding protein. Recombinant CbpB (rCbpB) was successfully obtained, and it was found that E. rhusiopathiae CbpB was located on the cell surface of E. rhusiopathiae. Moreover, CbpB exhibited binding activity to porcine endothelial cells. Recombinant CbpB successfully bound to host plasminogen but was unable to bind to fibronectin. In conclusion, our work suggested that CbpB is a virulence factor of E. rhusiopathiae.

Evaluation of the protective efficacy of four newly identified surface proteins of Erysipelothrix rhusiopathiae

Erysipelothrix rhusiopathiae is the causative agent of animal erysipelas and human erysipeloid. Bacterial surface proteins are promising vaccine candidates. We recently identified 3 E. rhusiopathiae surface proteins (GAPDH, HP0728, and HP1472) and characterized their roles as virulence factors. However, their efficacy as protective antigens is still unknown. The N-terminal region of a previously identified surface protein, CbpB (CbpB-N), is speculated to be a protective antigen, but this needs to be verified. The aim of this study was to evaluate the protective efficacy of GAPDH, HP0728, HP1472, and CbpB-N. Immunization with recombinant GAPDH provided complete protection in a mouse model, recombinant CbpB-N provided partial protection, while recombinant HP0728 and HP1472 provided no protection. Recombinant GAPDH also provided good protection in a pig model. GAPDH antiserum exhibited significant blood bactericidal activity against E. rhusiopathiae. In conclusion, GAPDH and CbpB-N were found to be protective antigens of E. rhusiopathiae, and GAPDH is a promising vaccine candidate.

Identification of the Chromosomal Region Essential for Serovar-Specific Antigen and Virulence of Serovar 1 and 2 Strains of Erysipelothrix rhusiopathiae

Erysipelothrix rhusiopathiae causes swine erysipelas, an infection characterized by acute septicemia or chronic endocarditis and polyarthritis. Among 17 E. rhusiopathiae serovars, determined based on heat-stable peptidoglycan antigens, serovars 1 and 2 are most commonly associated with the disease; however, the molecular basis for the association between these serovars and virulence is unknown. To search for the genetic region defining serovar 1a (Fujisawa) strain antigenicity, we examined the 15-kb chromosomal region encompassing a putative pathway for polysaccharide biosynthesis, which was previously identified in the E. rhusiopathiae Fujisawa strain. Six transposon mutants of Fujisawa strain possessing a mutation in this region lost antigenic reactivity with serovar 1a-specific rabbit serum. Sequence analysis of this region in wild-type strains of serovars 1a, 1b, and 2 and serovar N, which lacks serovar-specific antigens, revealed that gene organization was similar among the strains and that serovar 2 strains showed variation. Serovar N strains displayed the same gene organization as the serovar 1a, 1b, or 2 strain and possessed certain mutations in this region. In two of the analyzed serovar N strains, restoration of the mutations via complementation with sequences derived from serovar 1a and 2 strains recovered antigenic reactivity with 1a- and 2-specific rabbit serum, respectively. Several gene mutations in this region resulted in altered capsule expression and attenuation of virulence in mice. These results indicate a functional connection between the biosynthetic pathways for the capsular polysaccharide and peptidoglycan antigens used for serotyping, which may explain variation in virulence among strains of different serovars.

Erysipelothrix rhusiopathiae recruits host plasminogen via the major protective antigen SpaA

Erysipelothrix rhusiopathiae is the causative agent of animal erysipelas and human erysipeloid. Some pathogenic bacteria are able to recruit host plasminogen and then use the plasminogen system for migration across tissue barriers or for nutritional demands during infection. However, there is no study on E. rhusiopathiae recruitment of plasminogen. SpaA has long been known to be a major protective antigen of E. rhusiopathiae, but its roles in virulence have not yet been well clarified. The aim of this study was to detect the activity of E. rhusiopathiae to recruit host plasminogen and evaluate the ability of SpaA to act as a receptor in the recruitment process. It was found that E. rhusiopathiae could recruit host plasminogen. SpaA could specifically bind host plasminogen. Anti-SpaA serum could significantly decrease the activity of E. rhusiopathiae to recruit plasminogen. In addition, this binding activity was lysine dependent. In conclusion, E. rhusiopathiae was able to recruit host plasminogen via SpaA. To our knowledge, this is the first report on E. rhusiopathiae recruitment of host plasminogen and the receptor in the process.

Multiplex PCR assay for the simultaneous detection and differentiation of clonal lineages of Erysipelothrix rhusiopathiae serovar 1a strains currently circulating in Japan

The species Erysipelothrix rhusiopathiae displays genetic heterogeneity; however, E. rhusiopathiae serovar 1a strains currently circulating in Japan exhibit remarkably low levels of genetic diversity and group into clonal sublineages of Lineage IVb (IVb-1 and IVb-2). In the present study, based on whole genome sequencing data, we designed primers for a multiplex PCR assay to simultaneously detect and differentiate the sublineages of E. rhusiopathiae strains. Among the one hundred and twenty-seven isolates of various serovar strains, including isolates from a wide range of hosts and geographic origins, the PCR assay could successfully detect and differentiate the serovar 1a strains belonging to the sublineages.

Glyceraldehyde-3-phosphate dehydrogenase acts as an adhesin in Erysipelothrix rhusiopathiae adhesion to porcine endothelial cells and as a receptor in recruitment of host fibronectin and plasminogen

Erysipelothrix rhusiopathiae is the causative agent of animal erysipelas and human erysipeloid. Previous studies suggested glyceraldehyde 3-phosphate dehydrogenase (GAPDH) plays a role in the pathogenesis of E. rhusiopathiae infection. We studied E. rhusiopathiae GAPDH interactions with pig vascular endothelial cells, fibronectin, and plasminogen. Recombinant GAPDH (rGAPDH) was successfully obtained, and it was shown that it plays a role in E. rhusiopathiae adhesion to pig vascular endothelial cells. Moreover, rGAPDH could bind fibronectin and plasminogen in a dose-dependent manner. To our knowledge, this is the first study demonstrating that a moonlighting protein plays a role in pathogenesis of E. rhusiopathiae infections.

Identification of Bacterial Surface Antigens by Screening Peptide Phage Libraries Using Whole Bacteria Cell-Purified Antisera

Bacterial surface proteins can be good vaccine candidates. In the present study, we used polyclonal antibodies purified with intact Erysipelothrix rhusiopthiae to screen phage-displayed random dodecapeptide and loop-constrained heptapeptide libraries, which led to the identification of mimotopes. Homology search of the mimotope sequences against E. rhusiopthiae-encoded ORF sequences revealed 14 new antigens that may localize on the surface of E. rhusiopthiae. When these putative surface proteins were used to immunize mice, 9/11 antigens induced protective immunity. Thus, we have demonstrated that a combination of using the whole bacterial cells to purify antibodies and using the phage-displayed peptide libraries to determine the antigen specificities of the antibodies can lead to the discovery of novel bacterial surface antigens. This can be a general approach for identifying surface antigens for other bacterial species.

Phosphorylcholine and SpaA, a choline-binding protein, are involved in the adherence of Erysipelothrix rhusiopathiae to porcine endothelial cells, but this adherence is not mediated by the PAF receptor

A crucial event in the initiation of many bacterial infections is the adherence of the bacteria to host cells, and bacterial surface structures and their interactions with host cell receptors play an important role in this process. Erysipelothrix rhusiopathiae is the causative agent of swine erysipelas, which may cause acute septicemia or chronic endocarditis and polyarthritis. To study the pathogenic mechanism of the widespread vascular disease observed in the acute form of swine erysipelas, we investigated the role of phosphorylcholine (PCho), a component of the E. rhusiopathiae capsule, in bacterial adherence to porcine endothelial cells (PECs) in vitro. We found that adherence of E. rhusiopathiae strain Fujisawa to PECs was twice that of adherence to control COS-7 cells and that the adherence rates of PCho-defective mutants were approximately 30-50% lower than those of the Fujisawa strain. The adherence of the Fujisawa strain to COS-7 cells transfected with the porcine platelet-activating factor receptor (PAFR) gene, which encodes a G protein-coupled receptor that has been shown to directly bind to Streptococcus pneumoniae via PCho in the bacterial cell wall, was not enhanced. Treatment with a PAFR antagonist (WEB-2086) did not inhibit bacterial adherence to PECs. Incubation of the bacterial cells with an antibody against PCho or SpaA, a choline-binding protein anchored to PCho of the Fujisawa strain, reduced the adherence of the strain to PECs. This effect was not observed when PCho-defective mutants were used. These results suggest that E. rhusiopathiae adheres to PECs via PCho and SpaA and that the PCho-mediated adherence is independent of PAFR.