Actinobacillus pleuropneumoniae: pathobiology and pathogenesis of infection

Evaluation the kill rate and mutant selection window of danofloxacin against Actinobacillus pleuropneumoniae in a peristaltic pump model

BACKGROUND

Actinobacillus pleuropneumoniae is a serious pathogen in pigs. The abundant application of antibiotics has resulted in the gradual emergence of drugresistant bacteria, which has seriously affected treatment of disease. To aid measures to prevent the emergence and spread of drug-resistant bacteria, herein, the kill rate and mutant selection window (MSW) of danofloxacin (DAN) against A. pleuropneumoniae were evaluated.

METHODS

For the kill rate study, the minimum inhibitory concentration (MIC) was tested using the micro dilution broth method and time-killing curves of DAN against A. pleuropneumoniae grown in tryptic soy broth (TSB) at a series drug concentrations (from 0 to 64 MIC) were constructed. The relationships between the kill rate and drug concentrations were analyzed using a Sigmoid Emax model during different time periods. For the MSW study, the MIC99 (the lowest concentration that inhibited the growth of the bacteria by ≥ 99%) and mutant prevention concentration (MPC) of DAN against A. pleuropneumoniae were measured using the agar plate method. Then, a peristaltic pump infection model was established to simulate the dynamic changes of DAN concentrations in pig lungs. The changes in number and sensitivity of A. pleuropneumoniae were measured. The relationships between pharmacokinetic/pharmacodynamic parameters and the antibacterial effect were analyzed using the Sigmoid Emax model.

RESULTS

In kill rate study, the MIC of DAN against A. pleuropneumoniae was 0.016 µg/mL. According to the kill rate, DAN exhibited concentration-dependent antibacterial activity against A. pleuropneumoniae. A bactericidal effect was observed when the DAN concentration reached 4-8 MIC. The kill rate increased constantly with the increase in DAN concentration, with a maximum value of 3.23 Log10 colony forming units (CFU)/mL/h during the 0-1 h period. When the drug concentration was in the middle part of the MSW, drugresistant bacteria might be induced. Therefore, the dosage should be avoided to produce a mean value of AUC24h/MIC99 (between 31.29 and 62.59 h. The values of AUC24h/MIC99 to achieve bacteriostatic, bactericidal, and eradication effects were 9.46, 25.14, and > 62.59 h, respectively.

CONCLUSION

These kill rate and MSW results will provide valuable guidance for the use of DAN to treat A. pleuropneumoniae infections.

Serotype diversity and antimicrobial susceptibility profiles of Actinobacillus pleuropneumoniae isolated in Italian pig farms from 2015 to 2022

Actinobacillus pleuropneumoniae (APP) is a bacterium frequently associated with porcine pleuropneumonia. The acute form of the disease is highly contagious and often fatal, resulting in significant economic losses for pig farmers. Serotype diversity and antimicrobial resistance (AMR) of APP strains circulating in north Italian farms from 2015 to 2022 were evaluated retrospectively to investigate APP epidemiology in the area. A total of 572 strains isolated from outbreaks occurring in 337 different swine farms were analysed. The majority of isolates belonged to serotypes 9/11 (39.2%) and 2 (28.1%) and serotype diversity increased during the study period, up to nine different serotypes isolated in 2022. The most common resistances were against tetracycline (53% of isolates) and ampicillin (33%), followed by enrofloxacin, florfenicol and trimethoprim/sulfamethoxazole (23% each). Multidrug resistance (MDR) was common, with a third of isolates showing resistance to more than three antimicrobial classes. Resistance to the different classes and MDR varied significantly depending on the serotype. In particular, the widespread serotype 9/11 was strongly associated with florfenicol and enrofloxacin resistance and showed the highest proportion of MDR isolates. Serotype 5, although less common, showed instead a concerning proportion of trimethoprim/sulfamethoxazole resistance. Our results highlight how the typing of circulating serotypes and the analysis of their antimicrobial susceptibility profile are crucial to effectively manage APP infection and improve antimicrobial stewardship.

Lung microdialysis and in vivo PK/PD integration of cefquinome against Actinobacillus pleuropneumoniae in a porcine experimental lung infection model

This study aim to explore the application of microdialysis in pharmacokinetic (PK) and pharmacodynamic (PD) integration of cefquinome against Actinobacillus pleuropneumoniae in a porcine experimental lung infection model. The model was established via intratracheal inoculation where average bacterial counts (CFU) in the lungs of infected pigs reached 6.57 log10 CFU/g after 3 h. The PK profiles of unbound cefquinome in lung dialysates were determined following intramuscular injection of single doses of 0.125, 0.25, 0.5, 1, 2, and 4 mg/kg. Lung dialysate samples were collected using microdialysis at a flow rate of 1.5 μL/min until 24 h. The PD studies were conducted over 24 h based on 10 intermittent dosing regimens and total daily doses ranged from 0.25 to 4 mg/kg and dosage intervals included 12 and 24 h. The lung tissue was collected after 24 h of treatment and homogenized for bacterial counts. The relationships between PK/PD parameters derived from lung dialysates and drug efficacy were analyzed using an inhibitory sigmoid Emax model. The percentage of time the free drug concentration exceeded the minimum inhibitory concentration (%fT > MIC) was the PK/PD index best describing the antimicrobial activity (R2 = 0.96) in the porcine experimental infection model. The %fT > MIC values required to achieve net bacterial stasis, 1, 2 and 3 log10 CFU/g reductions in the lung were 22.45, 28.86, 37.62, and 56.46%, respectively. Cefquinome exhibited time-dependent characteristics against A. pleuropneumoniae in vivo. These results provide valuable insights into the application of microdialysis in PK/PD integration model studies and optima regimen of cefquinome for the treatment of porcine respiratory diseases caused by A. pleuropneumoniae.

Perception about the Major Health Challenges in Different Swine Production Stages in Spain

One of the main challenges for the sustainability and productivity of the Spanish swine industry is health instability, resulting in significant economic losses. Information on the main swine diseases which affect the Spanish pig industry could help in optimizing the efforts within control programs. This study determined the frequency of occurrence of the main diseases in Spain and the main control tool used, based on perceptions from veterinarians and consultants in a specific survey. Results showed that Streptococcus (S.) suis, E. coli, and coccidia are the most frequent pathogens in the gestation and lactation phase, whereas the most important were Porcine Reproductive and Respiratory Syndrome virus (PRRSV). In the nursery phase, the most frequent were S. suis, E. coli, and PRRSV, the latter being the most important for the participants. Finally, in the fattening phase, PRRSV and Actinobacillus pleuropneumoniae were the most frequent and important pathogen, respectively. Statistical differences among responses were detected with respect to the location and the gestation and lactation phases by farm size. Regarding the tools used for controlling the diseases, vaccination was the main strategy in all production phases, except in the fattening period, in which antibiotherapy was the most common response from the participants. Finally, the improvement of management practices was the most proposed tool, suggesting its importance within control programs.

Examination of the Virulence of Actinobacillus pleuropneumoniae Serovar 16 in Pigs

Different virulence variants of A. pleuropneumoniae are involved in the etiology of porcine pleuropneumonia. The purpose of the present trial was examination of the virulence of the Actinobacillus pleuropneumoniae A-85/14 strain, the type strain of serovar 16, in an animal challenge experiment. Thirty 12-week-old piglets seronegative for A. pleuropneumoniae were allocated into three trial groups each of 10 animals, and they were infected intranasally with 106, 107, or 108 colony forming units (cfu) of the strain, respectively. Clinical signs were recorded twice a day, and the animals were euthanized 6 days after the infection. Typical clinical signs and postmortem lesions of porcine pleuropneumonia were seen in the animals of each trial group; however, they were generally mild, and no significant differences could be seen between the three groups. Even 106 colony forming units of A. pleuropneumoniae A-85/14 strain could induce clinical signs and lesions. Based on these results, the type strain of serovar 16 of A. pleuropneumoniae must be regarded as a typical pathogenic strain of the species.

Effects of OxyR regulator on oxidative stress, Apx toxin secretion and virulence of Actinobacillus pleuropneumoniae

Introduction

Actinobacillus pleuropneumoniae, the causative agent of porcine pleuropneumonia, poses a significant threat to global swine populations due to its high prevalence, mortality rates, and substantial economic ramifications. Understanding the pathogen's defense mechanisms against host-produced reactive oxygen species is crucial for its survival, with OxyR, a conserved bacterial transcription factor, being pivotal in oxidative stress response.

Methods

This study investigated the presence and role of OxyR in A. pleuropneumoniae serovar 1-12 reference strains. Transcriptomic analysis was conducted on an oxyR disruption mutant to delineate the biological activities influenced by OxyR. Additionally, specific assays were employed to assess urease activity, catalase expression, ApxI toxin secretion, as well as adhesion and invasion abilities of the oxyR disruption mutant on porcine 3D4/21 and PT cells. A mice challenge experiment was also conducted to evaluate the impact of oxyR inactivation on A. pleuropneumoniae virulence.

Results

OxyR was identified as a conserved regulator present in A. pleuropneumoniae serovar 1-12 reference strains. Transcriptomic analysis revealed the involvement of OxyR in multiple biological activities. The oxyR disruption resulted in decreased urease activity, elevated catalase expression, enhanced ApxI toxin secretion-attributed to OxyR binding to the apxIBD promoter-and reduced adhesion and invasion abilities on porcine cells. Furthermore, inactivation of oxyR reduced the virulence of A. pleuropneumoniae in a mice challenge experiment.

Discussion

The findings highlight the pivotal role of OxyR in influencing the virulence mechanisms of A. pleuropneumoniae. The observed effects on various biological activities underscore OxyR as an essential factor contributing to the pathogenicity of this bacterium.

Native ApxIIA secreted by Actinobacillus pleuropneumoniae induces apoptosis in porcine alveolar macrophages dependent on concentration and acylation

Actinobacillus pleuropneumoniae is an important swine respiratory pathogen causing substantial economic losses to the global pig industry. The Apx toxins of A. pleuropneumoniae belong to the RTX toxin family and are major virulence factors. In addition to hemolysis and/or cytotoxicity via pore-forming activity, RTX toxins, such as ApxIA of A. pleuropneumoniae, have been reported to cause other effects on target cells, e.g., apoptosis. A. pleuropneumoniae ApxIIA is expressed by most serotypes and has moderate hemolytic and cytotoxic activities. In this study, porcine alveolar macrophages (3D4/21) were stimulated with different concentrations of purified native ApxIIA from the serotype 7 strain AP76 which only secretes ApxIIA. By observation of nuclear condensation via fluorescent staining and detection of apoptosis and necrosis by flow cytometry, it was found that high and low concentrations of native ApxIIA mainly caused necrosis or apoptosis of 3D4/21 cells, respectively. ApxIIA purified from an AP76 mutant with a deleted acetyltransferase gene (apxIIC) did not induce necrosis nor apoptosis. Western blot analysis using specific antibodies showed that a cleaved caspase 3 and activated capase 9 was detected after treatment of cells with a low concentration of native ApxIIA, while general or specific inhibitors of caspase 3, 8, 9 blocked these effects. ApxIIA-induced apoptosis of macrophages may be a mechanism of A. pleuropneumoniae to escape host immune clearance.

De novo identification of bacterial antigens of a clinical isolate by combining use of proteosurfaceomics, secretomics, and BacScan technologies

Background

Emerging infectious diseases pose a significant threat to both human and animal populations. Rapid de novo identification of protective antigens from a clinical isolate and development of an antigen-matched vaccine is a golden strategy to prevent the spread of emerging novel pathogens.

Methods

Here, we focused on Actinobacillus pleuropneumoniae, which poses a serious threat to the pig industry, and developed a general workflow by integrating proteosurfaceomics, secretomics, and BacScan technologies for the rapid de novo identification of bacterial protective proteins from a clinical isolate.

Results

As a proof of concept, we identified 3 novel protective proteins of A. pleuropneumoniae. Using the protective protein HBS1_14 and toxin proteins, we have developed a promising multivalent subunit vaccine against A. pleuropneumoniae.

Discussion

We believe that our strategy can be applied to any bacterial pathogen and has the potential to significantly accelerate the development of antigen-matched vaccines to prevent the spread of an emerging novel bacterial pathogen.

Actinobacillus pleuropneumoniae, surface proteins and virulence: a review

Actinobacillus pleuropneumoniae (App) is a globally distributed Gram-negative bacterium that produces porcine pleuropneumonia. This highly contagious disease produces high morbidity and mortality in the swine industry. However, no effective vaccine exists to prevent it. The infection caused by App provokes characteristic lesions, such as edema, inflammation, hemorrhage, and necrosis, that involve different virulence factors. The colonization and invasion of host surfaces involved structures and proteins such as outer membrane vesicles (OMVs), pili, flagella, adhesins, outer membrane proteins (OMPs), also participates proteases, autotransporters, and lipoproteins. The recent findings on surface structures and proteins described in this review highlight them as potential immunogens for vaccine development.

Pulmonary lesions with asteroid bodies in a pig experimentally infected with Actinobacillus pleuropneumoniae serovar 15

Five pigs experimentally infected with Actinobacillus pleuropneumoniae serovar 15 isolated in our previous study were pathologically examined. One pig died at 2 days post inoculation (dpi) and four pigs were euthanized at 7 dpi. Autopsy revealed fibrinohemorrhagic pleuropneumonia in all pigs. Histopathologically, the lesions were characterized by extensive hemorrhage and necrosis, fibrin deposition, and multifocal abscesses composed of numerous neutrophils including oat cells and numerous Gram-negative bacilli. In one survived pig, asteroid body formation was confirmed in the lung. The bacteria within the abscesses and asteroid bodies were immunohistochemically positive for antiserum raised against A. pleuropneumoniae serovar 15. This is the first report describing porcine pleuropneumonia with asteroid bodies in a pig experimentally infected with A. pleuropneumoniae serovar 15.

Tea Polyphenols Protects Tracheal Epithelial Tight Junctions in Lung during Actinobacillus pleuropneumoniae Infection via Suppressing TLR-4/MAPK/PKC-MLCK Signaling

Actinobacillus pleuropneumoniae (APP) is the causative pathogen of porcine pleuropneumonia, a highly contagious respiratory disease in the pig industry. The increasingly severe antimicrobial resistance in APP urgently requires novel antibacterial alternatives for the treatment of APP infection. In this study, we investigated the effect of tea polyphenols (TP) against APP. MIC and MBC of TP showed significant inhibitory effects on bacteria growth and caused cellular damage to APP. Furthermore, TP decreased adherent activity of APP to the newborn pig tracheal epithelial cells (NPTr) and the destruction of the tight adherence junction proteins β-catenin and occludin. Moreover, TP improved the survival rate of APP infected mice but also attenuated the release of the inflammation-related cytokines IL-6, IL-8, and TNF-α. TP inhibited activation of the TLR/MAPK/PKC-MLCK signaling for down-regulated TLR-2, TLR4, p-JNK, p-p38, p-PKC-α, and MLCK in cells triggered by APP. Collectively, our data suggest that TP represents a promising therapeutic agent in the treatment of APP infection.

TurboID screening of ApxI toxin interactants identifies host proteins involved in Actinobacillus pleuropneumoniae-induced apoptosis of immortalized porcine alveolar macrophages

Actinobacillus pleuropneumoniae (APP) is a gram-negative pathogenic bacterium responsible for porcine contagious pleuropneumonia (PCP), which can cause porcine necrotizing and hemorrhagic pleuropneumonia. Actinobacillus pleuropneumoniae-RTX-toxin (Apx) is an APP virulence factor. APP secretes a total of four Apx toxins, among which, ApxI demonstrates strong hemolytic activity and cytotoxicity, causing lysis of porcine erythrocytes and apoptosis of porcine alveolar macrophages. However, the protein interaction network between this toxin and host cells is still poorly understood. TurboID mediates the biotinylation of endogenous proteins, thereby targeting specific proteins and local proteomes through gene fusion. We applied the TurboID enzyme-catalyzed proximity tagging method to identify and study host proteins in immortalized porcine alveolar macrophage (iPAM) cells that interact with the exotoxin ApxI of APP. His-tagged TurboID-ApxIA and TurboID recombinant proteins were expressed and purified. By mass spectrometry, 318 unique interacting proteins were identified in the TurboID ApxIA-treated group. Among them, only one membrane protein, caveolin-1 (CAV1), was identified. A co-immunoprecipitation assay confirmed that CAV1 can interact with ApxIA. In addition, overexpression and RNA interference experiments revealed that CAV1 was involved in ApxI toxin-induced apoptosis of iPAM cells. This study provided first-hand information about the proteome of iPAM cells interacting with the ApxI toxin of APP through the TurboID proximity labeling system, and identified a new host membrane protein involved in this interaction. These results lay a theoretical foundation for the clinical treatment of PCP.

The morphology and metabolic changes of Actinobacillus pleuropneumoniae during its growth as a biofilm

Actinobacillus pleuropneumoniae is an important swine respiratory pathogen. Previous studies have suggested that growth as a biofilm is a natural state of A. pleuropneumoniae infection. To understand the survival features involved in the biofilm state, the growth features, morphology and gene expression profiles of planktonic and biofilm A. pleuropneumoniae were compared. A. pleuropneumoniae in biofilms showed reduced viability but maintained the presence of extracellular polymeric substances (EPS) after late log-phase. Under the microscope, bacteria in biofilms formed dense aggregated structures that were connected by abundant EPS, with reduced condensed chromatin. By construction of Δpga and ΔdspB mutants, polymeric β-1,6-linked N-acetylglucosamine and dispersin B were confirmed to be critical for normal biofilm formation. RNA-seq analysis indicated that, compared to their planktonic counterparts, A. pleuropneumoniae in biofilms had an extensively altered transcriptome. Carbohydrate metabolism, energy metabolism and translation were significantly repressed, while fermentation and genes contributing to EPS synthesis and translocation were up-regulated. The regulators Fnr (HlyX) and Fis were found to be up-regulated and their binding motifs were identified in the majority of the differentially expressed genes, suggesting their coordinated global role in regulating biofilm metabolism. By comparing the transcriptome of wild-type biofilm and Δpga, the utilization of oligosaccharides, iron and sulfur and fermentation were found to be important in adhesion and aggregation during biofilm formation. Additionally, when used as inocula, biofilm bacteria showed reduced virulence in mouse, compared with planktonic grown cells. Thus, these results have identified new facets of A. pleuropneumoniae biofilm maintenance and regulation.

Novel Multiplex PCR Assay and Its Application in Detecting Prevalence and Antibiotic Susceptibility of Porcine Respiratory Bacterial Pathogens in Guangxi, China

Porcine respiratory disease complex (PRDC) is a serious disease caused by multiple pathogens which inflicts huge economic losses on the pig industry. Investigating the epidemiology of porcine respiratory bacterial pathogens (PRBPs) in specific geographic areas and exploring the antibiotic susceptibility of local strains will contribute to the prevention and control of PRDC. However, the epidemiology of PRBPs in Guangxi Province remains unclear, and existing diagnostic methods have multiple limitations, such as high costs and the detection of only a single pathogen at a time. In this study, we developed a multiplex PCR assay for Streptococcus suis, Glaesserella parasuis, Actinobacillus pleuropneumoniae, Pasteurella multocida, and Mycoplasma hyopneumoniae, and investigated the prevalence of PRBPs in pigs with respiratory symptoms in Guangxi Province. The isolates from positive samples were subjected to susceptibility tests to 16 antibiotics. Our results indicated that of the 664 samples from pigs with respiratory symptoms, 433 (65.21%), 320 (48.19%), 282 (42.47%), 23 (3.46%), and 9 (1.36%), respectively, carried each of these 5 pathogens; 533 samples were positive; and 377 (56.78%) carried multiple pathogens simultaneously. The dominant PRBPs in pigs with respiratory symptoms in Guangxi province were S. suis, G. parasuis, and A. pleuropneumoniae, which frequently co-infected swine herds. Most of the isolates (A. pleuropneumoniae, G. parasuis, S. suis, and P. multocida) were sensitive to cefquinome, ceftiofur, trimethoprim-sulfamethoxazole (TMP-SMX), and tiamulin antibiotics. We developed a rapid specific multiplex PCR assay for PRBPs. Our findings provide new information on the epidemiology of PRBPs in Guangxi Province and offer a reference for developing drug targets against PRDC. IMPORTANCE Pigs are closely associated with humans as the most common food animals and the vectors of numerous pathogens. PRDC, caused by multiple pathogens, is a serious disease that can cause growth retardation in swine and even sudden death. Due to the droplet transmission of PRBP and the similar clinical signs of different pathogen infections, most pig farms struggle to identify and control PRBPs, leading to the abuse of antibiotics. In addition, some PRBPs have the potential to infect humans and threaten human health. Therefore, this study developed a multiplex PCR method targeting PRBPs, investigated the prevalence of these pathogens, and tested their antibiotic susceptibility. Our studies have important implications for public health safety and the development of the pig industry.

Adh Promotes Actinobacillus pleuropneumoniae Survival in Porcine Alveolar Macrophages by Inhibiting CHAC2-Mediated Respiratory Burst and Inflammatory Cytokine Expression

Actinobacillus pleuropneumoniae (A. pleuropneumoniae) causes porcine pleuropneumonia that seriously endangers pig's health. Adh, located in the head region of trimeric autotransporter adhesion of A. pleuropneumoniae, affects bacterial adhesion and pathogenicity. However, how Adh mediates A. pleuropneumoniae immune invasion is still unclear. Here, we established the A. pleuropneumoniae strain L20 or L20 ΔAdh-infected porcine alveolar macrophages (PAM) model, and applied protein overexpression, RNA interference, qRT-PCR, Western blot and immunoflourescence techniques to dissect the effects of Adh on PAM during A. pleuropneumoniae infection. We found that Adh could increase the A. pleuropneumoniae adhesion and intracellular survival in PAM. Gene chip analysis of piglet lungs further showed that Adh significantly induced cation transport regulatory-like protein 2 (CHAC2) expression, whose overexpression suppressed the phagocytic capacity of PAM. Furthermore, CHAC2 overexpression dramatically increased glutathione (GSH) expression, decreased reactive oxygen species (ROS), and promoted A. pleuropneumoniae survival in PAM, while the knockdown of CHAC2 reversed these phenomena. Meanwhile, CHAC2 silence activated the NOD1/NF-κB pathway, resulting in an increase in IL-1β, IL-6, and TNF-α expression, whereas this effect was weakened by CHAC2 overexpression and addition of NOD1/NF-κB inhibitor ML130. Moreover, Adh enhanced the secretion of LPS of A. pleuropneumoniae, which regulated the expression of CHAC2 via TLR4. In conclusion, through a LPS-TLR4-CHAC2 pathway, Adh inhibits respiratory burst and inflammatory cytokines expression to promote A. pleuropneumoniae survival in PAM. This finding may provide a novel target for the prevention and treatment of A. pleuropneumoniae.

Pathological Features and Genomic Characterization of an Actinobacillus equuli subsp. equuli Bearing Unique Virulence-Associated Genes from an Adult Horse with Pleuropneumonia

Actinobacillus equuli subsp. equuli is the etiological agent of sleepy foal disease, an acute form of fatal septicemia in newborn foals. A. equuli is commonly found in the mucous membranes of healthy horses' respiratory and alimentary tracts and rarely causes disease in adult horses. In this study, we report a case of a 22-year-old American Paint gelding presenting clinical signs associated with an atypical pattern of pleuropneumonia subjected to necropsy. The gross and histopathological examinations revealed a unilateral fibrinosuppurative and hemorrhagic pleuropneumonia with an infrequent parenchymal distribution and heavy isolation of A. equuli. The whole genome sequence analysis indicated that the isolate shared 95.9% homology with the only other complete genome of A. equuli subsp. equuli available in GenBank. Seven virulence-associated genes specific to the isolate were identified and categorized as iron acquisition proteins, lipopolysaccharides (LPS), and capsule polysaccharides. Moreover, four genes (glf, wbaP, glycosyltransferase family 2 protein, and apxIB) shared higher amino acid similarity with the invasive Actinobacillus spp. than the reference A. equuli subsp. equuli genome. Availability of the whole genome sequence will allow a better characterization of virulence determinants of A. equuli subsp. equuli, which remain largely elusive.

Effects of oil-based adjuvants on the immune response of pigs after dermal administration of antigen and evaluation of the immunization level after a subsequent Actinobacillus pleuropneumoniae challenge in pigs

Route of vaccine delivery can greatly impact the immunogenicity, efficacy and safety of the vaccine. Four groups of piglets were immunised transdermally (t.d.), intradermally (i.d.) or intramuscularly (i.m.) with the same doses of antigen in combination with a water-in-oil-in-water emulsion adjuvant Montanide™ ISA 201 VG or with a microemulsion adjuvant Montanide™ IMS 1313 VG N ST (Seppic, France). The last group was left without vaccination as a control group. All animals were subsequently exposed to the infection induced by Actinobacillus pleuropneumoniae (App). The immune response was evaluated with respect to the intensity of systemic and mucosal antibody formation, their isotype characterisation and rate of cell-mediated immunity. These findings were compared with the intensity of adverse local reactions and level of protection in experimental challenge. Monitoring of the local reaction at the injection site after each administration showed that microemulsion adjuvant IMS 1313 was less reactogenic than the water-in-oil-in-water emulsion ISA 201. In terms of efficacy, both dermal administrations were less immunogenic than the i.m route. The i.m. injection induced higher anti-App9 IgG and IgM titres. Nevertheless, IgG1 and IgG2 isotypes analysis revealed a close immunological profile between i.m. and i.d. routes. The concentration of IFN-γ from peripheral blood after in vitro restimulation with the specific antigen was only increased in the i.m. group at the day of challenge (D35) and two weeks after (D49). Interestingly, the smallest gross pulmonary lesions were observed in the i.d. vaccinated group (3.4%) compared to the control group (39.4%) and to groups with other routes of administration. Taken together, these results suggest that i.d. administration of vaccines is a promising approach. Even the i.d. vaccine was more reactogenic and slightly less immunogenic than the i.m. vaccine, its protection effectiveness seemed to be superior.

Comparative Efficacy in Challenge Dose Models of a Toxin Expressing Whole-Cell Vaccine against Eight Serovars of Actinobacillus pleuropneumoniae in Pigs

Actinobacillus pleuropneumoniae is a major economically significant bacterial respiratory pig pathogen, and whole cell vaccines are used to prevent disease. However, there is little data available on multi-serovar whole cell vaccine protection. Therefore, we determined the protective efficacies of a whole-cell A. pleuropneumoniae serovar 1 and 2 vaccine comprising ApxI-III toxins (C-vaccine, Coglapix^®, Ceva, France) against serovars 1, 2, 4, 5, 6, 7, 9/11, and 13. The infection doses used induced disease representative of endemic field conditions, and standard protocols were used for all studies. Protection against homologous serovars 1 and 2 significantly reduced lung lesion scores (LLS) compared to positive controls: p = 0.00007 and p = 0.00124, respectively. The protection against heterologous serovars 4, 5, 6, 7, 9/11, and 13 also significantly reduced LLS: range p = 2.9 × 10^-10 to p = 0.00953. As adjudged by the estimated random effect, reproducibility between studies was high. A highly significant serovar-independent reduction of pathological lung lesions by the C-vaccine was found for all the serovars tested (1, 2, 4, 5, 6, 7, 9/11, and 13). We conclude that the C-vaccine gives high serovar-independent protection against disease and is suitable for this use in the field.

Polymorphisms in Pattern Recognition Receptor Genes Are Associated with Respiratory Disease Severity in Pig Farms

Reduced productivity caused by infections, particularly respiratory diseases, is a serious problem in pig farming. We have previously reported polymorphisms in porcine pattern recognition receptor genes affecting molecular functions and demonstrated that the 2197A/C polymorphism in the nucleotide-binding oligomerization domain containing 2 (NOD2) gene influences porcine circovirus 2-induced mortality. Here, we investigated how these polymorphisms affect respiratory disease-induced lesions, using samples from a slaughterhouse dealing with pigs from two farms. Lung lesions were evaluated using two scoring systems, Goodwin (GW) and slaughterhouse pleuritis evaluation system (SPES), to determine the influence of Mycoplasma hyopneumoniae (Mhp) and Actinobacillus pleuropneumoniae (App), respectively. SPES scores were significantly higher when the 1205T allele of Toll-like receptor 5 (TLR5-1205T), rather than TLR5-1205C, was present. On the farm with more severe Mhp invasion, lower GW lesion scores were significantly associated with the presence of the NOD-like receptor family pyrin domain containing 3 (NLRP3)-2906G allele; where App invasion was worse, lower SPES scores were significantly associated with the presence of the NOD2-2197C allele. Combinations of polymorphisms in pattern recognition receptor genes can therefore be utilized for breeding for resistance against respiratory diseases in pigs. DNA markers of these polymorphisms can thus be used to improve productivity by reducing respiratory diseases due to bacterial pathogens in pig livestock.

Explorative Field Study on the Use of Oral Fluids for the Surveillance of Actinobacillus pleuropneumoniae Infections in Fattening Farms by an Apx-Real-Time PCR

Simple Summary

Oral fluid sampling (OFS) is an animal friendly and easy way for surveillance purposes in domestic swine populations, especially concerning respiratory diseases. In case of Actinobacillus (A.) pleuropneumoniae surveillance, measures are usually combined with burdensome sampling for animals and humans. In the present study, we evaluated the suitability of oral fluids (OFs) for surveillance purposes of A. pleuropneumoniae infections in fattening pigs using an Apx-toxin real-time PCR. We were able to demonstrate that the examination of OFs by an Apx-toxin real-time PCR is suitable for A. pleuropneumoniae surveillance in the field in an animal friendly and easy way. These results might contribute to an increased compliance of laboratory diagnostic measures on pig farms and thereby to increased animal welfare due to less burdensome sampling and improved animal health.

Abstract

Oral fluids (OFs) represent a cost effective and reliable tool for surveillance purposes, mostly regarding viruses. In the present study, we evaluated the suitability of OFs for surveillance purposes concerning Actinobacillus (A.) pleuropneumoniae infections in fattening pigs under field conditions. OFs were examined with an Apx-toxin real-time PCR that detects the genes encoding for Apx I-, Apx III-, and Apx IV-toxin. For this purpose, we conducted a pen-wise collection of OFs over one fattening period from fattening pigs of two farms (farm A and B) with a known history of A. pleuropneumoniae infection. Lung lesions were determined at slaughter to estimate the extend of pulmonary lesions and pleural affection. Apx III- and Apx IV-toxin DNA were present in the OFs of both farms whereas Apx I-toxin DNA was present on farm A only. We were able to detect Apx I-, Apx III-, and Apx IV-toxin DNA in different patterns directly after introduction of the new pigs in the farms and over the entire study period. In summary, or results indicate the suitability of OFS for the early detection and surveillance of A. pleuropneumoniae in fattening farms.

The two-component system CpxA/CpxR is critical for full virulence in Actinobacillus pleuropneumoniae

Actinobacillus pleuropneumoniae, a major bacterial porcine respiratory tract pathogen causing pig pleuropneumonia, has resulted in high economic losses worldwide. The mutation of the two-component system CpxAR strongly impacted the virulence of A. pleuropneumoniae, but the underlying regulatory mechanism remained unclear. Here, we found that CpxAR positively regulated the cpxDCBA gene cluster involved in polysaccharide capsule export. A capsular layer was confirmed in wild-type cells by transmission electron microscopy, whereas cpxAR and cpxD mutants were non-capsulated. The mutants for polysaccharide capsule export gene cpxD exhibited non-capsulated and were strongly impaired in virulence for mice, indicating a major role of CPS export system in virulence. We then demonstrated that CpxR directly regulated the transcription of the CPS export gene cluster cpxDCBA. Taken together, our data suggested that CpxAR is a key modulator of capsule export that facilitates A. pleuropneumoniae survival in the host.

Genetic Diversity of Actinobacillus pleuropneumoniae Serovars in Hungary

Simple Summary

Actinobacillus pleuropneumoniae causes severe pneumonia in pigs, resulting in high economic losses. A total of 114 isolates from pneumonia were characterized by the examination of biotype, serovar, antibiotic resistance genes, and genes of toxin production. Analyzing their genetic relationship, 16 groups of related isolates were found. The genetic diversity was different in the different groups, however. It was remarkably small in the case of serovar 13, which was unusually frequent in Hungary. Therefore, representative isolates of serovar 13 were subjected to whole-genome sequencing, confirming low diversity. Antibiotic resistance was frequently found in isolates of serovar 13 but was less frequent in other serovars. The unusually high frequency and low diversity of serovar 13 suggest a clonal spread in Hungary, which may have been facilitated by a high frequency of resistance to beta-lactams and tetracyclines.

Abstract

A total of 114 Actinobacillus pleuropneumoniae isolates from porcine hemorrhagic necrotic pleuropneumonia were characterized by the examination of biotype, serovar, antibiotic resistance genes, and genes of toxin production. Pulsed-field gel electrophoresis was used to analyze their genetic relationship, which identified 16 clusters. Serovar 2 (50 isolates), serovar 13 (25 isolates), serovar 9 (11 isolates), and serovar 16 (7 isolates) were the most frequent serovars. Serovar 2 formed nine distinguishable clusters; serovar 13 and serovar 16 were less diverse, exhibiting two potentially related subclusters; serovar 9 was represented by a single cluster. Remarkably small differences were seen in the core genome when nine representative isolates of serovar 13 were subjected to whole-genome sequencing. Tetracycline resistance was relatively frequent in the two clusters of serovar 13; one of them was also frequently resistant against beta-lactams. Resistance in other serovars was sporadic. All isolates carried the apxIV gene. The toxin profiles of serovar 2 were characterized by the production of ApxII and ApxIII toxins, except for a small cluster of three isolates: serovar 9 and serovar 16 isolates produced ApxI and ApxII toxins. Serovar 13 carried apxII and apxIBD genes, indicating the production of the ApxII toxin, but not of ApxI or ApxIII. The unusually high frequency and low diversity of serovar 13 are not explained by its virulence properties, but the high frequency of resistance to beta-lactams and tetracyclines may have played a role in its spread. The emergence of serovar 16 may be facilitated by its high virulence, also explaining its high clonality.

The Metabolic Adaptation in Response to Nitrate Is Critical for Actinobacillus pleuropneumoniae Growth and Pathogenicity under the Regulation of NarQ/P

Nitrate metabolism is an adaptation mechanism used by many bacteria for survival in anaerobic environments. As a by-product of inflammation, nitrate is used by the intestinal bacterial pathogens to enable gut infection. However, the responses of bacterial respiratory pathogens to nitrate are less well understood. Actinobacillus pleuropneumoniae is an important bacterial respiratory pathogen of swine. Previous studies have suggested that adaptation of A. pleuropneumoniae to anaerobiosis is important for infection. In this work, A. pleuropneumoniae growth and pathogenesis in response to the nitrate were investigated. Nitrate significantly promoted A. pleuropneumoniae growth under anaerobic conditions in vitro and lethality in mice. By using narQ and narP deletion mutants and single-residue-mutated complementary strains of ΔnarQ, the two-component system NarQ/P was confirmed to be critical for nitrate-induced growth, with Arg50 in NarQ as an essential functional residue. Transcriptome analysis showed that nitrate upregulated multiple energy-generating pathways, including nitrate metabolism, mannose and pentose metabolism, and glycerolipid metabolism via the regulation of NarQ/P. Furthermore, narQ, narP, and its target gene encoding the nitrate reductase Nap contributed to the pathogenicity of A. pleuropneumoniae. The Nap inhibitor tungstate significantly reduced the survival of A. pleuropneumoniae in vivo, suggesting that Nap is a potential drug target. These results give new insights into how the respiratory pathogen A. pleuropneumoniae utilizes the alternative electron acceptor nitrate to overcome the hypoxia microenvironment, which can occur in the inflammatory or necrotic infected tissues.

New Insights into Neutrophil Extracellular Trap (NETs) Formation from Porcine Neutrophils in Response to Bacterial Infections

Actinobacillus pleuropneumoniae (A.pp, Gram negative) and Streptococcus (S.) suis (Gram positive) can cause severe diseases in pigs. During infection, neutrophils infiltrate to counteract these pathogens with phagocytosis and/or neutrophil extracellular traps (NETs). NETs consist of a DNA-backbone spiked with antimicrobial components. The NET formation mechanisms in porcine neutrophils as a response to both of the pathogens are not entirely clear. The aim of this study was to investigate whether A.pp (serotype 2, C3656/0271/11) and S. suis (serotype 2, strain 10) induce NETs by NADPH oxidase- or CD18-dependent mechanisms and to characterize phenotypes of NETs in porcine neutrophils. Therefore, we investigated NET induction in porcine neutrophils in the presence and absence of NET inhibitors and quantified NETs after 3 h. Furthermore, NETosis and phagocytosis were investigated by transmission electron microscopy after 30 min to characterize different phenotypes. A.pp and S. suis induce NETs that are mainly ROS-dependent. A.pp induces NETs that are partially CD18-dependent. Thirty minutes after infection, both of the pathogens induced a vesicular NET formation with only slight differences. Interestingly, some neutrophils showed only NET-marker positive phagolysosomes, but no NET-marker positive vesicles. Other neutrophils showed vesicular NETs and only NET-marker negative phagolysosomes. In conclusion, both of the pathogens induce ROS-dependent NETs. Vesicular NETosis and phagocytosis occur in parallel in porcine neutrophils in response to S. suis serotype 2 and A.pp serotype 2.

Cytokine expression by CD163+ monocytes in healthy and Actinobacillus pleuropneumoniae-infected pigs

Distinct monocyte subpopulations have been previously described in healthy pigs and pigs experimentally infected with Actinobacillus pleuropneumoniae (APP). The CD163+ subpopulation of bone marrow (BM), peripheral blood (PB) and lung monocytes was found to play an important role in the inflammatory process. The inflammation is accompanied by elevation of inflammatory cytokines. The aim of the study was to evaluate the contribution of CD163+ monocytes and macrophages to cytokine production during APP-induced lung inflammation. Cytokine production was assessed by flow cytometry (FC) and quantitative PCR (qPCR) in CD163+ monocytes and by qPCR, immunohistochemistry/fluorescence in lungs and tracheobronchial lymph nodes (TBLN). Despite the systemic inflammatory response after APP infection, BM and PB CD163+ monocytes did not express elevated levels of a wide range of cytokines compared to control pigs. In contrast, significant amounts of IL-1β, IL-6, IL-8 and TNF-α were produced in lung lesions and IL-1β in the TBLN. At the protein level, TNF-α was expressed by both CD163+ monocytes and macrophages in lung lesions, whereas IL-1β, IL-6 and IL-8 expression was found only in CD163+ monocytes; no CD163+ macrophages were found to produce these cytokines. Furthermore, the quantification of CD163+ monocytes expressing the two cytokines IL-1β and IL-8 that were most elevated was performed. In lung lesions, 36.5% IL-1β positive CD163+ monocytes but only 18.3% IL-8 positive CD163+ monocytes were found. In conclusion, PB and BM CD163+ monocytes do not appear to contribute to the elevated cytokine levels in plasma. On the other hand, CD163+ monocytes contribute to inflammatory cytokine expression, especially IL-1β at the site of inflammation during the inflammatory process.

A Combinatorial Vaccine Containing Inactivated Bacterin and Subunits Provides Protection Against Actinobacillus pleuropneumoniae Infection in Mice and Pigs

Actinobacillus pleuropneumoniae (APP) is the etiological agent of porcine contagious pleuropneumonia (PCP) that causes great economic losses in the swine industry. Currently, vaccination is still a commonly used strategy for the prevention of the disease. Commercially available vaccines of this disease, including inactivated bacterins and subunit vaccines, have clinical limitations such as side effects and low cross-protection. In this study, a combinatorial vaccine (Bac-sub) was developed, which contained inactivated bacterial cells of a serovar 1 strain and three recombinant protoxins (rApxIA, rApxIIA, and rApxIIIA). Its side effects, immune protection, and cross-protection were evaluated and compared with a commercial subunit vaccine and a commercial trivalent bacterin in a mouse infection model. The results revealed that the Bac-sub vaccine showed no obvious side effects, and induced higher levels of Apx toxin-specific IgG, IgG1, and IgG2a than the commercial vaccines after booster. After a challenge with virulent strains of serovars 1, 5, and 7, the Bac-sub vaccine provided greater protection (91.76%, 100%, and 100%, respectively) than commercial vaccines. Much lower lung bacterial loads (LBLs) and milder lung lesions were observed in the Bac-sub-vaccinated mice than in those vaccinated with the other two vaccines. The protective efficacy of the Bac-sub vaccine was further evaluated in pigs, which showed that vaccinated pigs displayed significantly milder clinical symptoms and lung lesions than the unvaccinated pigs after the challenge. Taken together, Bac-sub is a safe and effective vaccine that could provide high protection against A. pleuropneumoniae infection in both mice and pigs.

Isolation of Biotype 1 Serotype 12 and Detection of Actinobacillus pleuropneumoniae from Wild Boars

Actinobacillus pleuropneumoniae is a major pathogen of swine, which can cause severe pleuropneumonia in pigs, but sometimes the disease can be generalized. Diseases caused by A. pleuropneumoniae are frequent all over the world, resulting in high losses among domestic pigs. However, our knowledge on the occurrence of A. pleuropneumoniae in wild boars and feral pigs is limited. We aimed to examine the carriage of A. pleuropneumoniae by hunted wild boars. The presence of A. pleuropneumoniae was examined in tonsils of 68 hunted wild boars collected at a game processing unit. An in-house designed species-specific PCR test was used to detect the gene of Apx IV toxin, and the samples were inoculated on a modified selective agar. A. pleuropneumoniae was detected in 10 animals (14.7%) by PCR and one A. pleuropneumoniae serotype 12 strain was isolated. The antibiotic resistance pattern of the strain resembled field strains that were isolated from farmed pigs in Hungary. This is the first case for the detection of A. pleuropneumoniae not only using PCR or ELISA, but also its isolation, identification, and serotyping.

Hypersecretion of OmlA antigen in Corynebacterium glutamicum through high-throughput based development process

Outer membrane lipoprotein A (OmlA) is a vaccine antigen against porcine contagious pleuropneumonia (PCP), a disease severely affecting the swine industry. Here, we aimed to systematically potentiate the secretory production of OmlA in Corynebacterium glutamicum (C. glutamicum), a widely used microorganism in the food industry, by establishing a holistic development process based on our high-throughput culture platform. The expression patterns, expression element combinations, medium composition, and induction conditions were comprehensively screened or optimized in microwell plates (MWPs), followed by fermentation parameter optimization in a 4 × 1 L parallel fermentation system (CUBER4). An unprecedented yield of 1.01 g/L OmlA was ultimately achieved in a 5-L bioreactor following the scaling-up strategy of fixed oxygen mass transfer coefficient (k_La), and the produced OmlA antigen showed well-protective immunity against Actinobacillus pleuropneumoniae challenge. This result provides a rapid and reliable pipeline to achieve the hyper-production of OmlA, and possibly other recombinant vaccines, in C. glutamicum. KEY POINTS: • Established a holistic development process and applied it to potentiate the secretion of OmlA. • The secretion of OmlA reached an unprecedented yield of 1.01 g/L. • The recombinant OmlA antigen induced efficient protective immunity.

Rapid Detection of Actinobacillus pleuropneumoniae From Clinical Samples Using Recombinase Polymerase Amplification

Actinobacillus pleuropneumoniae (APP) is the causative agent of porcine pleuropneumonia, resulting in high economic impact worldwide. There are currently 19 known serovars of APP, with different ones being predominant in specific geographic regions. Outbreaks of pleuropneumonia, characterized by sudden respiratory difficulties and high mortality, can occur when infected pigs are brought into naïve herds, or by those carrying different serovars. Good biosecurity measures include regular diagnostic testing for surveillance purposes. Current gold standard diagnostic techniques lack sensitivity (bacterial culture), require expensive thermocycling machinery (PCR) and are time consuming (culture and PCR). Here we describe the development of an isothermal point-of-care diagnostic test - utilizing recombinase polymerase amplification (RPA) for the detection of APP, targeting the species-specific apxIVA gene. Our APP-RPA diagnostic test achieved a sensitivity of 10 copies/μL using a strain of APP serovar 8, which is the most prevalent serovar in the UK. Additionally, our APP-RPA assay achieved a clinical sensitivity and specificity of 84.3 and 100%, respectively, across 61 extracted clinical samples obtained from farms located in England and Portugal. Using a small subset (n = 14) of the lung tissue samples, we achieved a clinical sensitivity and specificity of 76.9 and 100%, respectively) using lung imprints made on FTA cards tested directly in the APP-RPA reaction. Our results demonstrate that our APP-RPA assay enables a suitable rapid and sensitive screening tool for this important veterinary pathogen.

Kill Rate and Evaluation of Ex Vivo PK/PD Integration of Cefquinome Against Actinobacillus pleuropneumoniae

We wished to study the detailed and precise antibacterial activity of cefquinome against Actinobacillus pleuropneumoniae (APP) in vitro and ex vivo. We analyzed the relationships between kill rate and cefquinome concentration in broth and between pharmacokinetic/pharmacodynamic (PK/PD) parameters and antibacterial effect in serum and tissue cage fluid (TCF) of piglets. Cefquinome exhibited time-dependent antibacterial activity against APP according to the kill rate. The maximum kill rate was 0.48 log₁₀ CFU/mL/h at the 0-9-h period in broth. In the ex vivo PK/PD study, the maximum concentration (C_max), time to reach the maximum concentration (T_max), terminal half-life (T_1/2β), and area under the concentration time curve (AUC_infinity) were 5.65 μg/ml, 0.58 h, 2.24 h, and 18.48 μg·h/ml in serum and 1.13 μg/ml, 2.60 h, 12.22 h, and 20.83 μg·h/ml in TCF, respectively. The values of area under the curve during 24 h/minimum inhibitory concentration (AUC_24h/MIC) for bacteriostatic, bactericidal, and bacterial eradication effects were 18.94, 246.8, and 1013.23 h in serum and 4.20, 65.81, and 391.35 h in TCF, respectively. Our findings will provide a valuable basis for optimization of dosage regimens when applying cefquinome to treat APP infection.

Streptococcus pluranimalium 2N12 Exerts an Antagonistic Effect Against the Swine Pathogen Actinobacillus pleuropneumoniae by Producing Hydrogen Peroxide

Actinobacillus pleuropneumoniae is the causal agent of porcine pleuropneumonia, a highly contagious and often deadly respiratory disease that causes major economic losses in the swine industry worldwide. The aim of the present study was to investigate the hydrogen peroxide (H₂O₂)-dependent antagonistic activity of Streptococcus pluranimalium 2N12 (pig nasal isolate) against A. pleuropneumoniae. A fluorimetric assay showed that S. pluranimalium produces H₂O₂ dose- and time-dependently. The production of H₂O₂ increased in the presence of exogenous lactate, suggesting the involvement of lactate oxidase. All 20 strains of A. pleuropneumoniae tested, belonging to 18 different serovars, were susceptible to H₂O₂, with minimal inhibitory concentrations and minimal bactericidal concentrations ranging from 0.57 to 2.3 mM. H₂O₂, as well as a culture supernatant of S. pluranimalium, killed planktonic cells of A. pleuropneumoniae. Treating the culture supernatant with catalase abolished its bactericidal property. H₂O₂ was also active against a pre-formed biofilm-like structure of A. pleuropneumoniae albeit to a lesser extent. A checkerboard assay was used to show that there were antibacterial synergistic interactions between H₂O₂ and conventional antibiotics, more particularly ceftiofur. Based on our results and within the limitations of this in vitro study, the production of H₂O₂ by S. pluranimalium could be regarded as a potential protective mechanism of the upper respiratory tract against H₂O₂-sensitive pathogens such as A. pleuropneumoniae.

Rationally designed mariner vectors for functional genomic analysis of Actinobacillus pleuropneumoniae and other Pasteurellaceae species by transposon-directed insertion-site sequencing (TraDIS)

Comprehensive identification of conditionally essential genes requires efficient tools for generating high-density transposon libraries that, ideally, can be analysed using next-generation sequencing methods such as Transposon Directed Insertion-site Sequencing (TraDIS). The Himar1 (mariner) transposon is ideal for generating near-saturating mutant libraries, especially in AT-rich chromosomes, as the requirement for integration is a TA dinucleotide, and this transposon has been used for mutagenesis of a wide variety of bacteria. However, plasmids for mariner delivery do not necessarily work well in all bacteria. In particular, there are limited tools for functional genomic analysis of Pasteurellaceae species of major veterinary importance, such as swine and cattle pathogens, Actinobacillus pleuropneumoniae and Pasteurella multocida, respectively. Here, we developed plasmids, pTsodCPC9 and pTlacPC9 (differing only in the promoter driving expression of the transposase gene), that allow delivery of mariner into both these pathogens, but which should also be applicable to a wider range of bacteria. Using the pTlacPC9 vector, we have generated, for the first time, saturating mariner mutant libraries in both A. pleuropneumoniae and P. multocida that showed a near random distribution of insertions around the respective chromosomes as detected by TraDIS. A preliminary screen of 5000 mutants each identified 8 and 14 genes, respectively, that are required for growth under anaerobic conditions. Future high-throughput screening of the generated libraries will facilitate identification of mutants required for growth under different conditions, including in vivo, highlighting key virulence factors and pathways that can be exploited for development of novel therapeutics and vaccines.

An in vitro study of ApxI from Actinobacillus pleuropneumoniae serotype 10 and induction of NLRP3 inflammasome-dependent cell death

BACKGROUND

Actinobacillus pleuropneumoniae (AP) is the causative agent of porcine pleuropneumonia. Apx exotoxins are the most important virulence factors associated with the induction of lesions. ApxI is highly cytotoxic on a wide range of cells. Besides the induction of necrosis and apoptosis of ApxI on porcine alveolar macrophages (PAMs), its role in pyroptosis, a caspase-1-dependent form of cell death, has not been reported. The aim of this study was to analyse if NLRP3 inflammasome participates in cell death induced by ApxI.

METHODS

PAMs, the porcine alveolar macrophage cell line 3D4/21 and a porcine aortic endothelial cell line were used in this study. We used Z-VAD-FMK and Ac-YVAD-cmk to inhibit caspase-1. Glyburide and MCC950 were used to inhibit the NLRP3 inflammasome. A lactate dehydrogenase release assay was used to measure the percentage of cell death. Caspase-1 expression was analysed by immunofluorescence. End-point RT-PCR was used to analyse the expression of NLRP3 mRNA.

RESULTS

Rapid cell death in PAMs, 3D4/21 cells and the endothelial cell line were induced by ApxI. This cell death decreased by using caspase-1 and NLRP3 inflammasome inhibitors and by blocking the K+ efflux. Expression of NLRP3 mRNA was induced by ApxI in alveolar macrophages while it was constitutive in the endothelial cell line. Detection of caspase-1 in alveolar macrophages was higher after ApxI treatment and it was blocked by MCC950 or heat inactivation.

CONCLUSIONS

To the best of the authors' knowledge, we have described for the first time in vitro induction of ApxI associated pyroptosis in alveolar macrophages and endothelial cells, a rapid cell death that depends on the activation of caspase-1 via the NLRP3 inflammasome.

Global metabolic regulation in Vibrio parahaemolyticus under polymyxin B stimulation

Vibrio parahaemolyticus is responsible for infection diseases of people who consume the contaminated seafood, but its metabolic regulation profile in response to colistin, the last treatment option for multidrug-resistant Gram-negative bacteria, remains unclear. In this study, the metabolic regulation profile of V. parahaemolyticus ATCC33846 under polymyxin B stimulation has been investigated. V. parahaemolyticus exposed to polymyxin B resulted in 4597 differentially transcribed genes, including 673 significantly up-regulated genes and 569 significantly down-regulated genes. In V. parahaemolyticus under polymyxin B stimulation, the cellular antioxidant systems to prevent bacteria from oxidant stress was activated, the synthesis of some nonessential macromolecules was reduced, and the assembly and modification of lipopolysaccharide and peptidoglycan to resist the attack from other antibiotics were promoted. These findings provide new insights into polymyxin B-related stress response in V. parahaemolyticus which should be useful for developing novel drugs for infection.

Actinobacillus pleuropneumoniae Surviving on Environmental Multi-Species Biofilms in Swine Farms

Actinobacillus pleuropneumoniae is the etiologic agent of porcine contagious pleuropneumonia, an important respiratory disease for the pig industry. A. pleuropneumoniae has traditionally been considered an obligate pig pathogen. However, its presence in the environment is starting to be known. Here, we report the A. pleuropneumoniae surviving in biofilms in samples of drinking water of swine farms from Mexico. Fourteen farms were studied. Twenty drinking water samples were positive to A. pleuropneumoniae distributed on three different farms. The bacteria in the drinking water samples showed the ability to form biofilms in vitro. Likewise, A. pleuropneumoniae biofilm formation in situ was observed on farm drinkers, where the biofilm formation was in the presence of other bacteria such as Escherichia coli, Stenotrophomonas maltophilia, and Acinetobacter schindleri. Our data suggest that A. pleuropneumoniae can inhabit aquatic environments using multi-species biofilms as a strategy to survive outside of their host.

Advanced Research in Porcine Reproductive and Respiratory Syndrome Virus Co-infection With Other Pathogens in Swine

The porcine reproductive and respiratory syndrome virus (PRRSV) is the pathogen causing epidemics of porcine reproductive and respiratory syndrome (PRRS), and is present in every major swine-farming country in the world. Previous studies have demonstrated that PRRSV infection leads to a range of consequences, such as persistent infection, secondary infection, and co-infection, and is common among pigs in the field. In recent years, coinfection of PRRSV and other porcine pathogens has occurred often, making it more difficult to define and diagnose PRRSV-related diseases. The study of coinfections may be extremely suitable for the current prevention and control in the field. However, there is a limited understanding of coinfection. Therefore, in this review, we have focused on the epidemiology of PRRSV coinfection with other pathogens in swine, both in vivo and in vitro.

Application of the MISTEACHING(S) disease susceptibility framework to Actinobacillus pleuropneumoniae to identify research gaps: an exemplar of a veterinary pathogen

Historically, the MISTEACHING (microbiome, immunity, sex, temperature, environment, age, chance, history, inoculum, nutrition, genetics) framework to describe the outcome of host-pathogen interaction, has been applied to human pathogens. Here, we show, using Actinobacillus pleuropneumoniae as an exemplar, that the MISTEACHING framework can be applied to a strict veterinary pathogen, enabling the identification of major research gaps, the formulation of hypotheses whose study will lead to a greater understanding of pathogenic mechanisms, and/or improved prevention/therapeutic measures. We also suggest that the MISTEACHING framework should be extended with the inclusion of a 'strain' category, to become MISTEACHINGS. We conclude that the MISTEACHINGS framework can be applied to veterinary pathogens, whether they be bacteria, fungi, viruses, or parasites, and hope to stimulate others to use it to identify research gaps and to formulate hypotheses worthy of study with their own pathogens.

Molecular and functional characterization of HtrA protein in Actinobacillus pleuropneumoniae

Actinobacillus pleuropneumoniae (A.pleuropneumoniae) causes serious economic loss for the swine industry. A high-temperature requirements A (HtrA)-like protease and its homologs have been reported to be involved in protein quality control and expression of important immunoprotective antigens in many pathogens. In this study, we showed that HtrA of A.pleuropneumoniae exhibited both chaperone and proteolytic activities. Moreover, Outer membrane protein P5 (OmpP5) in A.pleuropneumoniae and Heat shock protein 90 (Hsp90) in porcine lung tissues were first discovered and identified as specific proteolytic substrates for rHtrA. The maximum cleavage activity occurs at 50 ℃ in a time-dependent manner. In addition, rHtrA mainly induced IgG 2a subtype of IgG and Th1 (IFN-γ, IL-2) response in a mice model, and promoted a significant proliferation of spleen lymphocytes compare with negative control (P < 0.05). The survival rates of 37.5 % were observed against A.pleuropneumoniae strain. Together, these data demonstrate that rHtrA plays a multi-functional role in A.pleuropneumoniae.

IFN-γ-/- Mice Resist Actinobacillus pleuropneumoniae Infection by Promoting Early Lung IL-18 Release and PMN-I Accumulation

Porcine pleuropneumonia is a common infectious disease of pigs caused by Actinobacillus pleuropneumoniae Interferon gamma (IFN-γ) expression increases in the lung of pigs after A. pleuropneumoniae infection, but the role of IFN-γ during the infection is still obscure. In this study, an IFN-γ^-/- mouse infection model was established, and bacterial load, levels of inflammatory cytokines, and types of neutrophils in the lungs were studied at different times post-A. pleuropneumoniae infection. We found that wild-type (WT) mice were more susceptible to A. pleuropneumoniae than IFN-γ^-/- mice. At 6 h postinfection (hpi), the expression of interleukin 18 (IL-18) and IL-1β in the lungs of IFN-γ^-/- mice was significantly increased compared to WT mice. The bacterial load and levels of inflammatory cytokines (IL-1β and IL-6) of IFN-γ^-/- mice were significantly reduced at 12 hpi compared to WT mice. After an initial loss, the numbers of lung polymorphonuclear (PMN)-I cells dramatically increased in the lungs of IFN-γ^-/- but not WT mice, whereas PMN-II cells continually decreased. Finally, in vivo administration of IL-18 significantly reduced clinical scores and bacterial load in the lungs of A. pleuropneumoniae-infected mice. This study identifies IFN-γ as a target for regulating the inflammatory response in the lung and provides a basis for understanding the course of clinical bacterial pneumonia and for the formulation of treatment protocols.

Proposal of Actinobacillus pleuropneumoniae serovar 19, and reformulation of previous multiplex PCRs for capsule-specific typing of all known serovars

Two serologically and molecularly non-typeable isolates of the porcine lung pathogen Actinobacillus pleuropneumoniae have been identified from diseased swine in two different continents. Genome sequencing was carried out to identify their diagnostically relevant genotypes. Both isolates are biovar 1 and encode genes for production of ApxIV and ApxII (apxIICA structural genes, and apxIBD export genes). They both possess the same novel type II capsule locus (most similar to serovar 1, but with two capsule genes not previously found in A. pleuropneumoniae) but differ in their O-Ag loci. Strain 7213384-1 from Denmark, which we propose as the reference strain for serovar 19, has a serogroup 3/6/8/15 O-Ag locus; the Canadian isolate A08-013 has a serogroup 4/7 O-Ag locus. We have expanded the second of our two previously described A. pleuropneumoniae mPCRs to include capsule gene-specific primers for definitive detection of serovars 13-14 and 16-19.

Exposure to Sublethal Ciprofloxacin Induces Resistance to Ciprofloxacin and Cross-Antibiotics, and Reduction of Fitness, Biofilm Formation, and Apx Toxin Secretion in Actinobacillus pleuropneumoniae

Actinobacillus pleuropneumoniae, the etiological agent of porcine pleuropneumonia, is increasingly resistant to antibiotics. However, little is known about the mechanisms of antibiotic resistance in this pathogen. In this study, we experimentally evolved the reference strain of both A. pleuropneumoniae serovar 1 and serovar 7, the most prevalent serovars worldwide, to quinolone resistance by sequential exposure to subinhibitory concentrations of ciprofloxacin. The adaptive ciprofloxacin-resistant mutants of A. pleuropneumoniae serovar 1 and serovar 7 had a minimum inhibitory concentration (MIC) increment from 0.004 to 1 or 2 μg/mL, respectively. Adaptation to ciprofloxacin was shown to confer quinolone resistance with a 32- to 512-fold increase (serovars 1 and 7, respectively) as well as cross-resistance to ampicillin with an increased MIC by 16,384- and 64-fold (serovars 1 and 7, respectively). The genetic analysis of quinolone resistance-determining region mutations showed that substitutions occurred in gyrA (S83A) and parC (D84N) of serovar 1, and gyrA (D87N) of serovar 7. The ciprofloxacin-resistant mutants showed significantly reduced bacterial fitness. The mutants also showed changes in efflux ability and biofilm formation. Notably, the transcription and secretion levels of Apx toxins were dramatically reduced in ciprofloxacin-resistant mutants compared with their wild-type strains. Altogether, these results demonstrated marked phenotypic changes in ciprofloxacin-resistant mutants of A. pleuropneumoniae. The results stress the need for further studies on the impact of both the genotypic and phenotypic characteristics of A. pleuropneumoniae following exposure to subinhibitory concentrations of antibiotics.

Research Relevant Background Lesions and Conditions: Ferrets, Dogs, Swine, Sheep, and Goats

Animal models provide a valuable tool and resource for biomedical researchers as they investigate biological processes, disease pathogenesis, novel therapies, and toxicologic studies. Interpretation of animal model data requires knowledge not only of the processes/diseases being studied but also awareness of spontaneous conditions and background lesions in the model that can influence or even confound the study results. Species, breed/stock, sex, age, anatomy, physiology, diseases (noninfectious and infectious), and neoplastic processes are model features that can impact the results as well as study interpretation. Here, we review these features in several common laboratory animal species, including ferret, dog (beagle), pig, sheep, and goats.

Actinobacillus pleuropneumoniae: The molecular determinants of virulence and pathogenesis

Actinobacillus pleuropneumoniae, the causative agent of porcine pleuropneumonia, is responsible for high economic losses in swine herds across the globe. Pleuropneumonia is characterized by severe respiratory distress and high mortality. The knowledge about the interaction between bacterium and host within the porcine respiratory tract has improved significantly in recent years. A. pleuropneumoniae expresses multiple virulence factors, which are required for colonization, immune clearance, and tissue damage. Although vaccines are used to protect swine herds against A. pleuropneumoniae infection, they do not offer complete coverage, and often only protect against the serovar, or serovars, used to prepare the vaccine. This review will summarize the role of individual A. pleuropneumoniae virulence factors that are required during key stages of pathogenesis and disease progression, and highlight progress made toward developing effective and broadly protective vaccines against an organism of great importance to global agriculture and food production.

Design and Characterization of a Novel Tool for the Antigenic Enrichment of Actinobacillus pleuropneumoniae Outer Membrane

Production and isolation of recombinant proteins are costly and work-intensive processes, especially in immunology when tens or hundreds of potential immunogens need to be purified for testing. Here we propose an alternative method for fast screening of immunogen candidates, based on genetic engineering of recombinant bacterial strains able to express and expose selected antigens on their outer membrane. In Actinobacillus pleuropneumoniae, a Gram-negative porcine pathogen responsible for extensive economic losses worldwide, we identified a conserved general secretion pathway (GSP) domain in the N-terminal part of the outer membrane protein ApfA (ApfA stem: ApfAs). ApfAs was used as an outer membrane anchor, to which potential immunogens can be attached. To enable confirmation of correct positioning, ApfAs, was cloned in combination with the modified acyl carrier protein (ACP) fluorescent tag ACP mini (ACPm) and the putative immunogen VacJ. The chimeric construct was inserted in the pMK-express vector, subsequently transformed into A. pleuropneumoniae for expression. Flow cytometry, fluorescence imaging and mass spectrometry analysis were employed to demonstrate that the outer membrane of the transformed strain was enriched with the chimeric ApfAs-ACPm-VacJ antigen. Our results confirmed correct positioning of the chimeric ApfAs-ACPm-VacJ antigen and supported this system's potential as platform technology enabling antigenic enrichment of the outer membrane of A. pleuropneumoniae.

Actinobacillus pleuropneumoniae: a review of an economically important pathogen

Actinobacillus pleuropneumoniae is one of the causative agents of porcine pleuropneumonia, which is an economically important respiratory disease of pig production. Clinical signs vary based on the severity of disease and lung lesions present, but include fever and severe respiratory signs including coughing and laboured breathing. Numerous serotypes exist which vary in their virulence, and virulence of serotypes has also been shown to be vary between countries. It is important to establish which serotypes are present and active on a farm as well as carrying out seroprofiling to determine the correct time for implementation of control measures such as vaccination. Understanding of transmission routes is vital, including the role of carrier animals on the farm which are persistently infected and can shed the bacteria, therefore infecting other animals. Therefore, as with all infectious diseases, good standards of internal and external biosecurity are important in controlling the disease on farm. Vaccination has been shown to be effective on affected farms in preventing outbreaks, reducing clinical signs if they occur, and most important to the farmer, preventing losses in mortality, feed conversion ratio and growth. Therefore, vaccines are often a good choice for controlling pleuropneumonia on farm and reducing the need for treatment using antimicrobials.

Actinobacillus pleuropneumoniae Interaction With Swine Endothelial Cells

Actinobacillus pleuropneumonia is a swine (host) specific respiratory pathogen and the etiological agent of swine pleuropneumonia which affects pigs of all ages, many being asymptomatic carriers. This pathogen has high morbidity and mortality rates which generates large economic losses for the pig industry. Actinobacillus pleuropneumoniae is a widely studied bacterium, however its pathogenesis is not yet fully understood. The prevalence of the 18 serotypes of A. pleuropneumoniae varies by geographic region, in North American area, more specifically in Mexico, serotypes 1, 3, 5b, and 7 show higher prevalence. Actinobacillus pleuropneumoniae is described as a strict extracellular pathogen with tropism for lower respiratory tract. However, this study depicts the ability of these serotypes to adhere to non-phagocytic cells, using an endothelial cell model, as well as their ability to internalize them, proposing it could be considered as an intracellular pathogen.

Genomic Analysis of IgG Antibody Response to Common Pathogens in Commercial Sows in Health-Challenged Herds

Losses due to infectious diseases are one of the main factors affecting productivity in the swine industry, motivating the investigation of disease resilience-related traits for genetic selection. However, these traits are not expected to be expressed in the nucleus herds, where selection is performed. One alternative is to use information from the commercial level to identify and select nucleus animals genetically superior for coping with pathogen challenges. In this study, we analyzed the genetic basis of antibody (Ab) response to common infectious pathogens in health-challenged commercial swine herds as potential indicator traits for disease resilience, including Ab response to influenza A virus of swine (IAV), Mycoplasma hyopneumoniae (MH), porcine circovirus (PCV2), and Actinobacillus pleuropneumoniae (APP; different serotypes). Ab response was measured in blood at entry into gilt rearing, post-acclimation (∼40 days after entering the commercial herd), and parities 1 and 2. Heritability estimates for Ab response to IAV, MH, and PCV2 ranged from 0 to 0.76. Ab response to APP ranged from 0 to 0.40. The genetic correlation (r _G ) of Ab response to IAV with MH, PCV2, PRRSV, and APP_mean (average Ab responses for all serotypes of APP) were positive (>0.29) at entry. APP_mean was negatively correlated with PCV2 and MH at entry and parity 2 but positively correlated with MH at post-acclimation and parity 1. Genomic regions associated with Ab response to different APP serotypes were identified on 13 chromosomes. The region on chromosome 14 (2 Mb) was associated with several serotypes of APP, explaining up to 4.3% of the genetic variance of Ab to APP7 at entry. In general, genomic prediction accuracies for Ab response were low to moderate, except average Ab response to all infectious pathogens evaluated. These results suggest that genetic selection of Ab response in commercial sows is possible, but with variable success depending on the trait and the time-point of collection. Future work is needed to determine genetic correlations of Ab response with disease resilience, reproductive performance, and other production traits.

Serovar-dependent differences in Hfq-regulated phenotypes inActinobacillus pleuropneumoniae

The RNA chaperone Hfq regulates diverse processes in numerous bacteria. In this study, we compared phenotypes (growth rate, adherence, response to different stress conditions and virulence in Galleria mellonella) of wild-type (WT) and isogenic hfq mutants of three serovars (1, 8 and 15) of the porcine pathogen Actinobacillus pleuropneumoniae. Similar growth in rich broth was seen for all strains except Ap1∆hfq, which showed slightly reduced growth throughout the 24 h time course, and the complemented Ap8∆hfqC mutant had a prolonged lag phase. Differences were seen between the three serovar WT strains regarding adherence, stress response and virulence in G. mellonella, and deletion of hfq affected some, but not all of these phenotypes, depending on serovar. Complementation by expression of cloned hfq from an endogenous promoter only restored some WT phenotypes, indicating that complex regulatory networks may be involved, and that levels of Hfq may be as important as presence/absence of the protein regarding its contribution to gene regulation. Our results support that Hfq is a pleiotropic global regulator in A. pleuropneumoniae, but serovar-related differences exist. These results highlight the importance of testing multiple strains/serovars within a given species when determining contributions of global regulators, such as Hfq, to expression of complex phenotypes.

Fecal microbiota transplantation from warthog to pig confirms the influence of the gut microbiota on African swine fever susceptibility

African swine fever virus (ASFV) is the causative agent of a devastating hemorrhagic disease (ASF) that affects both domestic pigs and wild boars. Conversely, ASFV circulates in a subclinical manner in African wild pigs, including warthogs, the natural reservoir for ASFV. Together with genetic differences, other factors might be involved in the differential susceptibility to ASF observed among Eurasian suids (Sus scrofa) and African warthogs (Phacochoerus africanus). Preliminary evidence obtained in our laboratory and others, seems to confirm the effect that environmental factors might have on ASF infection. Thus, domestic pigs raised in specific pathogen-free (SPF) facilities were extremely susceptible to highly attenuated ASFV strains that were innocuous to genetically identical domestic pigs grown on conventional farms. Since gut microbiota plays important roles in maintaining intestinal homeostasis, regulating immune system maturation and the functionality of the innate/adaptive immune responses, we decided to examine whether warthog fecal microbiota transplantation (FMT) to domestic pigs affects host susceptibility to ASFV. The present work demonstrates that warthog FMT is not harmful for domestic weaned piglets, while it modifies their gut microbiota; and that FMT from warthogs to pigs confers partial protection against attenuated ASFV strains. Future work is needed to elucidate the protective mechanisms exerted by warthog FMT.