A Unique Capsule Locus in the Newly Designated Actinobacillus pleuropneumoniae Serovar 16 and Development of a Diagnostic PCR Assay

Evaluation of the mutant selection window of danofloxacin against Actinobacillus pleuropneumoniae in an in vitro dynamic model

Introduction

The rapid emergence and widespread spread of multidrug-resistant bacteria is a serious threat to the health of humans and animals. The pharmacokinetic/pharmacodynamic (PK/PD) integration model based on mutant selection window (MSW) theory is an important method to optimize the dosage regimen to prevent the emergence and spread of drug-resistant bacteria. Actinobacillus pleuropneumoniae (AP) is a pathogen that can cause pleuropneumonia in pigs.

Methods

We employed an in vitro dynamic infection model (DIM) to study the prevention of drug-resistant mutations of danofloxacin against AP. A peristaltic pump was applied to establish an in vitro DIM to simulate the PK of danofloxacin in plasma, and to study the MSW of danofloxacin against AP. A peristaltic-pump in vitro infection model was established to simulate dynamic changes in the danofloxacin concentration in pig plasma. PK and PD data were obtained. Then, the relationship between PK/PD parameters and antibacterial activity was analyzed by the sigmoid E_max model.

Results and discussion

The area under the curve during 24 h/ the minimum concentration that inhibits colony formation by 99% (AUC_24h/MIC₉₉) had the best-fitting relationship with antibacterial activity. The AUC_24h/MIC₉₉ values for a bacteriostatic effect, bactericidal effect, and eradication effect were 2.68, 33.67, and 71.58 h, respectively. We hope these results can provide valuable guidance when using danofloxacin to treat AP infection.

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.

In silico capsule locus typing for serovar prediction of Actinobacillus pleuropneumoniae

Actinobacillus pleuropneumoniae is a causative agent of pleuropneumonia in pigs of all ages. A . pleuropneumoniae is divided into 19 serovars based on capsular polysaccharides (CPSs) and lipopolysaccharides. The serovars of isolates are commonly determined by serological tests and multiplex PCR. This study aimed to develop a genomic approach for in silico A. pleuropneumoniae typing by screening for the presence of the species-specific apxIV gene in whole-genome sequencing (WGS) reads and identifying capsule locus (KL) types in genome assemblies. A database of the A . pleuropneumoniae KL, including CPS synthesis and CPS export genes, was established and optimized for Kaptive. To test the developed genomic approach, WGS reads of 189 A . pleuropneumoniae isolates and those of 66 samples from 14 other bacterial species were analysed. ariba analysis showed that apxIV was detected in all 189 A . pleuropneumoniae samples. These apxIV -positive WGS reads were de novo assembled into genome assemblies and assessed. A total of 105 A . pleuropneumoniae genome assemblies that passed the quality assessment were analysed by Kaptive analysis against the A . pleuropneumoniae KL database. The results showed that 97 assemblies were classified and predicted as 13 serovars, which matched the serovar information obtained from the literature. The six genome assemblies from previously nontypable isolates were typed and predicted as serovars 17 and 18. Notably, one of the two “Actinobacillus porcitonsillarum ” samples was apxIV positive, and its genome assembly was typed as KL03 with high identity and predicted as A . pleuropneumoniae serovar 3. Collectively, a genomic approach was established and could accurately determine the KL type of A . pleuropneumoniae isolates using WGS reads. This approach can be used with high-quality genome assemblies for predicting A . pleuropneumoniae serovars and for retrospective analysis.

Development of a novel high resolution melting assay for identification and differentiation of all known 19 serovars of Actinobacillus pleuropneumoniae

Actinobacillus pleuropneumoniae is the etiological agent of porcine pleuropneumonia, a respiratory infectious disease responsible for global economic losses in the pig industry. From a monitoring perspective as well as due to the different courses of disease associated with the various serovars, it is essential to distinguish them in different herds or countries. In this study, we developed a novel high resolution melting (HRM) assay based on reference strains for each of the 19 known serovars and additional 15 clinical A. pleuropneumoniae isolates. The novel HRM comprises the species‐specific APP‐HRM1 and two serovar‐specific HRM assays (APP‐HRM2 and APP‐HRM3). APP‐HRM1 allowed polymerase chain reaction (PCR) amplification of apxIV resulting in an A. pleuropneumoniae specific melting curve, while nadV specific primers differentiated biovar 2 from biovar 1 isolates. Using APP‐HRM2 and APP‐HRM3, 13 A. pleuropneumoniae serovars can be determined by inspecting the assigned melting temperature. In contrast, serovar 3 and 14, serovar 9 and 11, and serovar 5 and 15 have partly overlapping melting temperatures and thus represent a challenge to accurately distinguish them. Consequently, to unambiguously ensure the correct assignment of the serovar, it is recommended to perform the serotyping HRM assay using a positive control for each serovar. This rapid and user‐friendly assay showed high sensitivity with 1.25 fg–125 pg of input DNA and a specificity of 100% to identify A. pleuropneumoniae. Characteristic melting patterns of amplicons might allow detecting new serovars. The novel HRM assay has the potential to be implemented in diagnostic laboratories for better surveillance of this pathogen.

Comparative genomics of 26 complete circular genomes of 18 different serotypes of Actinobacillus pleuropneumoniae

Actinobacillus pleuropneumoniae is a Gram-negative, rod-shaped bacterium of the family Pasteurellaceae causing pig pleuropneumonia associated with great economic losses worldwide. Nineteen serotypes with distinctive lipopolysaccharide (LPS) and capsular (CPS) compositions have been described so far, yet complete circular genomes are publicly available only for the reference strains of serotypes 1, 4 and 5b, and for field strains of serotypes 1, 3, 7 and 8. We aimed to complete this picture by sequencing the reference strains of 17 different serotypes with the MinION sequencer (Oxford Nanopore Technologies, ONT) and on an Illumina HiSeq (Illumina) platform. We also included two field isolates of serotypes 2 and 3 that were PacBio- and MinION-sequenced, respectively. Genome assemblies were performed following two different strategies, i.e. PacBio- or ONT-only de novo assemblies polished with Illumina reads or a hybrid assembly by directly combining ONT and Illumina reads. Both methods proved successful in obtaining accurate circular genomes with comparable qualities. blast-based genome comparisons and core-genome phylogeny based on core genes, SNP typing and multi-locus sequence typing (cgMLST) of the 26 circular genomes indicated well-conserved genomes across the 18 different serotypes, differing mainly in phage insertions, and CPS, LPS and RTX-toxin clusters, which, consistently, encode serotype-specific antigens. We also identified small antibiotic resistance plasmids, and complete subtype I-F and subtype II-C CRISPR-Cas systems. Of note, highly similar clusters encoding all those serotype-specific traits were also found in other pathogenic and commensal Actinobacillus species. Taken together with the presence of transposable elements surrounding these loci, we speculate a dynamic intra- and interspecies exchange of such virulence-related factors by horizontal gene transfer. In conclusion, our comprehensive genomics analysis provides useful information for diagnostic test and vaccine development, but also for whole-genome-based epidemiological studies, as well as for the surveillance of the evolution of antibiotic resistance and virulence genes in A. pleuropneumoniae.

Seroprevalence of Actinobacillus pleuropneumoniae infection in pigs from Bulgaria

Actinobacillus pleuropneumoniae (App) is the etiological agent of porcine pleuropneumonia. The purpose of the study was to present a serological report on App prevalence among pigs in industrial farms in Bulgaria. Seventy-two pigs from four industrial farms in four districts of Bulgaria – Eastern Bulgaria (Razgrad and Yambol districts) and Western Bulgaria (Lovech and Sofia districts) were included. Animals were divided in two age groups: weaners and fattening pigs. A commercial enzyme-linked immunosorbent assay (ELISA, INgezim APP MIX, Eurofins Ingenasa, Madrid, Spain) for the detection of antibodies against App parasuis in porcine serum was used. Microtitrе plate was coated with App antigen of the serovars 1, 2, 9 and 11. Positive results for anti-App antibodies were detected in 32 (44.4%) of all 72 tested sera. The overall seropositivity in weaners and fattening pigs was 22.2% (8/36), and 66.7% (24/36), respectively. The highest App seropositivity in pigs was found in Eastern Bulgaria - 61.1% (22/36; P<0.001) in comparison to App seropositivity in Western Bulgaria – 27.8% (10/36; P=0.137). This study on anti-App prevalence among pigs in Bulgaria gives new insights on App epidemiology in our country.

Proposal of a subtype of serovar 4, K4b:O3, of Actinobacillus pleuropneumoniae based on serological and genotypic analysis

The aim of this study was to investigate an isolate of Actinobacillus pleuropneumoniae, named 14-760, which was serologically not classifiable among the recognised serovars of A. pleuropneumoniae. It reacted with the antisera raised against serovars 3, 6, 8, 15 and 17 in the agar gel precipitation (AGP) test, and was positive in the capsular serovar 4-specific PCR (cps4B PCR) assay. The isolate contains a type II capsule locus similar to serovar 4 but with variations in the length of four intergeneric regions (modF-cpxA, cpxD-cpsA, cpsC-a 114 bp orf, and lysA-ydeN), and three gene sequences (modF, cpsC and ydeN). The main difference found between the K4 and K4b cps genes is the additional 35 AAs found in type 4b due to a 4 bp insert in cps4bC. The LPS O-Ag locus is highly similar to that of reference strains of serovars 3, 6, 8, 15, 17 and 19. Isolate 14-760 is biovar 1 and contains solely the structural genes required for toxin ApxII production (apxIICA), and the type I secretion system (apxIBD) for the export of ApxII. Antiserum against isolate 14-760 adsorbed with antigen prepared from serovars 8, 15 or 17 reference strains remained reactive with isolate 14-760, but not with antigens prepared from serovars 1-18. Taken together, our results indicate the existence of a subtype of A. pleuropneumoniae, serovar 4, that we called "K4b:O3″, and we propose isolate 14-760 as the reference strain.

Rapid Detection and Typing of Actinobacillus pleuropneumoniae Serovars Directly From Clinical Samples: Combining FTA® Card Technology With Multiplex PCR

Actinobacillus pleuropneumoniae (APP), the causative agent of porcine pleuropneumonia, is highly contagious and responsible for high morbidity, mortality, and economic losses in the swine industry worldwide, but quick serotyping and diagnosis are still not widely available. In this study, we sought to validate the use of Whatman FTA® cards for collection and processing of A. pleuropneumoniae isolates, or porcine lung tissue samples, for direct use in diagnostic multiplex PCRs. We have optimized the processing of 3-mm discs punched from FTA® cards loaded with cultured A. pleuropneumoniae, or imprinted on lesioned regions of lung tissue, with only three distilled water washes before addition into our APP-multiplex PCR (mPCR) assay for rapid, low-cost identification and serotyping. DNA captured on FTA® cards generated the same diagnostic PCR results as DNA extracted using commercial kits for 85 A. pleuropneumoniae clinical isolate cultures and 22 lung samples. Additionally, bacterial DNA bound to FTA® cards was detectable by PCR after 6 months of storage at 37°C. This study provides simple, efficient, rapid, and practical sample processing for detection and molecular serotyping of A. pleuropneumoniae.

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.

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.

Basal-Level Effects of (p)ppGpp in the Absence of Branched-Chain Amino Acids in Actinobacillus pleuropneumoniae

The (p)ppGpp-mediated stringent response (SR) is a highly conserved regulatory mechanism in bacterial pathogens, enabling adaptation to adverse environments, and is linked to pathogenesis. Actinobacillus pleuropneumoniae can cause damage to the lungs of pigs, its only known natural host. Pig lungs are known to have a low concentration of free branched-chain amino acids (BCAAs) compared to the level in plasma. We had investigated the role for (p)ppGpp in viability and biofilm formation of A. pleuropneumoniae Now, we sought to determine whether (p)ppGpp was a trigger signal for the SR in A. pleuropneumoniae in the absence of BCAAs. Combining transcriptome and phenotypic analyses of the wild type (WT) and an relA spoT double mutant [which does not produce (p)ppGpp], we found that (p)ppGpp could repress de novo purine biosynthesis and activate antioxidant pathways. There was a positive correlation between GTP and endogenous hydrogen peroxide content. Furthermore, the growth, viability, morphology, and virulence were altered by the inability to produce (p)ppGpp. Genes involved in the biosynthesis of BCAAs were constitutively upregulated, regardless of the existence of BCAAs, without accumulation of (p)ppGpp beyond a basal level. Collectively, our study shows that the absence of BCAAs was not a sufficient signal to trigger the SR in A. pleuropneumoniae (p)ppGpp-mediated regulation in A. pleuropneumoniae is different from that described for the model organism Escherichia coli Further work will establish whether the (p)ppGpp-dependent SR mechanism in A. pleuropneumoniae is conserved among other veterinary pathogens, especially those in the Pasteurellaceae family.IMPORTANCE (p)ppGpp is a key player in reprogramming transcriptomes to respond to nutritional challenges. Here, we present transcriptional and phenotypic differences of A. pleuropneumoniae grown in different chemically defined media in the absence of (p)ppGpp. We show that the deprivation of branched-chain amino acids (BCAAs) does not elicit a change in the basal-level (p)ppGpp, but this level is sufficient to regulate the expression of BCAA biosynthesis. The mechanism found in A. pleuropneumoniae is different from that of the model organism Escherichia coli but similar to that found in some Gram-positive bacteria. This study not only broadens the research scope of (p)ppGpp but also further validates the complexity and multiplicity of (p)ppGpp regulation in microorganisms that occupy different biological niches.

Etiology, epidemiology, pathology, and advances in diagnosis, vaccine development, and treatment of Gallibacterium anatis infection in poultry: a review

Gallibacterium anatis is a Gram-negative bacterium of the Pasteurellaceae family that resides normally in the respiratory and reproductive tracts in poultry. It is a major cause of oophoritis, salpingitis, and peritonitis, decreases egg production and mortality in hens thereby severely affecting animal welfare and overall productivity by poultry industries across Europe, Asia, America, and Africa. In addition, it has the ability to infect wider host range including domesticated and free-ranging avian hosts as well as mammalian hosts such as cattle, pigs and human. Evaluating the common virulence factors including outer membrane vesicles, fimbriae, capsule, metalloproteases, biofilm formation, hemagglutinin, and determining novel factors such as the RTX–like toxin GtxA, elongation factor-Tu, and clustered regularly interspaced short palindromic repeats (CRISPR) has pathobiological, diagnostic, prophylactic, and therapeutic significance. Treating this bacterial pathogen with traditional antimicrobial drugs is discouraged owing to the emergence of widespread multidrug resistance, whereas the efficacy of preventing this disease by classical vaccines is limited due to its antigenic diversity. It will be necessary to acquire in-depth knowledge on important virulence factors, pathogenesis and, concerns of rising antibiotic resistance, improvised treatment regimes, and novel vaccine candidates to effectively tackle this pathogen. This review substantially describes the etio-epidemiological aspects of G. anatis infection in poultry, and updates the recent development in understanding the pathogenesis, organism evolution and therapeutic and prophylactic approaches to counter G. anatis infection for safeguarding the welfare and health of poultry.

A bivalent fusion vaccine composed of recombinant Apx proteins shows strong protection against Actinobacillus pleuroneumoniae serovar 1 and 2 in a mouse model

Actinobacillus pleuropneumonia (APP) causes porcine pleuropneumoniae, resulting in severe economic losses in the swine industry. Since there are diverse serotypes of APP, it is necessary for vaccines to induce cross-protection. In this report, we developed a bivalent fusion vaccine, the L vaccine composed of ApxIA and ApxIIA fragments. According to the experimental results of the L vaccine, recombinant protein specific-IgG antibody level increased significantly as well as Apx toxin specific-IgG antibody, suggesting toxin-neutralizing effect. Also, the production of both IgG1 and IgG2a indicates this fusion vaccine induces Th1 and Th2 immune reactions. In addition, lymphocytes were proliferated and immune related-cytokines of TNF-α, IL-12, IFN-γ and IL-5 were detected in the serum after the vaccination. The L vaccine showed a perfect cross-protection against APP serovar 1 and 2 that each secrete different Apx exotoxins. These findings reveal that the fusion L vaccine induces specific humoral and cellular immunity, leading to a perfect cross-protection against A. pleuropneumoniae infections in a murine model.

Direct detection of Actinobacillus pleuropneumoniae in swine lungs and tonsils by real-time recombinase polymerase amplification assay

Actinobacillus pleuropneumoniae is the etiological agent of swine contagious pleuropneumoniae, which is distributed globally and associated with severe economic losses in the pig rearing industry. In this study, a real-time recombinase polymerase amplification assay (real-time RPA) based on the apxIVA gene was developed to rapid detect A. pleuropneumoniae. Real-time RPA was performed successfully in Genie III at the constant temperature of 39 °C for 20 min. The developed assay was highly specific for A. pleuropneumoniae, and the sensitivity at 95% probability was 536 fg of A. pleuropneumoniae genomic DNA. The real-time RPA for A. pleuropneumoniae was further evaluated on the 112 clinical swine lung and tonsil samples, and 25 (22.3%), 27 (24.1%), and 12 (10.7%) samples were positive for A. pleuropneumoniae by the real-time RPA, real-time PCR and bacterial isolation, respectively. With a real-time PCR as the reference method, the real-time RPA showed a diagnostic specificity of 98.8%, a diagnostic sensitivity of 88.9%, a positive predicative value of 96.0%, a negative predictive value of 96.5%, and a kappa value of 0.900. The above data demonstrated the well potentiality and usefulness of the developed real-time RPA assay in the reliable detection of A. pleuropneumoniae, especially in resource limited settings.

Actinobacillus pleuropneumoniae serotypes in Hungary

A total of 255 Actinobacillus pleuropneumoniae isolates were collected from 634 lung samples representing 70 swine herds in Hungary between January 2012 and June 2016. On the basis of the indirect haemagglutination test 77 independent strains were included in the evaluation after the elimination of duplicate or multiple serotypes from the same herd. In the case of 7 herds strains of two different serotypes were identified. Fourteen Hungarian A. pleuropneumoniae isolates from the culture collection of the Department of Microbiology and Infectious Diseases, isolated before 2012, were also included in the evaluation (one each from 12 herds and two each from two herds, where two serotypes occurred). Out of the altogether 91 A. pleuropneumoniae strains 72 strains belonged to biotype I and 19 strains could be allocated to biotype II. In Hungary, the most common serotypes were serotype 2 (39.5%), 13 (15.4%), 8 (8.8%) and 16 (8.8%), but serotypes 9 (5.5%), 11 (3.3%) and 12 (3.3%) were also isolated. Twelve strains (13.2%) were untypable.

Incorporation of Actinobacillus pleuropneumoniae in Preformed Biofilms by Escherichia coli Isolated From Drinking Water of Swine Farms

Actinobacillus pleuropneumoniae, the etiological agent of porcine pleuropneumonia, represents one of the most important health problems in the swine industry worldwide and it is included in the porcine respiratory disease complex. One of the bacterial survival strategies is biofilm formation, which are bacterial communities embedded in an extracellular matrix that could be attached to a living or an inert surface. Until recently, A. pleuropneumoniae was considered to be an obligate pathogen. However, recent studies have shown that A. pleuropneumoniae is present in farm drinking water. In this study, the drinking water microbial communities of Aguascalientes (Mexico) swine farms were analyzed, where the most frequent isolated bacterium was Escherichia coli. Biofilm formation was tested in vitro; producing E. coli biofilms under optimal growth conditions; subsequently, A. pleuropneumoniae serotype 1 (strains 4074 and 719) was incorporated to these biofilms. Interaction between both bacteria was evidenced, producing an increase in biofilm formation. Extracellular matrix composition of two-species biofilms was also characterized using fluorescent markers and enzyme treatments. In conclusion, results confirm that A. pleuropneumoniae is capable of integrates into biofilms formed by environmental bacteria, indicative of a possible survival strategy in the environment and a mechanism for disease dispersion.

Galactose-1-phosphate uridyltransferase (GalT), an in vivo-induced antigen of Actinobacillus pleuropneumoniae serovar 5b strain L20, provided immunoprotection against serovar 1 strain MS71

GALT is an important antigen of Actinobacillus pleuropneumoniae (APP), which was shown to provide partial protection against APP infection in a previous study in our lab. The main purpose of the present study is to investigate GALT induced cross-protection between different APP serotypes and elucidate key mechanisms of the immune response to GALT antigenic stimulation. Bioinformatic analysis demonstrated that galT is a highly conserved gene in APP, widely distributed across multiple pathogenic strains. Homologies between any two strains ranges from 78.9% to 100% regarding the galT locus. Indirect enzyme-linked immunosorbent assay (ELISA) confirmed that GALT specific antibodies could not be induced by inactivated APP L20 or MS71 whole cell bacterin preparations. A recombinant fusion GALT protein derived from APP L20, however has proven to be an effective cross-protective antigen against APP sevorar 1 MS71 (50%, 4/8) and APP sevorar 5b L20 (75%, 6/8). Histopathological examinations have confirmed that recombinant GALT vaccinated animals showed less severe pathological signs in lung tissues than negative controls after APP challenge. Immunohistochemical (IHC) analysis indicated that the infiltration of neutrophils in the negative group is significantly increased compared with that in the normal control (P<0.001) and that in surviving animals is decreased compared to the negative group. Anti-GALT antibodies were shown to mediate phagocytosis of neutrophils. After interaction with anti-GALT antibodies, survival rate of APP challenged vaccinated animals was significantly reduced (P<0.001). This study demonstrated that GALT is an effective cross-protective antigen, which could be used as a potential vaccine candidate against multiple APP serotypes.

Comparative sequence analysis of the capsular polysaccharide loci of Actinobacillus pleuropneumoniae serovars 1-18, and development of two multiplex PCRs for comprehensive capsule typing

Problems with serological cross-reactivity have led to development of a number of PCRs (individual and multiplex) for molecular typing of Actinobacillus pleuropneumoniae, the causative agent of porcine pleuropneumonia. Most of these assays were developed for detection of specific amplicons within capsule biosynthetic genes before the availability of complete sequences for the different serovars. Here we describe comparative analysis of the complete capsular loci for all 18 serovars of A. pleuropneumoniae, and development of two multiplex PCRs for comprehensive capsule typing of this important pig pathogen.

Antimicrobial Resistance in Pasteurellaceae of Veterinary Origin

Members of the highly heterogeneous family Pasteurellaceae cause a wide variety of diseases in humans and animals. Antimicrobial agents are the most powerful tools to control such infections. However, the acquisition of resistance genes, as well as the development of resistance-mediating mutations, significantly reduces the efficacy of the antimicrobial agents. This article gives a brief description of the role of selected members of the family Pasteurellaceae in animal infections and of the most recent data on the susceptibility status of such members. Moreover, a review of the current knowledge of the genetic basis of resistance to antimicrobial agents is included, with particular reference to resistance to tetracyclines, β-lactam antibiotics, aminoglycosides/aminocyclitols, folate pathway inhibitors, macrolides, lincosamides, phenicols, and quinolones. This article focusses on the genera of veterinary importance for which sufficient data on antimicrobial susceptibility and the detection of resistance genes are currently available (Pasteurella, Mannheimia, Actinobacillus, Haemophilus, and Histophilus). Additionally, the role of plasmids, transposons, and integrative and conjugative elements in the spread of the resistance genes within and beyond the aforementioned genera is highlighted to provide insight into horizontal dissemination, coselection, and persistence of antimicrobial resistance genes. The article discusses the acquisition of diverse resistance genes by the selected Pasteurellaceae members from other Gram-negative or maybe even Gram-positive bacteria. Although the susceptibility status of these members still looks rather favorable, monitoring of their antimicrobial susceptibility is required for early detection of changes in the susceptibility status and the newly acquired/developed resistance mechanisms.

Proposal of serovars 17 and 18 of Actinobacillus pleuropneumoniae based on serological and genotypic analysis

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Identification of two new serovars of Actinobacillus pleuropneumoniae.

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Serological confirmation of specific reactivity with homologous antisera.

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Characterization of the capsule loci of serovars 17 and 18.

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Development of PCRs for molecular diagnostics.

The aim of this study was to investigate isolates of Actinobacillus pleuropneumoniae previously designated serologically either as non-typable (NT) or as ‘K2:07’, which did not produce serovar-specific amplicons in PCR assays.

We used whole genome sequencing to identify the capsule (CPS) loci of six previously designated biovar 1 NT and two biovar 1 ‘K2:O7’ isolates of A. pleuropneumoniae from Denmark, as well as a recent biovar 2 NT isolate from Canada. All of the NT isolates have the same six-gene type I CPS locus, sharing common cpsABC genes with serovars 2, 3, 6, 7, 8, 9, 11 and 13. The two ‘K2:O7’ isolates contain a unique three-gene type II CPS locus, having a cpsA gene similar to that of serovars 1, 4, 12, 14 and 15. The previously NT isolates share the same O-antigen genes, found between erpA and rpsU, as serovars 3, 6, 8, and 15. Whereas the ‘K2:O7’ isolates, have the same O-antigen genes as serovar 7, which likely contributed to their previous mis-identification. All of the NT and ‘K2:O7’ isolates have only the genes required for production of ApxII (apxIICA structural genes, and apxIBD export genes).

Rabbit polyclonal antisera raised against representative isolates with these new CPS loci demonstrated distinct reactivity compared to the 16 known serovars. The serological and genomic results indicate that the isolates constitute new serovars 17 (previously NT) and 18 (previously ‘K2:O7’). Primers designed for amplification of specific serovar 17 and 18 sequences for molecular diagnostics will facilitate epidemiological tracking of these two new serovars of A. pleuropneumoniae.

Identification of four type II toxin-antitoxin systems in Actinobacillus pleuropneumoniae

Toxin-antitoxin (TA) systems are small genetic elements that are widely prevalent in the genomes of bacteria and archaea. These modules have been identified in various bacteria and proposed to play an important role in bacterial physiology and virulence. However, their presence in the genomes of Actinobacillus species has received no attention. In this study, we describe the identification of four type II TA systems in Actinobacillus pleuropneumoniae, the causative agent of porcine pleuropneumonia. Reverse transcription PCR analysis revealed that the genes encoding the toxin and antitoxin are co-transcribed. Overexpression of each toxin inhibited the growth of Escherichia coli, and the toxic effect could be counteracted by its cognate antitoxin. The pull-down experiments demonstrated that each toxin interacts with its cognate antitoxin in vivo. The promoter activity assays showed that each antitoxin could autoregulate either positively or negatively the TA operon transcription. In addition, the APJL_0660/0659 TA system is present in half of the detected serovars of A. pleuropneumoniae, while the others are present in all. Collectively, we identified four type II TA systems in A. pleuropneumoniae, and this study has laid the foundation for further functional study of these TA systems.

Investigation of putative invasion determinants of Actinobacillus species using comparative genomics

Actinobacillus spp. are Gram-negative bacteria associated with mucosal membranes. While some are commensals, others can cause important human and animal diseases. A. pleuropneumoniae causes severe fibrinous hemorrhagic pneumonia in swine but not systemic disease whereas other species invade resulting in septicemia and death. To understand the invasive phenotype of Actinobacillus spp., complete genomes of eight isolates were obtained and pseudogenomes of five isolates were assembled and annotated. Phylogenetically, A. suis isolates clustered by surface antigen type and were more closely related to the invasive A. ureae, A. equuli equuli, and A. capsulatus than to the other swine pathogen, A. pleuropneumoniae. Using the LS-BSR pipeline, 251 putative virulence genes associated with serum resistance and invasion were detected. To our knowledge, this is the first genome-wide study of the genus Actinobacillus and should contribute to a better understanding of host tropism and mechanisms of invasion of pathogenic Actinobacillus and related genera.

In vivo testing of novel vaccine prototypes against Actinobacillus pleuropneumoniae

Actinobacillus pleuropneumoniae (A. pleuropneumoniae) is a Gram-negative bacterium that represents the main cause of porcine pleuropneumonia in pigs, causing significant economic losses to the livestock industry worldwide. A. pleuropneumoniae, as the majority of Gram-negative bacteria, excrete vesicles from its outer membrane (OM), accordingly defined as outer membrane vesicles (OMVs). Thanks to their antigenic similarity to the OM, OMVs have emerged as a promising tool in vaccinology. In this study we describe the in vivo testing of several vaccine prototypes for the prevention of infection by all known A. pleuropneumoniae serotypes. Previously identified vaccine candidates, the recombinant proteins ApfA and VacJ, administered individually or in various combinations with the OMVs, were employed as vaccination strategies. Our data show that the addition of the OMVs in the vaccine formulations significantly increased the specific IgG titer against both ApfA and VacJ in the immunized animals, confirming the previously postulated potential of the OMVs as adjuvant. Unfortunately, the antibody response raised did not translate into an effective protection against A. pleuropneumoniae infection, as none of the immunized groups following challenge showed a significantly lower degree of lesions than the controls. Interestingly, quite the opposite was true, as the animals with the highest IgG titers were also the ones bearing the most extensive lesions in their lungs. These results shed new light on A. pleuropneumoniae pathogenicity, suggesting that antibody-mediated cytotoxicity from the host immune response may play a central role in the development of the lesions typically associated with A. pleuropneumoniae infections.

Detection of Actinobacillus Pleuropneumoniae ApxIV Toxin Antibody in Serum and Oral Fluid Specimens from Pigs Inoculated Under Experimental Conditions

Introduction

The prevention and control of Actinobacillus pleuropneumoniae in commercial production settings is based on serological monitoring. Enzyme-linked immunosorbent assays (ELISAs) have been developed to detect specific antibodies against a variety of A. pleuropneumoniae antigens, including long-chain lipopolysaccharides (LPS) and the ApxIV toxin, a repeats-in-toxin (RTX) exotoxin unique to A. pleuropneumoniae and produced by all serovars. The objective of this study was to describe ApxIV antibody responses in serum and oral fluid of pigs.

Material and Methods

Four groups of pigs (six pigs per group) were inoculated with A. pleuropneumoniae serovars 1, 5, 7, or 12. Weekly serum samples and daily oral fluid samples were collected from individual pigs for 56 days post inoculation (DPI) and tested by LPS and ApxIV ELISAs. The ApxIV ELISA was run in three formats to detect immunlgobulins M, G, and A (IgM, IgG and IgA) while the LPS ELISA detected only IgG.

Results

All pigs inoculated with A. pleuropneumoniae serovars 1 and 7 were LPS ELISA serum antibody positive from DPI 14 to 56. A transient and weak LPS ELISA antibody response was observed in pigs inoculated with serovar 5 and a single antibody positive pig was observed in serovar 12 at ≥35 DPI. Notably, ApxIV serum and oral fluid antibody responses in pig inoculated with serovars 1 and 7 reflected the patterns observed for LPS antibody, albeit with a 14 to 21 day delay.

Conclusion

This work suggests that ELISAs based on ApxIV antibody detection in oral fluid samples could be effective in population monitoring for A. pleuropneumoniae.

An anti-Propionibacterium acnes antibody shows heterologous resistance to an Actinobacillus pleuropneumoniae infection independent of neutrophils in mice

Porcine contagious pleuropneumonia is a highly fatal respiratory disease that is caused by Actinobacillus pleuropneumoniae (APP) and results in tremendous economic losses for the pig breeding industry worldwide. Previous studies have demonstrated that Propionibacterium acnes (PA) could effectively prevent APP infection in mice and pigs. The humoral immune response played a primary role during this process and anti-PA antibody could mediate macrophages to kill the bacteria. However, the role of neutrophils in this process is currently unknown. In this study, mice were injected with cyclophosphamide to deplete neutrophils and then passively immunized with anti-PA serum or negative serum. Mice were subsequently challenged with APP serotype 1. The results showed that the mice exhibited less bacterial colonization, less lung damage, and a high survival rate, which were immunized with the anti-PA antibody whether neutrophils were depleted or not. Worse still, the presence of neutrophils increased the damage to the mice after challenge. These results suggest that the activity of the anti-PA antibody against APP infection was independent of neutrophils. These findings have important significance for understanding the mechanisms of humoral immunity conferred by heterologous immunization and lay a good foundation for preventing APP infection.

Identification and characterization of serovar-independent immunogens in Actinobacillus pleuropneumoniae

Despite numerous actions to prevent disease, Actinobacillus pleuropneumoniae (A. pleuropneumoniae) remains a major cause of porcine pleuropneumonia, resulting in economic losses to the swine industry worldwide. In this paper, we describe the utilization of a reverse vaccinology approach for the selection and in vitro testing of serovar-independent A. pleuropneumoniae immunogens. Potential immunogens were identified in the complete genomes of three A. pleuropneumoniae strains belonging to different serovars using the following parameters: predicted outer-membrane subcellular localization; ≤ 1 trans-membrane helices; presence of a signal peptide in the protein sequence; presence in all known A. pleuropneumoniae genomes; homology with other well characterized factors with relevant data regarding immunogenicity/protective potential. Using this approach, we selected the proteins ApfA and VacJ to be expressed and further characterized, both in silico and in vitro. Additionally, we analysed outer membrane vesicles (OMVs) of A. pleuropneumoniae MIDG2331 as potential immunogens, and compared deletions in degS and nlpI for increasing yields of OMVs compared to the parental strain. Our results indicated that ApfA and VacJ are highly conserved proteins, naturally expressed during infection by all A. pleuropneumoniae serovars tested. Furthermore, OMVs, ApfA and VacJ were shown to possess a high immunogenic potential in vitro. These findings favour the immunogen selection protocol used, and suggest that OMVs, along with ApfA and VacJ, could represent effective immunogens for the prevention of A. pleuropneumoniae infections in a serovar-independent manner. This hypothesis is nonetheless predictive in nature, and in vivo testing in a relevant animal model will be necessary to verify its validity.

Actinobacillus pleuropneumoniae biofilms: Role in pathogenicity and potential impact for vaccination development

Actinobacillus pleuropneumoniae is a Gram-negative bacterium that belongs to the family Pasteurellaceae. It is the causative agent of porcine pleuropneumonia, a highly contagious respiratory disease that is responsible for major economic losses in the global pork industry. The disease may present itself as a chronic or an acute infection characterized by severe pathology, including hemorrhage, fibrinous and necrotic lung lesions, and, in the worst cases, rapid death. A. pleuropneumoniae is transmitted via aerosol route, direct contact with infected pigs, and by the farm environment. Many virulence factors associated with this bacterium are well characterized. However, much less is known about the role of biofilm, a sessile mode of growth that may have a critical impact on A. pleuropneumoniae pathogenicity. Here we review the current knowledge on A. pleuropneumoniae biofilm, factors associated with biofilm formation and dispersion, and the impact of biofilm on the pathogenesis A. pleuropneumoniae. We also provide an overview of current vaccination strategies against A. pleuropneumoniae and consider the possible role of biofilms vaccines for controlling the disease.

Update on Actinobacillus pleuropneumoniae-knowledge, gaps and challenges

Porcine pleuropneumonia, caused by the bacterial porcine respiratory tract pathogen Actinobacillus pleuropneumoniae, leads to high economic losses in affected swine herds in most countries of the world. Pigs affected by peracute and acute disease suffer from severe respiratory distress with high lethality. The agent was first described in 1957 and, since then, knowledge about the pathogen itself, and its interactions with the host, has increased continuously. This is, in part, due to the fact that experimental infections can be studied in the natural host. However, the fact that most commercial pigs are colonized by this pathogen has hampered the applicability of knowledge gained under experimental conditions. In addition, several factors are involved in development of disease, and these have often been studied individually. In a DISCONTOOLS initiative, members from science, industry and clinics exchanged their expertise and empirical observations and identified the major gaps in knowledge. This review sums up published results and expert opinions, within the fields of pathogenesis, epidemiology, transmission, immune response to infection, as well as the main means of prevention, detection and control. The gaps that still remain to be filled are highlighted, and present as well as future challenges in the control of this disease are addressed.

Application of an enzyme-linked immunosorbent assay for detection of antibodies to Actinobacillus pleuropneumoniae serovar 15 in pig sera

An indirect enzyme-linked immunosorbent assay (ELISA) using lipopolysaccharide extract as antigen was evaluated for detection of antibodies to Actinobacillus pleuropneumoniae serovar 15. The serovar 15 ELISA had a higher sensitivity and specificity than latex agglutination test for 63 and 80 sera from pigs experimentally infected and not infected with A. pleuropneumoniae, respectively. When the serovar 15 ELISA was applied to 454 field sera, high rates of seropositivity were found in pigs from farms infected with A. pleuropneumoniae serovar 15, but not in those from farms free of A. pleuropneumoniae serovar 15. The results suggest that the serovar 15 ELISA may be useful for the serological surveillance of infection with A. pleuropneumoniae serovar 15.

Pig Lung Immune Cytokine Response to the Swine Influenza Virus and the Actinobacillus Pleuropneumoniae Infection

Introduction

The aim of this study was to evaluate and compare the local innate immune response to the swine influenza virus (SIV) and Actinobacillus pleuropneumoniae (App) infection in pigs.

Material and Methods

The study was performed on 37 seven-week-old pigs, divided into four groups: App-infected (n=11), App+SIV-infected (n=11), SIV-infected (n=11), and control (n=4). Lung samples were collected, following euthanasia, on the 2^nd and 4^th dpi (three piglets per inoculated group) and on the 10^th dpi (remaining inoculated and control pigs). Lung concentrations of IL-1β, IL-6, IL-8, TNF-α, IL-10, IFN-α, and IFN-γ were analysed with the use of commercial porcine cytokine ELISA kits.

Results

Lung concentrations of IL-1β, IL-6, IL-8, TNF-α, IFN-α, and IFN-γ were induced in SIV-infected and App+SIV-infected pigs. In the lung tissue of App-infected pigs, only concentrations of IL-1β, IL-6, IL-8, and IFN-γ were elevated. Additionally, in App+SIV-infected pigs, significantly greater concentrations of IL-1β, IL-8, and IFN-α were found when compared with pigs infected with either SIV or App alone. In each tested group, the lung concentration of IL-10 remained unchanged during the entire study.

Conclusion

The results of the study indicate that the experimental infection of pigs with SIV or App alone and co-infection with both pathogens induced a local lung inflammatory response. However, the local cytokine response was considerably higher in co-infected pigs compared to single-infected pigs.