Detection and identification of Actinobacillus pleuropneumoniae serotypes 1, 2, and 8 by multiplex PCR

Enhancement of Apx Toxin Production in Actinobacillus pleuropneumoniae Serotypes 1, 2, and 5 by Optimizing Culture Condition

Actinobacillus pleuropneumoniae (APP) is a causative agent of porcine pleuropneumonia. Therefore, the development of an effective vaccine for APP is necessary. Here, we optimized the culture medium and conditions to enhance the production yields of Apx toxins in APP serotype 1, 2, and 5 cultures. The use of Mycoplasma Broth Base (PPLO) medium improved both the quantity and quality of the harvested Apx toxins compared with Columbia Broth medium. Calcium chloride (CaCl₂) was first demonstrated as a stimulation factor for the production of Apx toxins in APP serotype 2 cultures. Cultivation of APP serotype 2 in PPLO medium supplemented with 10 μg/ml of nicotinamide adenine dinucleotide (NAD) and 20 mM CaCl₂ yielded the highest levels of Apx toxins. These findings suggest that the optimization of the culture medium and conditions increases the concentration of Apx toxins in the supernatants of APP serotype 1, 2, and 5 cultures and may be applied for the development of vaccines against APP infection.

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.

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.

Real-time quantitative PCR for detection and identification of Actinobacillus pleuropneumoniae serotype 2

Introduction: Porcine pleuropneumonia inflicts important economic losses on most commercial herds. Detection of subclinical or chronic infection in animals still remains a challenge, as isolation and identification of A. pleuropneumoniae serotypes is difficult and quantification of the bacteria on agar plates is often almost impossible. The aim of the study was to develop and evaluate a serotype-specific quantitative TaqMan probe-based PCR for detection of serotype 2 in pig lungs, tonsils, and nasal swabs.

Material and Methods: The primers were designed from the capsular polysaccharide biosynthesis genes of A. pleuropneumoniae serotype 2. PCR specificity and sensitivity were evaluated using reference strains and several other bacterial species commonly isolated from pigs.

Results: The real-time qPCR for detection of A. pleuropneumoniae serotype 2 was highly specific and gave no false positives with other serotypes or different bacterial species of pig origin. The detection limit for pure culture was 1.2 × 104 CFU/mL, for lung tissue and nasal swabs it was 1.2 × 105 CFU/mL, and for tonsils - 1.2 × 105 CFU/mL.

Conclusion: The method can be used to serotype A. pleuropneumoniae isolates obtained during cultivation and to detect and identify A. pleuropneumoniae serotype 2 directly in nasal swabs and tonsil scrapings obtained from live pigs or lung tissue and tonsils collected post-mortem.

Development of a Multiplex PCR Assay for Rapid Molecular Serotyping of Haemophilus parasuis

Haemophilus parasuis causes Glässer's disease and pneumonia in pigs. Indirect hemagglutination (IHA) is typically used to serotype this bacterium, distinguishing 15 serovars with some nontypeable isolates. The capsule loci of the 15 reference strains have been annotated, and significant genetic variation was identified between serovars, with the exception of serovars 5 and 12. A capsule locus and in silico serovar were identified for all but two nontypeable isolates in our collection of >200 isolates. Here, we describe the development of a multiplex PCR, based on variation within the capsule loci of the 15 serovars of H. parasuis, for rapid molecular serotyping. The multiplex PCR (mPCR) distinguished between all previously described serovars except 5 and 12, which were detected by the same pair of primers. The detection limit of the mPCR was 4.29 × 10(5) ng/μl bacterial genomic DNA, and high specificity was indicated by the absence of reactivity against closely related commensal Pasteurellaceae and other bacterial pathogens of pigs. A subset of 150 isolates from a previously sequenced H. parasuis collection was used to validate the mPCR with 100% accuracy compared to the in silico results. In addition, the two in silico-nontypeable isolates were typeable using the mPCR. A further 84 isolates were analyzed by mPCR and compared to the IHA serotyping results with 90% concordance (excluding those that were nontypeable by IHA). The mPCR was faster, more sensitive, and more specific than IHA, enabling the differentiation of 14 of the 15 serovars of H. parasuis.

The challenge of detecting herds sub-clinically infected with Actinobacillus pleuropneumoniae

The introduction into a naïve herd of animals sub-clinically infected with Actinobacillus pleuropneumoniae (App) is frequently the cause of clinical pleuropneumonia and the identification of such infected herds is a priority in the control of disease. Different serological tests for App have been developed and a number of these are routinely used. Some are species-specific whereas others identify more specifically the serotype/serogroup involved which requires updated information about important serotypes recovered from diseased pigs in a given area/country. Serotyping methods based on molecular techniques have been developed lately and are ready to be used by most diagnostic laboratories. When non-conclusive serological results are obtained, direct detection of App from tonsils is sometimes attempted. This review addresses different techniques and approaches used to monitor herds sub-clinically infected by this important pathogen.

The genetic organization of the capsular polysaccharide biosynthesis region of Actinobacillus pleuropneumoniae serotype 14

The genetic organization of the gene involved in the capsular polysaccharide (CPS) biosynthesis of Actinobacillus pleuropneumoniae serotype 14 has been determined. The DNA region for the CPS biosynthesis of serotype 14 (cps14) comprised 9 open reading frames, designated as cps14AB₁B₂B₃CDEFG genes, encoding Cps14A to Cps14G protein, respectively. Cps14A was similar to CpsA of A. pleuropneumoniae serotypes 1, 4 and 12; the Cps14B₁ and Cps14B₂ were similar to CpsB of A. pleuropneumoniae serotypes 1, 4 and 12, suggesting that CPS structure of A. pleuropneumoniae serotype 14 would belong to Group I including A. pleuropneumoniae serotypes 1, 4, 12 and 15. Surprisingly, the overall nucleotide sequence, deduced amino acid sequence, and the genetic organization of the cps14 were nearly identical to those of Actinobacillus suis. This study will provide the molecular basic knowledge for development of diagnostics and vaccine of A. pleuropneumoniae serotype 14.

The genetic organization of the capsular polysaccharide biosynthesis region of Actinobacillus pleuropneumoniae serotype 15

Nucleotide sequence determination and analysis of the cps gene involved in the capsular polysaccharide biosynthesis of Actinobacillus pleuropneumoniae serotype 15 revealed the presence of three open reading frames, designated as cps15ABC genes. At the protein level, Cps15A and Cps15B showed considerably high homology to CpsA (67.0 to 68.7%) and CpsB (31.7 to 36.8%), respectively, of A. pleuropneumoniae serotypes 1, 4 and 12, revealing the common genetic organization of the cps among serotypes 1, 4, 12 and 15. However, Cps15C showed no homology to any proteins of A. pleuropneumoniae serotypes, indicating that cps15C may be specific to serotype 15. This study will provide the basic molecular knowledge necessary for the development of diagnostics and a vaccine for A. pleuropneumoniae serotype 15.

Evaluation of a multiplex PCR to identify and serotype Actinobacillus pleuropneumoniae serovars 1, 5, 7, 12 and 15

The aim of this study was to validate a multiplex PCR for the species identification and serotyping of Actinobacillus pleuropneumoniae serovars 1, 5, 7, 12 and 15. All 15 reference strains and 411 field isolates (394 from Australia, 11 from Indonesia, five from Mexico and one from New Zealand) of A. pleuropneumoniae were tested with the multiplex PCR. The specificity of this multiplex PCR was validated on 26 non-A. pleuropneumoniae species. The multiplex PCR gave the expected results with all 15 serovar reference strains and agreed with conventional serotyping for all field isolates from serovars 1 (n = 46), 5 (n = 81), 7 (n = 80), 12 (n = 16) and serovar 15 (n = 117). In addition, a species-specific product was amplified in the multiplex PCR with all 411 A. pleuropneumoniae field isolates. Of 25 nontypeable field isolates only two did not yield a serovar-specific band in the multiplex PCR. This multiplex PCR for serovars 1, 5, 7, 12 and 15 is species specific and capable of serotyping isolates from diverse locations. Significance and impact of the study: A multiplex PCR that can recognize serovars 1, 5, 7, 12 and 15 of A. pleuropneumoniae was developed and validated. This novel diagnostic tool will enable frontline laboratories to provide key information (the serovar) to guide targeted prevention and control programmes for porcine pleuropneumonia, a serious economic disease of pigs. The previous technology, traditional serotyping, is typically provided by specialized reference laboratories, limiting the capacity to respond to this key disease.

Multiplex PCR assay for unequivocal differentiation of Actinobacillus pleuropneumoniae serovars 1 to 3, 5 to 8, 10, and 12

An improved multiplex PCR, using redesigned primers targeting the serovar 3 capsule locus, which differentiates serovars 3, 6, and 8 Actinobacillus pleuropneumoniae isolates, is described. The new primers eliminate an aberrant serovar 3-indicative amplicon found in some serovar 6 clinical isolates. Furthermore, we have developed a new multiplex PCR for the detection of serovars 1 to 3, 5 to 8, 10, and 12 along with apxIV, thus extending the utility of this diagnostic PCR to cover a broader range of isolates.

Development of two real-time polymerase chain reaction assays to detect Actinobacillus pleuropneumoniae serovars 1-9-11 and serovar 2

Two real-time, or quantitative, polymerase chain reaction (qPCR) assays were developed to detect Actinobacillus pleuropneumoniae serovars 1-9-11 (highly related serovars with similar virulence potential) and serovar 2, respectively. The specificity of these assays was verified on a collection of 294 strains, which included all 16 reference A. pleuropneumoniae strains (including serovars 5a and 5b), 263 A. pleuropneumoniae field strains isolated between 1992 and 2009 in different countries, and 15 bacterial strains other than A. pleuropneumoniae. The detection levels of both qPCR tests were evaluated using 10-fold dilutions of chromosomal DNA from reference strains of A. pleuropneumoniae serovars 1 and 2, and the detection limit for both assays was 50 fg per assay. The analytical sensitivities of the qPCR tests were also estimated by using pure cultures and tonsils experimentally spiked with A. pleuropneumoniae. The detection threshold was 2.5 × 104 colony forming units (CFU)/ml and 2.9 × 105 CFU/0.1 g of tonsil, respectively, for both assays. These specific and sensitive tests can be used for the serotyping of A. pleuropneumoniae in diagnostic laboratories to control porcine pleuropneumonia.

Isolation of atypical genotype Actinobacillus pleuropneumoniae serotype 6 in Japan

We describe here isolation of genetically atypical serotype 6 Actinobacillus pleuropneumoniae in Japan indistinguishable by the multiplex PCR that can discriminate between immunologically cross-reactive serotypes 3, 6 and 8. Nucleotide sequence analysis of capsular export and biosynthesis genes revealed that the atypical isolates have capsular polysaccharide export and synthesis gene sequences that are distinct from those of the serotype 6 reference strain. The atypical strains contain a sequence that is identical with both serotype 3- and 6-specific primers, which causes cross-reactions in multiplex PCR.

Characterization of the omlA gene from different serotypes of Actinobacillus pleuropneumoniae: A new insight into an old approach

The OmlA protein is a virulence factor of Actinobacillus pleuropneumoniae, an important pathogen in pigs. The polymorphisms present in the omlA gene sequence of 15 reference serotypes of A. pleuropneumoniae and non-serotypable isolates were assessed to determine the possible evolutionary relationship among them and to validate the importance of this gene as a molecular marker for the characterization of this bacterium. Divergence among the 15 serotypes of A. pleuropneumoniae probably resulted initially from two major evolutionary events that led to subsequent differentiation into nine groups. This differentiation makes it possible to characterize most of the serotypes by using bionformatics, thereby avoiding problems with immunological cross-reactivity. A conserved α-helix common to all the serotypes was most likely involved in connecting the protein to the outer membrane and acting as a signal peptide. A previously unknown gene duplication was also identified and could contribute to the genetic variability that makes it difficult to serotype some isolates. Our data support the importance of the omlA gene in the biology of A. pleuropneumoniae and provide a new area of research into the OmlA protein.

Detection of Actinobacillus pleuropneumoniae in drinking water from pig farms

Actinobacillus pleuropneumoniae is the aetiological agent of porcine pleuropneumonia and is normally transmitted by aerosols and direct contact between animals. A. pleuropneumoniae has traditionally been considered an obligate pathogen of pigs and its presence in the environment has yet to be investigated. Here, the presence of A. pleuropneumoniae was detected in drinking water of pig farms in Mexico using a PCR specific for the RTX toxin gene, apxIV. The presence of A. pleuropneumoniae in farm drinking water was confirmed by indirect immunofluorescence using an A. pleuropneumoniae-specific polyclonal antibody and by fluorescent in situ hybridization. Viable bacteria from the farm drinking water were detected using the Live/Dead BacLight stain. Additionally, viable A. pleuropneumoniae was selected and isolated using the cAMP test and the identity of the isolated bacteria were confirmed by Gram staining, a specific polyclonal antibody and an A. pleuropneumoniae-specific PCR. Furthermore, biofilms were observed by scanning electron microscopy in A. pleuropneumoniae-positive samples. In conclusion, our data suggest that viable A. pleuropneumoniae is present in the drinking water of swine farms and may use biofilm as a strategy to survive in the environment.

Detection of Actinobacillus pleuropneumoniae in pigs by real-time quantitative PCR for the apxIVA gene

A real-time quantitative PCR (qPCR) for detection of the apxIVA gene of Actinobacillus pleuropneumoniae was validated using pure cultures of A. pleuropneumoniae and tonsillar and nasal swabs from experimentally inoculated Caesarean-derived/colostrum-deprived piglets and naturally infected conventional pigs. The analytical sensitivity was 5colony forming units/reaction. In comparison with selective bacterial examination using tonsillar samples from inoculated animals, the diagnostic sensitivity of the qPCR was 0.98 and the diagnostic specificity was 1.0. The qPCR showed consistent results in repeatedly sampled conventional pigs. Tonsillar brush samples and apxIVA qPCR analysis may be useful for further epidemiological studies and monitoring for A. pleuropneumoniae.

A new variant of Actinobacillus pleuropneumoniae serotype 3 lacking the entire apxII operon

Actinobacillus pleuropneumoniae is an important respiratory pathogen causing pleuropneumonia in pig. The species is genetically characterized by the presence of 4 RTX (Repeats in the Structural ToXin) toxin genes: apxI, apxII, and apxIII genes are differentially present in various combinations among the different serotypes, thereby defining pathogenicity; the apxIV gene is present in all serotypes. Polymerase chain reaction (PCR)-based apx gene typing is done in many veterinary diagnostic laboratories, especially reference laboratories. The present report describes the isolation of atypical A. pleuropneumoniae from 4 independent cases from 2 countries. All isolates were beta-nicotinamide adenine dinucleotide (β-NAD) dependent and nonhemolytic but showed strong co-hemolysis with the sphingomyelinase of Staphylococcus aureus on sheep blood agar. Classical biochemical tests as well as Matrix-assisted laser desorption ionization time-of-flight mass spectrometry and sequence-based analysis (16S ribosomal RNA [rRNA] and rpoB genes) identified them as A. pleuropneumoniae. Apx-toxin gene typing using 2 different PCR systems showed the presence of apxIV and only the apxIII operon (apxIIICABD). None of the apxI or apxII genes were present as confirmed by Southern blot analysis. The 16S rRNA and rpoB gene analyses as well as serotype-specific PCR indicate that the isolates are variants of serotype 3. Strains harboring only apxIV and the apxIII operon are possibly emerging types of A. pleuropneumoniae and should therefore be carefully monitored for epidemiological reasons.

Genomic differences between Actinobacillus pleuropneumoniae serotypes 1 and 3 and the diversity distribution among 15 serotypes

The limited information on the genetic differences among the 15 currently known serotypes of Actinobacillus pleuropneumoniae has significantly hampered the development of typing-based diagnostic methods and multivalent vaccines. In this study, we compared the genomic differences between A. pleuropneumoniae strains CVCC259 (serotype 1) and CVCC261 (serotype 3) by representational difference analysis. Of the eight differential DNA sequences in the CVCC259 strain and 11 differential DNA sequences in the CVCC261 strain that we identified, seven represent known virulent genes, 10 encode putative proteins, and two encode hypothetical proteins. We also investigated the distribution of these 19 sequences among the 15 serotypes, and each serotype showed a different distribution pattern. The autotransporter adhesin occurred as a novel putative virulence factor in serotypes 1, 5, 7, 8, 9, and 11. Notably, the presence of wzm and wzt in serotypes 1, 9, and 11 and the diverse distribution of wzz and wzy in the other serotypes suggest the presence of different O-antigen biosynthesis pathways among serotypes. The information on the differential distribution of these DNA sequences in the 15 serotypes of A. pleuropneumoniae may contribute to future research on the pathogenic mechanisms of different serotypes, typing-based diagnosis methods, and multivalent vaccines.

DNA microarray-based identification and typing of Actinobacillus pleuropneumoniae

A DNA microarray system was prepared and shown to facilitate identification and typing of Actinobacillus pleuropneumoniae. The DNA microarray, composed of 18 DNA polymerase chain reaction (PCR) amplicons printed on glass slides and arranged in 3 subarrays, was developed. These target DNA included 1 or multiple fragments of the outer membrane lipoprotein, apx toxin, capsular polysaccharide, and disulfide bound formation protein E (dsbE)-like genes of A. pleuropneumoniae. These arrayed target DNA retained their expected hybridization properties. The hybridization signal intensities ranged from the least-intense to the most-intense, 4626 to 9789 arbitrary fluorescence units, respectively. Cy3-probes of A. pleuropneumoniae strains labeled with multiplex PCR were hybridized to the DNA microarray. A total of 51 different A. pleuropneumoniae strains representing serotype 1 to 12 reference strains and clinical isolates were detected and typed by the DNA microarray. Twelve reference serotypes produced 11 distinct target DNA hybridization patterns, and hybridization patterns of serotypes 1 (n = 7), 3 (n = 5), and 7 (n = 6) field isolates were identical to hybridization patterns of reference serotypes 1, 3, and 7, respectively. Non-serotyped isolates 4, 6, and 11 (out of 21) from diseased pigs had identical hybridization patterns to reference serotypes 3, 7, and 1, respectively. The results show that the DNA microarray system described in the present study is a valuable tool for identifying and typing reference strains and isolates of A. pleuropneumoniae, and enables relatively rapid identification of non-serotyped isolates.

Effect of tulathromycin on the carrier status of Actinobacillus pleuropneumoniae serotype 2 in the tonsils of pigs

The effect of a single or double dose of tulathromycin was evaluated in pigs carrying Actinobacillus pleuropneumoniae serotype 2 in their tonsils. Twenty-nine pigs from a reinfected specific pathogen-free-herd were selected from animals testing positive in an A pleuropneumoniae serotype 2-specific pcr test on tonsil scrapings and they were divided into three groups. The pigs in group 1 were treated subcutaneously with 2.5 mg/kg tulathromycin on day 0, the pigs in group 2 were treated with 2.5 mg/kg tulathromycin on days 0 and 4, and the pigs in group 3 were left untreated as controls. The pigs were tested by pcr on tonsil scrapings on days 0, 4, 11 and 33, and on day 33 all the animals were euthanased. There were no significant differences between the numbers of PCR-positive animals in the three groups on any of the sampling dates.

PCR specific for Actinobacillus pleuropneumoniae serotype 3

Serotypes 3 and 8 of Actinobacillus pleuropneumoniae, the aetiological agent of porcine pleuropneumonia, have been reported to predominate in the UK. Direct serotyping of isolates of the organism is typically determined by the immunological reactivity of rabbit serum to its surface polysaccharides, but the method has limitations, for example, cross-reactions between serotypes 3, 6 and 8. This study describes the development of a serotype 3-specific pcr, based on the capsule locus, which can be used in a multiplex format with the organism's specific gene apxIV. The pcr test was evaluated on 266 strains of A pleuropneumoniae and 121 strains of other organisms, including all the major respiratory bacterial pathogens of pigs. The test was highly specific and sensitive and should be useful for differentiating strains of serotypes 3, 6 and 8, and in seroprevalence and epidemiological surveys in regions where serotype 3 is prevalent, such as the UK.

Development of a multiplex PCR test for identification of Actinobacillus pleuropneumoniae serovars 1, 7, and 12

A PCR assay for simultaneous species identification and separation of Actinobacillus pleuropneumoniae serovars 1, 7 and 12 was developed. Primers specific for genes involved in biosynthesis of the capsular polysaccharides (cps genes) of serovars 1, 7, and 12 were combined with a species-specific PCR test based on the omlA gene. The PCR test was evaluated with the serovar reference strains of A. pleuropneumoniae as well as 183 Danish field isolates. For all typable strains, a complete correspondence was found between results obtained with the multiplex PCR test and results from the traditional serotyping methods. Among eight serologically cross-reacting strains designated K1:O7, seven isolates produced amplicons of similar sizes as serovar 1 and one isolate produced amplicons of similar sizes as serovar 7. The species specificity of the assay was evaluated using a collection of 126 strains representing 25 different species within the family Pasteurellaceae including 45 field strains of the phylogenetically affiliated species Actinobacillus lignieresii. All these isolates tested negative for the cps genes by the multiplex PCR test except for 6 isolates of A. lignieresii. Five of these isolates produced an amplicon identical to the cps gene of serovar 7, whereas one isolate produced an amplicon identical to the cps gene of serovar 1. In addition, four isolates of Actinobacillus genomospecies 1 tested positive for the omlA gene but negative for the cps genes. The test represents a convenient and specific method for serotyping A. pleuropneumoniae in diagnostic laboratories.

Multiplex PCR that can distinguish between immunologically cross- reactive serovar 3, 6, and 8 Actinobacillus pleuropneumoniae strains

We describe a highly sensitive and specific multiplex PCR, based on capsular loci and the species specific apxIV gene, that unequivocally differentiates serovar 3, 6, and 8 Actinobacillus pleuropneumoniae strains that are cross-reactive in conventional immunological tests.

The genetic organisation of the capsule biosynthesis region of Actinobacillus pleuropneumoniae serotypes 1, 6, 7, and 12

The aim of the present study was to investigate the organisation of the genes (cps) involved in biosynthesis the capsular polysaccharide (CPS) of Actinobacillus pleuropneumoniae serotypes 6, 7, and 12 and to compare these to the corresponding genes previously described in other A. pleuropneumoniae serotypes. In serotypes 6 and 7 the sequenced DNA regions comprised five and four open reading frames, respectively, designated cps6ABCDE and cps7ABCD, whereas the sequenced DNA region in serotype 12 comprised only two open reading frames designated cps12AB. At the amino acid level, CpsA, CpsB, and CpsC of A. pleuropneumoniae serotypes 2, 6, 7, and 8 contained a high degree of homology. At the amino acid level Cps6D revealed a high degree of homology to Cps8D, whereas Cps7D contained a high degree of homology to the Cps2D. The deduced gene product of the partially sequenced cps6E gene showed no homology to any deduced gene products of any cps genes of A. pleuropneumoniae investigated so far. None of the deduced gene products of the cps genes involved in encapsulation of A. pleuropneumoniae serotypes 2, 6, 7, and 8 revealed homology to the deduced gene products of the cps genes of serotypes 1, 5A, and 12. For some genes, a local homology was found to genes probably involved in teichoic acid synthesis in other bacteria. The results obtained revealed a high degree of homology among the genes involved in CPS biosynthesis for serotypes 2, 6, 7, and 8 and a different group of homologous cps genes for serotypes 1 and 12. In some serotype 7 strains, including the serotype 7 reference strain, WF83, the cps genes were not located adjacent to the genes responsible for CPS export (cpx), probably due to genetic rearrangements.

Genetic diversity of Actinobacillus pleuropneumoniae assessed by amplified fragment length polymorphism analysis

Amplified fragment length polymorphism (AFLP) was evaluated as a method for genotypic characterization and subtyping within the bacterial species Actinobacillus pleuropneumoniae. A total of 155 isolates of A. pleuropneumoniae, representing the serotypic variation described to occur within this species, were analyzed. In order to elucidate the species boundaries, six strains of the phylogenetically closely related species Actinobacillus lignieresii were also included. Furthermore, the ability of AFLP to subtype was studied using 42 isolates of serovar 2 and the performance compared to that obtained by pulsed-field gel electrophoresis (PFGE). AFLP analysis provided a clear separation of A. lignieresii and A. pleuropneumoniae and divided the isolates of A. pleuropneumoniae into 20 clusters. Most of the serovars of A. pleuropneumoniae were represented by single and quite homogeneous clusters. The exceptions were serovars 10, K2:O7, and K1:O7, which were represented by two clusters each. In the cases where the serovars were represented by more than one cluster, the existence of these clusters was supported by additional phenotypic or genotypic properties. Furthermore, AFLP typing was able to allocate serologically nontypeable isolates to appropriate genetic groups within the species. Further investigations are needed to determine whether some of the clusters revealed through AFLP analysis represent additional serovars. When evaluated as a method for subtyping within serovar 2 of A. pleuropneumoniae, AFLP was found to achieve a degree of separation among isolates superior to that obtained by PFGE. However, a higher degree of separation between serovar 2 isolates was obtained by a combination of the two methods.

Use of miniaturized protein arrays for Escherichia coli O serotyping

Serological typing of Escherichia coli O antigens is a well-established method used for differentiation and identification of O serotypes commonly associated with disease. In this feasibility study, we have developed a novel somatic antibody-based miniaturized microarray chip, using 17 antisera, which can be used to detect bound whole-cell E. coli antigen with its corresponding immobilized antibody, to assess the feasibility of this approach. The chip was tested using the related 17 control strains, and the O types found by the microarray chip showed 100% correlation with the O types found by conventional typing. A blind trial was performed in which 100 E. coli isolates that had been O serotyped previously by the conventional assay were tested by the array approach. Overall, the O serotypes of 88% of isolates were correctly identified by the microarray method. For several isolates, ambiguity of O-type designation by microarray arose due to increased sensitivity of this method, allowing signal intensities of cross-reactions to be quantified. Investigation of discrepancies between conventional and microarray O serotyping indicated that some isolates upon storage had become untypeable and, therefore, gave poor signal intensity when tested by the microarray or retested by conventional means. For all 20 serotype O26 and O157 isolates, the apparent discrepancy in O serotyping was analyzed further by a third independent test, which confirmed the microarray results. Therefore, the use of miniaturized protein arrays increases the speed and efficiency of O serotyping in a cost-effective manner, and these preliminary findings suggest the microarray approach may have a higher accuracy than those of traditional O-serotyping methods.

In-source fragmentation and analysis of polysaccharides by capillary electrophoresis/mass spectrometry

In order to develop a robust and easy-to-use technique for characterization of bacterial polysaccharides, a pseudo-hydrolysis strategy was investigated. Based on in-source collision-induced dissociation, polysaccharide molecular ions were fragmented within the orifice-skimmer region of an electrospray ionization (ESI) mass spectrometer. The fragment ions thus generated were then analyzed similarly to the conventional ESI mass spectrometry approach. MS/MS scanning was applied to obtain product-ion spectra of the primary fragments for sequencing. To further improve the sensitivity and separation of polysaccharides from other components in the samples, a pressure-assisted capillary electrophoresis/mass spectrometry (CE/MS) system was employed. Using bacterial polysaccharides as model compounds, the mass spectra obtained for polysaccharide repeating units generated through chemical hydrolysis and in-source fragmentation were directly compared, both in positive and negative ion modes. With the additional separation of impurities provided by CE, the success of this technique has been demonstrated for structural analysis of O-chain polysaccharides (O-PS) and capsular polysaccharides (CPS). In-source fragmentation was applied to promote the formation of structurally relevant repeating units of heterogeneous CPS that would remain undetected using conventional ESI conditions. This approach was proven to be particularly useful for probing the subtle structural differences in monosaccharide composition and functionalities arising across bacterial serotypes.