Optimization and standardization of an enzyme-linked immunosorbent assay protocol for serodiagnosis of Actinobacillus pleuropneumoniae serotype 5

Long-chain LPS-based enzyme-linked immunosorbent assay to detect swine herds infected by Actinobacillus pleuropneumoniae serotype 17

Actinobacillus pleuropneumoniae serotype 17, one of the two most recent serotypes described, has been isolated from diseased pigs in North America. Yet, no serological test for surveillance has been developed so far. An enzyme-linked immunosorbent assay (ELISA) using the long-chain lipopolysaccharide antigen (LC-LPS) of this serotype is described. As predicted by previous genetic data on the O-antigen locus, cross reactions were observed between this serotype and serotypes 3, 6, 8, and 15. Although animals infected by serotype 17 would be detected using the current serotype 3 LC-LPS ELISA, better results may be obtained when plates are coated with the antigen purified from the homologous serotype.

In house ELISA based on recombinant ORF2 protein underline high prevalence of IgG anti-hepatitis E virus amongst blood donors in south Brazil

Hepatitis E Virus (HEV) is a zoonotic pathogen responsible for causing acute hepatitis in human, especially in developing countries. Diagnosis of HEV usually relies on the detection of antibodies mostly by enzyme-linked immunosorbent assay (ELISA). In the present study, we designed a new indirect ELISA (iELISA) based on a short recombinant peptide derived from the capsid protein (ORF2p) and demonstrated its potential for detecting human IgG against HEV genotype 3. The best polystyrene plate (Maxisorp^®), optimal ORF2p coating antigen concentration (0,67μg/well) and primary antibody dilution (1:100) were determined. This iELISA showed a sensitivity of 91.4% and specificity of 95.9%. The comparison of our in house iELISA with a commercial assay (RecomWell, Mikrogen^®) showed 94.25% of agreement and a kappa index of 0.88. The ORF2 recombinant ELISA was used to screen 780 blood donors for anti-HEV IgG and we found that 314 (40,25%) of these donors were IgG positive. This high prevalence of antibodies suggests, for the first time, that the Southern Brazil region might be endemic to Hepatitis E Virus genotype 3.

A modified enzyme-linked immunosorbent assay adapted for immunodetection of low amounts of water-insoluble proteins

A mixture of thiourea, urea and CHAPS (TUC) is an excellent solvent compatible with isoelectrofocusing (IEF) separation of water-insoluble protein extracts, and their subsequent two-dimensional gel electrophoresis is an important step in proteomic studies. The main aim of this work was to quantify extremely low amounts of water-insoluble proteins contained, for instance, in samples collected in bio-aerosol samplers. High CHAPS concentrations solubilize many proteins. However, enzyme-linked immunosorbent assay (ELISA), which is the most popular immunodetection method of quantifying antigens, is unfortunately not compatible with these high CHAPS concentrations and with the low protein concentrations of TUC extracts. The most common mixture used to solubilize these proteins contains 2 mol l(-1) thiourea, 7 mol l(-1) urea and 5% w/v CHAPS. This paper shows that these components inhibit the adsorption and/or recognition of proteins on microtitration plates, preventing antigen quantification under classic ELISA conditions. We have tried several solvents (ethanol, isopropanol, acetonitrile and trichloroacetic acid) to make the TUC-soluble proteins stick to the ELISA plates, and ethanol was shown to be the most appropriate. In this study, we have defined a new ELISA protocol allowing rapid and sensitive detection of low concentrations (60-500 ng ml(-1)) of water-insoluble proteins extracted with high concentrations of TUC.

Detection of antibodies to Actinobacillus pleuropneumoniae serotype 12 in pig serum using a blocking enzyme-linked immunosorbent assay

The objective was to develop a blocking enzyme-linked immunosorbent assay (ELISA) for detection of antibodies to Actinobacillus pleuropneumoniae (Ap) serotype 12 in pig serum. Lipopolysaccharide (LPS) from Ap serotype 12 was purified and used as antigen in the assay. Antibodies to the LPS antigen in samples of pig serum were detected by inhibition of the binding of polyclonal rabbit antibodies raised against Ap serotype 12. The assay was evaluated against sera from experimentally infected pigs, from pig herds naturally infected with Ap and from herds declared free of Ap serotype 12 infection. The blocking ELISA showed no cross-reaction when tested with sera from pigs experimentally infected with 12 other serotypes of Ap biotype 1. The sensitivity and specificity of the blocking ELISA on the herd level was evaluated by testing sera from pig herds naturally infected with Ap serotypes 2 and/or 12 and from herds declared free of infection with Ap. The Ap serotype 12 blocking ELISA showed a herd sensitivity of 0.77 (95% confidence interval, 0.62-0.88) and a herd specificity of 1.00 (0.95-1.00) with a cut-off value at 40% relative absorbance or 60% inhibition. The assay may be used advantageously as a confirmatory test in serological surveillance programmes for Ap infections in SPF systems for pig production.

Evaluation of an enzyme-linked immunosorbent assay for serological surveillance of infection with Actinobacillus pleuropneumoniae serotype 5 in pig herds

An indirect enzyme-linked immunoassay for serological surveillance of infection of pigs with Actinobacillus pleuropneumoniae (Ap) serotype 5 was developed. The antigen used was prepared from Ap serotype 5b strain L20. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) analysis showed that the antigen contained high molecular weight lipopolysaccharide (LPS) and presumably also capsular polysaccharide (CP). The Ap serotype 5 ELISA was tested using sera from pigs experimentally infected with the 12 different Ap serotypes of biotype 1 and with sera from herds naturally infected with Ap serotypes 5, 6, 7 and 12. Cross-reactions were shown in one pig from a herd naturally infected with Ap serotype 7 and in one pig from a herd naturally infected with Ap serotype 12. The herd sensitivities of the Ap5 ELISA and a complement fixation test (CFT) were both estimated to 1.0, on the basis of serum samples from six herds naturally infected with Ap serotype 5. The herd specificities of both tests were estimated to 0.98, based on serum samples from 123 pig herds (10 samples from each herd) from the Danish specific pathogen-free (SPF) programme for pig production.

Seroprevalence of Actinobacillus pleuropneumoniae serovars 2, 3 and 9 in slaughter pigs from Belgian fattening farms

Fifty randomly selected fattening pig herds were studied to investigate the epidemiological characteristics of infections with Actinobacillus pleuropneumoniae serovars 2, 3 and 9, and to identify risk factors for their within-herd seroprevalences. Information about 13 farm characteristics was obtained by means of a questionnaire and used to assess potential risk factors for the percentage of slaughter pigs with antibodies against each of the three serovars. The presence of antibodies was measured with an indirect ELISA. The median within-herd seroprevalence for serovar 2 was 58 per cent (range 0 to 100 per cent), for serovar 3, 53 per cent (range 10 to 95 per cent), and for serovar 9, 35 per cent (range 5 to 100 per cent). All but one farm tested positive for A pleuropneumoniae serovar 2, and all the farms were positive for A pleuropneumoniae serovars 3 and 9. There was a positive association (P < 0.05) between each pair of serovars. The within-herd seroprevalence of serovar 2 was significantly associated with the density of pig herds in the municipality (odds ratio [OR] = 1.60; P < 0.05) and with the absence of preventive medication at the start of the fattening period (OR = 2.77; P < 0.10). No significant risk factors were found for serovar 3. The percentage of pigs positive for serovar 9 was significantly associated with a slaughter date in June (OR = 2.30; P < 0.10) and with herds in which the finishing houses were not divided into separate compartments (OR = 2.99; P < 0.05).

Serologic detection of Actinobacillus pleuropneumoniae in swine by capsular polysaccharide-biotin-streptavidin enzyme-linked immunosorbent assay

Serologic detection of Actinobacillus pleuropneumoniae infections in swine have been problematic due to antigenic cross-reactivity of Apx toxins, lipopolysaccharide, and outer membrane proteins between A. pleuropneumoniae serotypes and other bacterial species. To maximize serologic specificity and sensitivity, we developed an assay that uses highly purified A. pleuropneumoniae capsular polysaccharide (CP) conjugated to biotin, which is then bound to streptavidin-coated enzyme-linked immunosorbent assay (CP-BS-ELISA) plates. This assay was used to test a panel of 240 serum samples from pigs prior to challenge, after challenge with bacterial species other than A. pleuropneumoniae, or after challenge with A. pleuropneumoniae serotype 1, 5, or 7. Overall assay results for the individual sera tested were reproducible on the same day and on separate days. The sensitivity of the assay was 100% by ELISAs with biotin-CPs of serotypes 1 and 7 and 87.5% by ELISAs with biotin-CP of serotype 5. Specificity was 100% by ELISAs with biotin-CPs of serotypes 1 and 5 and 94.5% by ELISAs with biotin-CP of serotype 7. The biotin-CPs of at least three A. pleuropneumoniae serotypes could be combined for use in a screening assay to detect antibodies to CPs from strains of different serotypes. In conclusion, the CP-BS-ELISA proved to be a serotype-specific and species-specific assay with high sensitivity for the identification of pigs exposed to A. pleuropneumoniae.

A novel enzyme-linked immunosorbent assay using the recombinant Actinobacillus pleuropneumoniae ApxII antigen for diagnosis of pleuropneumonia in pig herds

For the surveillance of pig herds infected with porcine pleuropneumonia, an enzyme-linked immunosorbent assay (ELISA) using the recombinant Actinobacillus pleuropneumoniae ApxII protein as species- but not serotype-specific antigen was developed. Using this ELISA, 243 of 400 animals from 22 A. pleuropneumoniae-infected herds were classified as seropositive.

Serodiagnosis of swine pleuropneumonia due to Actinobacillus pleuropneumoniae serotypes 7 and 4 using long-chain lipopolysaccharides

A saline boiled extract (SBE), capsular polysaccharides (CPS) and long-chain lipopolysaccharides (LC-LPS) of Actinobacillus pleuropneumoniae serotype 7 have been evaluated in ELISA for the serodiagnosis of swine pleuropneumonia caused by this serotype. Mean optical densities (ODs) obtained with the 3 antigens using sera from negative herds as well as from animals experimentally and naturally exposed to A. pleuropneumoniae serotypes 7 or 4 were not statistically different. The positive ELISA reaction with anti-serotype 4 sera was unexpected with the CPS, which are supposed to be serotype-specific; LPS traces present in the CPS appeared to be responsible for this reaction. In addition, sera from animals exposed to A. pleuropneumoniae serotypes 5 or 10 presented cross-reactions with the SBE and the CPS, but not with the LC-LPS. Cross-reactions were mainly due to rough LPS, as shown by immunoblotting. The LC-LPS is easily obtainable and can be used for the detection of antibodies in animals infected with A. pleuropneumoniae serotypes 7 and 4.

Evaluation of a saline boiled extract, capsular polysaccharides and long-chain lipopolysaccharides of Actinobacillus pleuropneumoniae serotype 1 as antigens for the serodiagnosis of swine pleuropneumonia

A saline boiled extract (SBE), capsular polysaccharides (CPS) and long-chain lipopolysaccharides (LC-LPS) of Actinobacillus pleuropneumoniae serotype 1 have been evaluated in ELISA for the serodiagnosis of swine pleuropneumonia caused by this serotype. Mean optical densities (ODs) obtained with the three antigens using sera from negative herds as well as from animals experimentally and naturally exposed to A. pleuropneumoniae serotypes 1, 9 or 11 were not significantly different. The positive ELISA reaction with anti-serotypes 9 and 11 was unexpected with the CPS, which are supposed to be serotype-specific; LPS, and to a lesser extent proteins, were present in the CPS and appeared to be responsible for this reaction. In addition, sera from animals exposed to a field strain of A. pleuropneumoniae serotype 3 and to Actinobacillus suis presented a significantly lower mean OD (P < 0.001) when LC-LPS were used. Cross-reacting antigens consisted mainly of LPS core-lipid A present in the SBE and CPS. The specificity and the sensitivity of the ELISA were evaluated using three different cut-off values (the OD plus two, three and four times the standard deviation or SD) obtained with 667 negative sera. The diagnostic sensitivity was of 81% with the three antigens and the different thresholds. The diagnostic specificity was of 84, 86 and 88% for the mean plus two, three and four times the SD respectively using the SBE and the CPS, while that obtained with the LC-LPS was of 96, 98 and 99% using the same thresholds. In conclusion, LC-LPS make an easily obtainable antigen and seem to retain the best specificity while minimizing losses of sensitivity.

Lipopolysaccharides of Actinobacillus pleuropneumoniae (serotype 1): a readily obtainable antigen for ELISA serodiagnosis of pig pleuropneumonia

A saline extract of boiled-formalinized whole cells of Actinobacillus pleuropneumoniae serotype 1 reference strain (Shope 4074) has been previously used as the antigen in an enzyme-linked immunosorbent assay (ELISA) for serodiagnosis of swine pleuropneumonia. Phenol extraction of this crude extract permitted the recovery of LPS with long O-chains in the aqueous phase. This antigen was shown to be specific for serotypes 1, 9 and 11 as they all possess structurally similar O-chains. Immunoblotting was used to identify the fraction present in the crude extract of strain 4074 responsible for cross-reactions observed in ELISA with a serum raised against a serotype 3 strain of A. pleuropneumoniae. The specific reactions in ELISA were shown to be associated with long O-chain LPS and the cross-reactions to LPS with short O-chains. LPS seem to be an important antigen for A. pleuropneumoniae serotype 1 as all homologous sera tested reacted with it. This antigen is easily recovered from the crude extract and it can be used in minute amounts (1-6 micrograms) for ELISA serodiagnosis of pig pleuropneumonia.

Detection of Actinobacillus pleuropneumoniae in the porcine upper respiratory tract as a complement to serological tests

Attempts were made to isolate Actinobacillus pleuropneumoniae from the nasal cavities and tonsils of 442 healthy pigs from 15 herds. Samples were streaked onto different media formulations. Serum samples were assayed for antibodies to A. pleuropneumoniae by enzyme-linked immunosorbent assay and complement fixation test. Actinobacillus pleuropneumoniae was isolated from the nasal cavities only in 24 pigs, from tonsils only in 90 pigs, and from both the nasal cavities and the tonsils in 11 pigs. A PPLO medium supplemented with lincomycin, bacitracin and crystal violet allowed recovery of A. pleuropneumoniae from more animals than a tryptic soy agar medium from both sites. Incubation of plates in an enriched CO2 atmosphere did not affect the recovery rate. Actinobacillus pleuropneumoniae belonging to serotypes 1, 2, 3, 5a, 5b, 7, 8, 10 and 12 were isolated, and, in several herds, more than one serotype were recovered. Serotypes of A. pleuropneumoniae were isolated from nine herds which were found seronegative to these. The isolation of A. pleuropneumoniae from the upper respiratory tract can be useful for detection of carrier pigs and complements serological screening.