Evaluation of an indirect ELISA for the detection of Salmonella in chicken meat

Avaliação de um ELISA indireto para detecção de Salmonella em carne de frango

FOOD MICROBIOLOGY

Evaluation of an indirect ELISA for the detection of Salmonella in chicken meat

Avaliação de um ELISA indireto para detecção de Salmonella em carne de frango

Andréa dos Santos Schneid^I; Kelly Lameiro Rodrigues^I; Davi Chemello^II; Eduardo Cesar Tondo^II; Marco Antônio Zacchia Ayub^II;José Antonio Guimarães Aleixo^I,*

^ICentro de Biotecnologia, Universidade Federal de Pelotas, Pelotas, RS, Brasil

^IIInstituto de Ciência e Tecnologia de Alimentos, Universidade Federal do Rio Grande do Sul, Porto Alegre, RS, Brasil

ABSTRACT

In this work, an indirect ELISA based on a monoclonal antibody (MAb) specific for an outer membrane protein of Salmonellaenterica serovar Enteritidis was used for detection of Salmonella in 154 samples of chicken meat. Its efficiency was determined through comparison with the results obtained from the conventional method. The prevalence of samples contaminated with Salmonella was 23% with the conventional culture method, and 26% with the ELISA. From thirty-five samples positive for Salmonella by the conventional method, 33 were also positive by ELISA. Seven other samples were only positive in the ELISA. Comparison of the results obtained in the two methods showed an ELISA sensitivity and specificity of 94%, and positive and negative predictive values of 82% and 98% respectively. The serotyping of the isolates revealed 31 Salmonella enterica serovar Enteritidis, 2 Salmonella enterica serovar Heidelberg, 1 Salmonella enterica serovar Choleraesuis and 1 Salmonella enterica sorovar 6,7:-:-.

Key words: Salmonella, monoclonal antibody, policlonal antibody, ELISA

RESUMO

Neste trabalho, um ELISA indireto baseado em um anticorpo monoclonal (MAb) especifico para proteína de membrane externa de Salmonellaenterica serovar Enteritidis foi usado para detecção de Salmonella em 154 amostras de carne de frango. Sua eficiência foi determinada através de comparação com os resultados obtidos pela metodologia convencional. A prevalência de amostras contaminadas com Salmonella foi de 23% pelo método de cultivo convencional, e 26% pelo ELISA. De 35 amostras positivas para Salmonella pela metodologia convencional, 32 também foram positivas no ELISA. Outras sete amostras foram positivas somente no ELISA. Comparando os resultados obtidos nos dois métodos, o ELISA demonstrou sensibilidade e especificidade de 94%, e valor preditivo positivo e negativo de 82% e 98% respectivamente. A sorotipagem dos isolados revelou 31 Salmonella enterica serovar Enteritidis, 2 Salmonella enterica serovar Heidelberg, 1 Salmonella enterica serovar Choleraesuis e 1 Salmonella enterica sorovar 6,7:-:-.

Palavras-chave: Salmonella, anticorpo monoclonal, anticorpo policlonal, ELISA

INTRODUCTION

Salmonella bacteria are often involved in outbreaks of gastroenteritis all over the world (21,28). As Salmonella is widely spread throughout the environment, contaminating water and a wide range of foods, it represents a serious problem for public health and it is therefore necessary to have fast and reliable methods of detection.

The conventional method of cultivation used in detection is reliable but slow as it includes stages of pre-enrichment, selective enrichment, cultivation in selective agars, biochemical characterisation of suspected isolates and a final serological confirmation (20). The relatively long time required to carry out analysis (4-7 days) as well as the costs incurred and the storage of foods suspected of being contaminated have stimulated the development of faster detection methods that can be useful in studies of the source of contamination and of outbreaks due to food contamination (7,15,19).

Among the methods that have already been put forward are DNA hybridisation (1,17), polymerase chain reaction (3,30,35), tests with specific bacteriophages (4), latex agglutination tests (9,11,32), immunomagnetic assays (29,34) and other types of immunoenzymatic assays (24,33,36,37).

The enzyme-linked immunosorbent assay (ELISA), a type of immunoenzymatic assay that uses solid phases to anchor the antibody-antigen complex, have been widely used because offer several advantages. One of these is the capacity to detect the presence of bacteria antigens without the cells having to be viable (24). Besides, many ELISA tests, mainly those that use polyclonal antibodies, show good sensitivity, but have restricted specificity for certain serovars. Others use a combination of polyclonal antibodies in order to obtain a wide spectrum of specificity. However, this method may potentialize the occurrence of unspecific reactions due to the nature and the way that these antibodies are obtained (6).

The use of monoclonal antibodies (MAbs) with high specificity and affinity has been a good alternative in making ELISA tests more reliable. The use of a combination of MAbs for specific Salmonella epitopes is also an important tool in order to increase the sensitivity of the ELISA (10).

This study aims to evaluate an ELISA system that uses a specific monoclonal antibody for a protein taken from the external membrane of Salmonella Enteritidis in the detection of Salmonella in naturally contaminated chicken.

MATERIALS AND METHODS

Sample collection and preparation

The flow chart of procedures (20) used during the collection and processing of samples is shown in Fig. 1. A total of 154 samples of refrigerated chicken cuts were obtained from a local shop. The samples, thighs and legs, which are normally on display in trays on refrigerated shelves, were bought and wrapped in plastic film and then placed in polystyrene with ice for transportation to the laboratory where the analyses were begun on the same day. The aseptic collection of 25g samples was carried out on a laminar flow bench. The samples were then placed in sterile plastic bags containing 225 mL of lactose broth (LB) (Difco Laboratories, Detroit, MI), homogenised for two minutes in Stomacher^TM, incubated at 37ºC for 18-24 h and from there on the detection stages of the two methods being compared were done.

Conventional detection method

After the incubation period, 0.1 mL and 1.0 mL of cultures in LB (Difco) were transferred, respectively, into Rappaport Vassiliadis (RV) (Difco Laboratories, Detroit, MI) and tetrationate (TT) (Difco) broths and incubated at a temperature between 42ºC (RV) and 43ºC (TT) for 18 - 24 h. At the end of the period of selective enrichment, the broths were plated in Hektoen enteric agar (Difco) and bismuth sulphite agar (Difco) and incubated at 37ºC for 18 - 24h in order to isolate suspected colonies. Biochemical identification was then carried out on 3 to 5 isolated colonies by cultivation at 37ºC for 24 h in triple sugar iron agar (TSI) (Difco), lisine iron agar (LIA) (Difco) and urea broth (Difco). Serological confirmation of isolates was carried out by slide agglutination using somatic polyvalent serum and in tubes using polyvalent flagellar serum (Probac, São Paulo, SP). The final serotype identification was carried out at the Adolfo Lutz Institute (São Paulo, Brazil).

ELISA procedures

After selective enrichment, portions of 0.5 mL of RV and TT were transferred into tubes containing 10 mL of brain heart infusion broth (BHI) in order to carry out post-enrichment for 6h at 37ºC in a water bath with agitation. The post-enriched cultures were heated (100ºC/10 min) and used to sensitize four wells of polystyrene plates with 50 µL/well for 1h at 37ºC. The plates were washed three times with phosphate-buffered saline (0.01 M, pH 7.2) containing 0.05% Tween 20 (PBST), and two wells received MAb 424H, that react specifically with a putative outer membrane protein from salmonellae (27), and the other two were used without MAb for control of unspecific reactions. The wells received 50 µL of the MAb diluted 1:1000 in PBST and left for 1h at 37ºC. After washing the plates another three times with PBST, 50 µL/well of a protein A-peroxide conjugate (Sigma, Saint Louis, MO) diluted 1:2000 in PBST was added and left for 1h at 37ºC. The plates were washed a further 5 times with PBST, and 50 µL well of a cromogene/substrate solution was added [40 mg of ortophenylenediamine dihydrochloride (OPD), 40 µL of hydrogen peroxide- H₂O₂ and 100 mL of citrate-phosphate buffer 0.05M, pH 4.5] and left to react for 15 min in the dark at room temperature. The optical density (OD) was then read at 450 nm in a MR 700 microplate reader (Dynatech Laboratories, INC., Virginia). The ELISA cut off for positive samples corresponded to a mean OD above 0.095, or the 96^th percentile of 20 negative samples (27). As ELISA positive control, each plate contained four wells sensitized with a thermo-extracted antigen from Salmonella Enteritidis ATCC 13076. The control antigen was obtained by heating a suspension of cells with an OD₆₀₀ of 1.00, prepared from a culture in Brain Heart Infusion broth (BHI; Difco Laboratories, Detroit, MI) at 37ºC for 6h. As ELISA negative control, four wells of a mixture of antigens heat-extracted from BHI cultures of chicken samples negative for salmonellae were used. The ELISA was considered to be acceptable when the mean OD of the positive control wells was between 1.0 and 1.2 and of the negative control wells was 0.095 or less.

Statistical analysis

The data were computed using the Epi Info programme version 6.04 (13). The Yates X₂ correction test with a confidence interval of 95% was used to see if there had been significant differences between the detection results of Salmonella in the samples of chicken meat using the ELISA and the conventional methodology. Sensitivity (s), specificity (sp), predictive positive value (ppv) and negative predictive value (npv) of the ELISA in the 154 samples tested were defined as: (s) probability of obtaining a positive result if the sample was truly positive, (sp) probability of obtaining a negative result if the sample was truly negative, (ppv) probability that an ELISA positive sample was truly positive, (npv) probability that an ELISA negative sample was truly negative. Accuracy, which is defined as the percentage of results that coincide in the two methods, was obtained using the rate between the number of coincidental results in the ELISA and the conventional method, and the total number of positive and negative results by the conventional method.

RESULTS

The results obtained from the analysis of the 154 chicken samples of are shown in Table 1. The prevalence of samples contaminated with Salmonella was 23% using the conventional method of cultivation and 26% using the indirect ELISA test. This difference was not statistically significant (P< 0.001). The ELISA gave 7 false positive results and only 2 false negative results. Among the 35 samples where Salmonella was detected, 31 were contaminated with Salmonella Enteritidis, 2 with Salmonella Heildelberg, 1 with Salmonella Choleraesuis and 1 with Salmonella enterica strain 6.7:-:-.

Compared to the results obtained by the conventional method, the ELISA accuracy was 94% with sensitivity and specificity also of 94%, and predictive positive and negative values of 82% and 98% respectively (Table 2).

DISCUSSION

When comparing the results obtained by the ELISA and the conventional method in the analysis of the 154 chicken samples for the detection of Salmonella, one can see that they were not statistically different. According to these results, using the conventional method the prevalence of Salmonella in chicken was 23% while using the ELISA it was 26%. Both detection systems revealed a high percentage of chicken cuts contaminated with Salmonella. This finding proves how important this type of food probably is in the transmission of this bacteria to humans in our community. Animal based foods, especially chicken, have been associated with the occurence of Salmonella in humans, Salmonella Enteritidis being the most frequently isolated strain in this type of food (8). Similar results were found in this study which showed that out of a total of 35 Salmonella positive samples, 31 were of the Salmonella Enteritidis strain which in turn corresponds to 88.6% of the strains isolated.

The results reveal that the ELISA and the conventional methodology behaved in a similar fashion in terms of efficiency. The ELISA was able to detect Salmonella in 40 samples, 7 more than the conventional system, a number of false positive results reasonably low (4.5%). This is reflected in the high values obtained for specificity (94%) and the positive prediction value (82%) (Table 2). Various studies comparing ELISA and conventional testing verify a tendency for ELISA tests to give false positive results (4,7,25). This tendency is generally attributed to cross reactions of antibodies with antigens of others microrganismspresent in the samples or to reactions with constituents in the food. However, the low number of Salmonella in the samples, cellular injury and the interference of natural flora in the multiplication of Salmonella in foods, are some factors that may contribute to difficulties or prevention of isolation thereby causing false negative results in the conventional method (14,23). Also, the isolation of pathogens in some agars may be made difficult by the presence of a large number of competitive bacteria, some of which are very similar to Salmonella and which may cause error when selecting suspect colonies (26,31).

Peplow et al. (25) stated that false negative results in rapid detection methods are easily proved by isolation of microorganisms using the conventional method and that, on the contrary, false positive results in rapid detection methods cannot be proved as they are, in reality, false negatives in the conventional methodology. It is therefore possible that the false positive results observed in our ELISA may be due to a detection fault in the conventional test. According to Bailey (2), the new technologies for detection of pathogens in foods are more sensitive than conventional methods besides eliminating the need for isolation to confirm the presence of pathogens.

The two false negative results given by the ELISA test (1.3%) directly influenced sensitivity (94%) and the predictive negative value (98%). One explanation for the false negatives may have been the low number of Salmonella cells present in the post enrichment phase. It is possible that the natural flora present in these two samples impaired the multiplication of Salmonella cells at levels that were detectable by ELISA (16). It is also possible that the MAb 424H in the reaction had not retained enough enzyme to form a sufficient quantity of product to be detected by the system. An alternative to avoid this type of error is to use a combination of antibodies that are specific for Salmonella antigens in order to amplify test results by the higher number of epitopes detected (5). Another alternative would be the improvement of enrichment protocols used in the ELISA test by inhibiting microbial competition and favouring the growth of Salmonella to increase the amount of antigen available for reaction with detector antibody (12,18,22,31).

In conclusion, the indirect ELISA with the MAb 424H used to detect the presence of Salmonella in chicken cuts allows positive results 30 hours before conventional testing. The ELISA has a sensitivity and specificity comparable to other ELISA tests already developed and commercially available. It has a good potential for use as a screening test in the quality control of the food industry.

Submitted: March 15, 2005; Approved: June 14, 2006

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Publication Dates

• Publication in this collection
  18 Dec 2006
• Date of issue
  Sept 2006

History

• Received
  15 Mar 2005
• Accepted
  14 June 2006