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Applied and Environmental Microbiology, May 2000, p. 2278-2281, Vol. 66, No. 5
0099-2240/00/$04.00+0
Copyright © 2000, American Society for Microbiology. All rights reserved.

Production and Characterization of Monoclonal Antibodies against Vegetative Cells of Bacillus cereus

Nadine Charni, Claude Perissol, Jean Le Petit, and Nathalie Rugani*

Laboratoire de Microbiologie, Service 452, UPRES A 6116 CNRS, Faculté des Sciences et Techniques de St Jérôme, Université Aix-Marseille, 13397 Marseille Cedex 20, France

Received 20 December 1999/Accepted 3 March 2000


    ABSTRACT
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Two monoclonal antibodies (MAbs) against Bacillus cereus were produced. The MAbs (8D3 and 9B7) were selected by enzyme-linked immunosorbent assay for their reactivity with B. cereus vegetative cells. They reacted with B. cereus vegetative cells while failing to recognize B. cereus spores. Immunoblotting revealed that MAb 8D3 recognized a 22-kDa antigen, while MAb 9B7 recognized two antigens with molecular masses of approximately 58 and 62 kDa. The use of MAbs 8D3 and 9B7 in combination to develop an immunological method for the detection of B. cereus vegetative cells in foods was investigated.


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Vegetative cells and spores of Bacillus cereus are present in the environment and can frequently be found in many raw, dried, and processed foods (4, 13, 19, 23, 25, 31, 41). This bacterium has been implicated in two different types of food poisoning, namely emetic and diarrheal (26, 28), and is responsible for numerous cases of food spoilage because of the production of lipases and proteases (7, 35). In addition, certain strains of B. cereus can grow at temperatures as low as 4 to 6°C (41, 42), and these psychrotrophic B. cereus strains are a health risk to the consumer since vegetative cells can produce enterotoxins mainly in the exponential phase (8, 15-17). It is impossible for the food industry to exclude B. cereus from their products because, as many studies have shown, B. cereus cells can survive heat processing and can grow in foods kept at refrigerated storage conditions. Thus, it is important to develop methods to detect the presence of B. cereus in order to eliminate the threat of food poisoning. Several selective plating methods described for detecting B. cereus require, with confirmatory testing, up to 4 days to perform (21, 24, 32, 33, 39, 40). Other efforts in B. cereus research have focused on detection of the organism by detection of enterotoxin-producing cells (1, 22, 29), and commercial kits designed to detect enterotoxic B. cereus via immunoassays have been developed (6, 7, 9). In the food industry, immunoassays are also used to detect spore-forming and non-spore-forming bacteria. Commercial immunoassay-based kits that use either polyclonal antibodies or monoclonal antibodies (MAbs) are available to detect Salmonella, Listeria, and other organisms. Immunoassays have been developed for the detection of Bacillus and Clostridium spores by using polyclonal antibodies and MAbs in enzyme-linked immunosorbent assays (ELISAs) (11, 36, 37). Immunoassays have also been developed for the vegetative cells of both spore-formers and nonsporeformers (20). To date, however, there are no commercially available ELISAs for the rapid detection of the vegetative cells of B. cereus in food products.

In this paper, we describe the production and characterization of two MAbs against B. cereus. These antibodies were selected by ELISA for their reactivity with B. cereus vegetative cells. The specificity of the selected antibodies was tested against bacterial cells of a variety of species within and across genera and spores of B. cereus. These antibodies, which are specific for vegetative cells, can be used to develop a rapid and sensitive method for the detection of strains of B. cereus in foods that potentially cause food poisoning.

Bacterial strains and culture conditions. The bacterial strains used in this study are shown in Table 1. All bacteria were grown at 30°C in Trypticase Soy medium (bioMerieux, Marcy l'Etoile, France). A spore suspension of B. cereus cells was prepared from an overnight culture of B. cereus vegetative cells that was inoculated on the sporulation medium described by Faille et al. (10). To remove vegetative cell remnants, spores were treated with a solution of thimerosal as described by Norris and Wolf (34). After centrifugation at 10,000 × g for 10 min, spores were then incubated in TEL buffer containing 100 mM Tris-HCl (pH 8), 5 mM EDTA, and 0.5% lysozyme at 50,000 U/mg for 1 h at 37°C.

                              
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  		TABLE 1.   Specificities of MAbs for B. cereus vegetative cells, as assessed by ELISA

For immunization procedure and immunochemical techniques, all bacterial cells in an exponential growing stage and B. cereus spores were harvested by centrifugation at 10,000 × g for 10 min at 4°C and washed twice in phosphate-buffered saline (PBS).

Production of MAbs. The MAbs were produced by the procedure described by Galfre and Milstein (12). Vegetative cells of B. cereus LMG 6923 (108/ml) were injected into BALB/c mice. Hybridomas were screened for antibody production by ELISA with vegetative cells of B. cereus LMG 6923 as antigens. Selected hybridomas were cloned at least twice by limiting dilution method. The MAb-secreting clones were propagated as ascitic fluid by the procedure of Harlow and Lane (18). The isotyping of MAbs was performed with a mouse monoclonal isotyping kit according to the manufacturer's instructions. Antibodies were concentrated by ammonium sulfate precipitation of ascites, and immunoglobulin G (IgG) antibody was purified by using a protein A column (18).

Immunochemical techniques. (i) ELISA. The ELISA was done according to the method described by Harlow and Lane (18), with a few modifications. Briefly, microtiter plates (Immulon-1; Dynatech, Chantilly, Va.) were coated overnight at 4°C with 108 bacterial cells or B. cereus spores per ml. After blocking with PBS containing 3% nonfat dry milk and 0.05% Tween 20, the plates were incubated with the MAbs. Peroxidase-conjugated goat anti-mouse IgG+IgM (Jackson Immunoresearch, Immunotech, Marseille, France) and o-phenylenediamine (Sigma Chemical Co., St. Quentin Fallavier, France) were used as secondary antibodies and substrate, respectively. Absorbance was read at 490 nm by using a microtiter plate reader (Metertech varepsilon  960 Instrument; BioBlock).

The ELISA procedure was also performed on plates coated with trypsin-protease-treated B. cereus vegetative cells to determine MAb specificity for protein antigens: wells at 108 cells/ml were treated with 1 mg of trypsin at 37°C for 4 h and were treated again with 100 µg of protease at 37°C overnight.

(ii) SDS-PAGE and immunoblotting. B. cereus LMG 6923 (108 vegetative cells) was extracted by the method of Matar et al. (30) and was subjected to electrophoresis through sodium dodecyl sulfate (SDS)-12% polyacrylamide gels as described by Laemmli (27).

Proteins separated by SDS-polyacrylamide gel electrophoresis (PAGE) were electroblotted by the method of Harlow and Lane (18). After transfer for 3 h at 1.2 A, the membrane was blocked for 1 h in PBS containing 5% nonfat dry milk and 0.5% Tween 20. MAbs and peroxidase-conjugated goat anti-mouse IgG+IgM (Jackson Immunoresearch) were used as primary and secondary antibodies, respectively. The blots were visualized with chemiluminescence (DuPont Co., Newtown, Conn.).

For glycoprotein determination, proteins separated by SDS-PAGE were exposed to periodic acid-Schiff staining (38). The proteins which stained positive as glycoproteins were determined and compared to the antigens determined by immunoblotting.

Periodate oxidation was used to determine MAb specificity for carbohydrate determinants (2). B. cereus cells exposed to various concentrations of sodium metaperiodate (0 to 0.05 M) were subjected to immunoblotting as indicated above.

(iii) ELISA capture system. ELISA capture system was performed with MAb 9B7 used as a specific capture antibody and with biotinylated MAb 8D3 used as a detector antibody (18). Briefly, microtiter plates were coated with 2 µg of MAb 9B7 for 2 h at 37°C. All dilutions were performed in ELISA buffers. Vegetative cell cultures of B. cereus were applied to wells for 1 h, and detection of bound antigen was performed by application of biotinylated MAb 8D3. Streptavidin-peroxidase (Sigma S 5512) was applied for 0.5 h. Absorbance was read at 490 nm after addition of the substrate o-phenylenediamine (Sigma Chemical Co.).

A total of nine hybridomas were screened by ELISA for their reactivities with B. cereus LMG 6923 vegetative cells. Of these, only two hybridomas secreted antibodies reactive with B. cereus. These MAbs, designated 8D3 and 9B7, were found to be IgG1 and IgM, with kappa light chains.

The specificity of the MAbs was examined by ELISA with a panel of select bacteria (Table 1). The results showed that the MAbs recognized not only vegetative cells of B. cereus LMG 6923, which was used for immunization, but also the vegetative cells of B. cereus originating from food or environmental samples. MAb 8D3 reacted strongly with all B. cereus strains and with Bacillus thuringiensis subsp. berliner and B. thuringiensis subsp. kurstaki. MAb 8D3 reacted weakly with B. thuringiensis DSM 2046, B. thuringiensis subsp. israelensis, and Bacillus mycoides. MAb 9B7 reacted strongly with B. cereus species and reacted weakly with B. thuringiensis subsp. berliner and B. thuringiensis subsp. kurstaki. However, B. cereus is closely related to B. thuringiensis and B. mycoides (3, 5). These data could explain the cross-reactivity of the MAbs with vegetative cells of B. thuringiensis or B. mycoides. None of these antibodies reacted with B. cereus LMG 6923 spores. In addition, both antibodies showed no reactivity to several members of the family of Enterobacteriaceae (Salmonella enteritidis, Escherichia coli, Proteus vulgaris, Citrobacter freundii) and other bacteria (Micrococcus luteus, Pseudomonas stutzeri, Listeria monocytogenes).

The antigenic reactivity of the MAbs was destroyed when ELISA plates were coated with trypsin-protease-treated vegetative cells of B. cereus. These results show that the antigens recognized by MAbs 8D3 and 9B7 are proteinaceous in nature.

MAbs were analyzed for the antigenic specificity by SDS-PAGE followed by immunoblotting (Fig. 1). B. cereus LMG 6923 vegetative cell extracts were used for electrophoretic studies. MAb 8D3 recognized an antigen with a molecular mass of 22 kDa. The antigens which react with MAb 9B7 have molecular masses of approximately 58 and 62 kDa. Periodic acid-Schiff staining indicated that the proteins which react with MAb 9B7 may be glycoproteins. Treatment of B. cereus cells with sodium metaperiodate had no effect on the detection of B. cereus by MAb 9B7, as assessed by immunoblotting. This indicates that the antigens which react with MAb 9B7 are not carbohydrates.


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  		FIG. 1.   Immunoblotting with MAbs against B. cereus LMG 6923 vegetative cells. Shown are molecular mass standards (in kilodaltons) (A) and B. cereus vegetative cells with MAbs 8D3 (B) and 9B7 (C).

ELISA experiments indicate that the proteins recognized by MAbs 8D3 and 9B7 are specific to vegetative cells, since the antibodies did not react with spores of B. cereus. Furthermore, as shown by SDS-PAGE and immunoblotting analysis, MAbs 8D3 and 9B7 react with different proteins on the vegetative cells. This result indicates that these MAbs are able to capture vegetative cells of B. cereus in an immunoassay. Our future research will focus on the use of these antibodies to develop immunoassays which will detect B. cereus cells in food products.

We need a rapid and reliable method to detect vegetative cells of B. cereus, since checking for this pathogenic organism is critical to ensure food safety. It is impossible for the food industry to completely avoid the presence of B. cereus in their products, and the consumption of foods containing 105 vegetative cells of B. cereus per ml will result in food poisoning. Thus, the detection method should be sensitive enough to be able to detect low numbers of B. cereus organisms (14).

We have therefore developed the following method to detect vegetative cells of B. cereus, which we tested in pure culture. We used MAbs 8D3 and 9B7 to develop an ELISA capture system. MAb 9B7 was used as a specific capture antibody and MAb 8D3 was used as a detector antibody. The results show that the ELISA capture system can detect and quantify vegetative cells of B. cereus (Table 2). We determined that the lower detection limit was on the order of 102 cells per ml and the upper detection limit was on the order of 108 cells per ml, with absorbance values of 0.224 to 1.805. So, this work appears to demonstrate the feasibility of detecting this pathogenic organism in food products with our ELISA capture system.

                              
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  		TABLE 2.   Detection of vegetative cells of B. cereus in pure culture by ELISA capture system


    ACKNOWLEDGMENTS

We thank J. Perrier for valuable discussions and helpful suggestions regarding this work.


    FOOTNOTES

* Corresponding author. Mailing address: Laboratoire de Microbiologie, Service 452, UPRES A 6116 CNRS, Faculté des Sciences et Techniques de St Jérôme, Université Aix-Marseille, Avenue Escadrille Normandie Niemen, 13397 Marseille Cedex 20, France. Phone: (33) 4 91 28 81 90. Fax: (33) 4 91 28 80 30. E-mail: Nathalie.Rugani{at}Microbio.u-3mrs.fr.


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Applied and Environmental Microbiology, May 2000, p. 2278-2281, Vol. 66, No. 5
0099-2240/00/$04.00+0
Copyright © 2000, American Society for Microbiology. All rights reserved.



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