Cell culture assays compared the cytotoxicity of the two species when grown at 8, 15 and 37 °C. Bacillus cereus cytotoxic virulence factors (diarrhoeal toxins) were also detected after growth at these temperatures, and the strains were tested for the ability to produce emetic B. cereus toxin (cereulide) and for the presence of cereulide-encoding genes. Three strains of B. cereus and four strains
of B. weihenstephanensis were used (Table 1). The B. cereus strains NVH 1230-88 and NVH 0075-95 were isolated at the Norwegian School of Veterinary Science learn more (NVH) from foodborne disease cases (Granum et al., 1996, 1999; Lund & Granum, 1996), and the B. cereus type strain ATCC 14579 was the NVH laboratory stock (NVH 262). The B. weihenstephanensis click here WSBC strains were generously provided from Prof. Scherer (Stenfors et al., 2002) and NVH 453-92 was isolated from a dairy product (Granum et al., 1993; Stenfors & Granum, 2001). All strains were kept as glycerol stocks at −70 °C. Bacterial strains were grown in broth cultures at 8, 15 and 37 °C (for B. cereus strains only 15 and 37 °C; the strains do not grow at 8 °C) (Table 1). Overnight cultures of 5 mL BHIG (brain heart infusion broth, Difco, with 1% w/v glucose), grown at 32 °C, were diluted 1 : 100 into BHIG and grown at the test temperatures with 100–110 r.p.m. of shaking. Samples were collected
at an OD600 nm between 2.5 and 3.0 by centrifugation of 1.5 mL of culture at 20 000 g for 3 min. Supernatants were frozen immediately at −20 °C the until the performance of cytotoxicity and diarrhoeal toxin assays. Cytotoxicity of culture supernatants from the different growth temperatures was tested on monolayers of Vero C1008 cells
(African green monkey kidney cells, ECACC no. 85020206). The assay measures cellular damage as the inhibition of protein synthesis in the Vero cells (Sandvig & Olsnes, 1982) and was performed as described in Stenfors Arnesen et al. (2007). Briefly, confluent monolayers of Vero cells were incubated for 2 h at 37 °C with the different bacterial culture supernatants (duplicates of 100 μL). The assay is part of routine screening for enterotoxin production of B. cereus at the Norwegian National Reference Laboratory for B. cereus (at NVH). Culture supernatants from the different growth temperatures were investigated for the presence of enterotoxin Hbl and Nhe components, using two antibody-based detection kits targeting these toxins [BCET-RPLA (Oxoid Ltd, UK) and TECRA-BDE (Tecra International Pty Ltd, Australia)], which detect the L2 component of Hbl and the NheA component of the Nhe toxin complexes, respectively (Beecher & Wong, 1994; Buchanan & Schultz, 1994; Day et al., 1994; Lund & Granum, 1996). To investigate whether the studied strains carried the genes necessary to produce cereulide, a PCR assay was performed using primers targeting ces genes as described by Ehling-Schulz et al. (2005a, b).