Cytotoxic activity and survival of spore-forming bacteria associated to bread-making process.

Raw materials used in bread-making process may be a rich source of spore-forming bacteria whose presence after cooking may represent a spoilage concern for bakery industries and a risk to consumer health. The aim of this study was to investigate the toxigenic potential of 54 spore-forming bacterial strains isolated from bread ingredients and bread, mainly of the Bacillus genus, and their resistance to a thermal treatment reproducing the bread cooking process to ascertain if they could represent a health concern for consumers. The potential toxigenicity of the strains was evaluated by screening the cytotoxic activity on HT-29 cells using bacterial culture filtrates after growing bacterial cells in BHI and in the bread-based medium BEB. The results showed a high cytotoxic activity of B. cereus strains, although it was lower in BEB medium. PCR analyses detected the presence of genes involved in the production of NHE, HBL or CytK toxins in B. cereus strains, while none of the tested strains contained the gene for cereulide production. Production of NHE and HBL toxins was also confirmed by specific immunoassays only for B. cereus strains. Cytotoxic activity of 13 strains belonging to B. amyloliquefaciens (7 strains), Paenibacillus spp. (3) B. mojavensis (1), B. simplex (1) and B. pumilus (1) was also detected. Interestingly, B. cereus strains assigned to phylogenetic group IV exhibited a thermal resistance markedly lower than B. cereus group III; furthermore, B. amyloliquefaciens strains almost completely survived the heat treatment, but showed a low cytotoxic activity. It is also relevant that single strains belonging to B. mojavensis and B. simplex showed a cytotoxic activity higher after growth in BEB than in BHI and a spore resistance enough to survive the bread cooking process. In conclusion, our study indicates that spore-forming bacteria could represent a risk to consumer health related to strains able to produce toxic substances and to survive bread cooking conditions.