Sporulation dynamics and spore heat resistance in wet and dry biofilms of Bacillus cereus

- Heat resistance of spores from liquid and agar-based media, and wet and dry biofilms was assessed.
- Air-exposure of preformed B. cereus NIZO 4080 and ATCC 10987 biofilms accelerated spore formation.
- Prolonged drying triggered spore germination in the biofilms of B. cereus ATCC 10987.
- Spores harvested from wet biofilms on stainless steel had the highest heat resistance.

Environmental conditions and growth history can affect the sporulation process as well as subsequent properties of formed spores. The sporulation dynamics was studied in wet and air-dried biofilms formed on stainless steel (SS) and polystyrene (PS) for Bacillus cereus ATCC 10987 and the undomesticated food isolate B. cereus NIZO 4080. After harvesting and maturation, the wet heat resistance of spores obtained from these biofilms was tested and compared to planktonic and agar plate-derived spores. Drying/air exposure of the preformed 24 h old biofilms accelerated spore formation for both strains and resulted in higher final spore percentages. Prolonged dry incubation of more than three days triggered germination of spores in the biofilms of ATCC 10987. Spores harvested from wet biofilms on SS displayed the highest heat resistance compared to liquid, agar plate and PS biofilm derived spores. The D95 °C values for these spores were 17 and 22 min for NIZO 4080 and ATCC 10987, respectively, which was 2 and 1.2 fold higher compared to planktonic spores of these strains. Spores obtained from dried biofilms of ATCC 10987 displayed reduced heat resistance compared to wet biofilm spores. The results indicate that environmental conditions encountered by biofilms affect sporulation dynamics and spore heat resistance, thus affecting subsequent quality issues and safety risks related to these biofilms.