The psychrotrophic cspA signature of the[i] Bacillus Cereus[/i] group characterizes growth and spoilage abilities at 10°c in whole egg but not in optimum medium.

Introduction: Bacterial spoilage involves visible modifications of texture, viscosity, colour, and/or flavour of liquid egg products. These later are widely used in various foodstuffs which microbiological quality is essential. Among the flora able to resist the mild heat treatments encountered by industrial egg products, the presence of the Bacillus cereus group is a recurring problem because of the growth and hydrolytic potentials of psychrotrophic strains of this group at refrigerated temperatures, leading to spoilage events during storage. The aim of the present study was to investigate the correlation between already known psychrotrophic genotypic and phenotypic signatures and whole egg spoilage ability at low (10°C) and medium (30°C) temperatures, using a collection of 68 psychrotrophic B. cereus group isolates coming from industrial pasteurized liquid whole egg products.Material and methods: Genetic profiles were designed by real-time PCR, according to specific cspA and 16S rDNA signatures (i.e. 16S rDNA-1m mesophilic and 16S rDNA-2p psychrotrophic signatures). Phenotypic profiles were designed by testing growth ability at 6°C and 43°C on BHI-YE agar. The spoilage potential was investigated in whole liquid egg at 10°C and 30°C, through measurement of the cell population (after 5 days and 18h, respectively), and investigation of the time leading to overall spoilage of the egg product, i.e. visible transformation of whole egg into a white slurry cream. As a relevant growth control, the level of population was investigated in optimum medium (BHI-YE) at both temperatures.Results: The average level of population was of the same order of magnitude in liquid whole egg and in BHI-YE at 30°C (8.2 ± 0.2 log CFU/mL). Surprisingly, this level was significantly higher in liquid whole egg (7.8 ± 0.5 log CFU/mL) than in BHI-YE (6.9 ± 0.3 log CFU/mL) at 10°C. Moreover, a strong correlation was highlighted between growth ability in whole egg at 10°C and presence of the cspA signature. Such a correlation was neither observed in BHI-YE regardless of the temperature, nor at 30°C regardless of the medium. Overall spoilage was observed for various incubation times, i.e from 11.3 h to 36 h, and from 5 d to 24 d at 30°C and 10°C, respectively. Once more, the presence of the cspA signature was strongly associated to specific thermal conditions : the bacteria exhibiting the cspA signature were those leading to the fastest whole egg spoilage at 10°C (P-values: 10-8) but not at 30°C. To conclude, the presence of the cspA signature highlights the isolates able to grow and spoil whole egg at 10°C, independently of their ability to grow at 10°C in optimum medium or to spoil whole egg at 30°C.Significance: This study provides new insights on psychrotrophic B. cereus group strains, by highlighting whole egg as a relevant model for evaluating spoilage kinetics, and the cspA signature as a relevant spoilage marker.