Bacillus cereus in fried rice meals: Natural occurrence, strain dependent growth and haemolysin (HBL) production

This study investigated B. cereus levels in fried rice meals in Milan (Italy) and the effect of a post contamination. Spores of three B. cereus (ATCC 14579, clinical isolate GPe2 and an isolate from fried rice R1) were inoculated in fried rice with ham, scrambled egg and peas, then stored at 10 °C and 15 °C for 5 days; the L2 component of haemolysin BL was quantified in the product. In a second trial, inoculated fried rice was submitted to microwave heating (2 min at 600 W) to investigate a possible decrease in bacterial loads. At 15 °C, spores germinated very quickly, followed by a fast growth especially of B. cereus R1 (+5.92 Log CFU g−1 from T0), statistically higher if compared to ATCC 14579 and GPe2 strains at 15 °C (P < 0.05 and P < 0.01). The production of HBL toxin by the R1 strain significantly started after 2 days in samples at 15 °C, while the production by ATCC and GPe2 strains was more modest and late. At 10 °C, R1 showed a significant fast growth (+4.64 Log CFU g−1 from T0) and significant HBL toxin production (from T4), if compared to GPe2 and ATCC strains that showed absent or very slow growth (P < 0.05).