The most probable number (MPN) method is recommended for routine surveillance of products in which small numbers of B. cereus are expected5. This method is also effective in testing foods that may contain a large population of competing species or in dehydrated food products in which the potential spores would outnumber vegetative cells and require additional nutrients to germinate.
Inoculate 3-tube MPN series in trypticase soy-polymyxin broth, using 1 mL inoculum of 10-1, 10-2, and 10-3 dilutions of sample with 3 tubes at each dilution. (Additional dilutions should also be tested if B. cereus population is expected to exceed 103/g.) Incubate tubes 48 ± 2 h at 30 + 2°C and observe for turbid growth, which is typical of B. cereus. Streak cultures from turbid, positive tubes onto separate agar plates (either Bacara or MYP) and incubate plates 18-24 h at 30°C. Pick one or more pink-orange (Bacara) or pink (MYP), lecithinase-positive colonies from each agar plate and transfer to BHI with 0.1% glucose for enterotoxin studies (Chapter 15) and nutrient agar slants for storage. Typical colonies grown on Bacara or MYP must be confirmed with biochemical testing as described in Sections F and G below.
Calculate MPN of B. cereus cells/g of sample (see BAM Appendix 2) based on the number of tubes at each dilution in which the presence of B. cereus was confirmed. Biochemical testing will be necessary to delineate to the species level, however, enterotoxins can