Enumeration of Bacillus cereus and Staphylococcus aureus in reference materials for food microbiology. BCR Food/trial 5

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A collaborative study was organised to test the newly developed reference materials (RM's) for Bacillus cereus and Staphylococcus aureus. Fourty one laboratories participated in this international collaborative study, each laboratory testing 5 capsules for each of the 2 types of RM. Each series of capsules contained an uncontaminated sample. For use each capsule had to be reconstituted according to a standardized procudure. Both types of RM were enumerated on one non-selective and three selective agars (one of which was chosen by the participating laboratory). The values found for repeatability (r) and reproducibility (R) were ca 1.4 and ca 1.8 respectively independent of the type of medium used. The stability of the RM's was extremely good due to the use of spores in this material. The r and R values were higher for S. aureus than for B. cereus. The r value was 1.6 and the R value, depending on the type of medium used, ranged from 2.0 to 3.4. The stability of this material was poor when stored at 22 degrees C but at -20 degrees C no decrease in contamination level was observed over 70 days of storage.