The certification of a reference material for the evaluation of methods for the enumeration of Bacillus cereus

A reference material containing Bacillus cereus was certified by the Community Bureau of Reference (BCR) for its number of colony‐forming particles (cfp) in 0·1 ml reconstituted capsule solution. To this end, a batch of approximately 15 000 capsules was produced and tested for its homogeneity and stability. The variation in the number of cfp between capsules (homogeneity) was not found to be significantly different from a Poisson distribution. Stability was tested for extended periods at storage temperature (−20 °C) and at various higher temperatures up to 37 °C for 4 weeks to simulate transport conditions. Only at 37 °C did a small but significant decrease in the number of cfp occur. At −20 °C, no decrease in the number of cfp was observed over a period of about 4 years. For certification, 12 laboratories determined the number of cfp on two agars: Mannitol Egg‐Yolk Polymyxin agar (MEYP, incubated at 30 °C) and Polymyxin pyruvate Egg‐yolk Bromothymol blue Agar (PEMBA, incubated at 37 °C). The certified geometric mean value on MEYP after 24 h of incubation was 53·4 cfp 0·1 ml−1 of the reconstituted capsule solution (95% confidence interval 51·7–55·2) and on PEMBA, 55·0 (95% confidence interval 52·8–57·4). Based on these certified values, user tables were constructed specifying the 95% confidence limits when testing a smaller number of capsules, as would be done in individual laboratories. Based on the information on homogeneity, stability and the certification study, the BCR decided to certify the material as CRM 528.