Evaluation of the ISO 7932 standard for the enumeration of Bacillus cereusin foods

Twenty, mostly European laboratories took part in a collaborative study to validate the general 1993 ISO 7932 standard For the enumeration of Bacillus cereus in foods (Anonymous, ISO 7932, Microbiology - General Guidance for the Enumeration of Bacillus cereus - Colony-count Technique at 30 degrees C . International Organisation for Standardization, Geneva, Switzerland, 1993 ). The objective was to determine the precision data in terms of repeatabil ity (r) and reproducibility (R) of the method using three different food ty pes at various inoculum levels. The results are intended for publication in Comite Europeen de Normalisation (CEN) and ISO standards. The method was c hallenged with three types of food product: fresh cheese, minced beef and p otato powder. Each participant received eight samples for each food type: b lind duplicates at four inoculum levels (target values of B. cereus of 0, 1 0(3) 10(4) and 10(5)-10(6) colony forming particles (cfp)/g). In addition, two reference materials (RMs, capsules containing milk powder, artificially contaminated with B. cereus) were included in the study. All rest material s were tested extensively for homogeneity and stability prior to the collab orative trial. In addition to determining the precision parameters of the I SO method, polymyxin pyruvate egg yolk bromothymol blue agar (PEMBA, incuba tion at 37 degrees C) was included in the study to evaluate possible differ ences in performance compared to mannitol egg yolk polymyxin agar (MEYP, in cubation at 30 degrees C) which is prescribed in the ISO standard. In this study no difference ih performance between MEYP and PEMBA medium was observ ed. Results from the glucose fermentation confirmation test indicated that generally 48 h was needed to obtain a yellow colour throughout the whole te st tube. A high number of false negative results with the Voges Proskauer ( VP) reaction, even after 48 h of incubation of the tubes, was observed in s ome laboratories. Values for r and R were therefore calculated without VP t est results. Further studies have been initiated by the ISO technical commi ttee in order to improve the performance of the confirmation tests. The pre cision of the test method appeared to be unaffected by the type of food or the concentration of B, cereus present in the test sample. The overall repe atability value found for the food samples was 0.29 log(10) units and the o verall reproducibility value was 0.42 log(10) units. For the reference mate rial, the repeatability was 0.11 log(10) units and the reproducibility 0.22 log(10) units. It was recommended to CEN and ISO to include these values i nto the revised horizontal standard for the enumeration of Bacillus cereus. (C) 2000 Elsevier Science B.V. All rights reserved.