Enumeration of Escherichia coli in reference materials for food microbiology. BCR/FOOD trial 6

Abstract

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A collaborative study was organised to test the reference material (RM) for Escherichia coli. Twenty eight laboratories participated in this international collaborative study, each laboratory testing 8 capsules. Each set of eight capsules contained one uncontaminated sample. The RM was enumerated by four methods, two for E.coli and two for Enterobacteriaceae. For both E.coli and Enterobacteriaceae one of two methods was standardized for all laboratories (called standard method or SM), the other method was chosen by the participating laboratory (called OWN method). The highest counts were obtained using the E.coli standard method. Sixteen of the 25 laboratories found higher counts using the SM than with their OWN method. The group of laboratories using a resuscitation step for the E.coli-OWN method found significantly more colonies than the group which did not. The average counts obtained for the Enterobacteriaceae methods were very low, too low for optimal use of the RM tested. This indicates that the methods for the enumeration of Enterobacteriaseae using VRBG agar have a low recovery. The values found for repeatability (r) and reproducibility (R) were high compared to the B.sereus and S.aureus RM's tested in the previous trial. The stability of the material, determined using the E.coli standard method, indicated that the material is not yet stable. The reference material is therefore as yeat not suitable for commencing the certification procedure.