English Abstract 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.
