Validation of ISO method 11290 - Part 2. Enumeration of Listeria monocytogenes in foods

The European and International Standard method for the enumeration of Liste ria monocytogenes, described in EN ISO 11290 Part 2: 1998 [EN ISO 11290-2 M icrobiology of Food and Animal Feedingstuffs-Horizontal Method for the Dete ction and Enumeration of L. monocytogenes: Part 2. Enumeration; Internation al Organisation for Standardisation, Geneva.] was validated by order of the European Commission (Standards, Measurement and Testing Fourth Framework P rogramme Project SMT4-CT96-2098). The objective was to determine the precis ion of the method in terms of repeatability (r) and reproducibility (R) usi ng three different food types inoculated with various levels of L. monocyto genes and a typical background flora. The results are intended for publicat ion in the associated standards. Cheese, meat, dried egg powder and referen ce materials were examined by 21 laboratories in 16 countries in Europe. Ea ch participant received eight test materials per food type: blind duplicate s at four inoculum levels (0, 10(2), 10(3), 10(4) cfu/a). In addition, two reference materials containing L. monocytogenes were included in the study. All test materials were subjected to stringent homogeneity and stability t esting before being used in the collaborative trial. Participants were requ ired to use only PALCAM agar for enumeration of L. monocytogenes, as prescr ibed by the reference method. Statistical analyses has been performed using a newly introduced approach for food microbiology (draft standard prEN ISO 16140 [prEN ISO 16140 Microbiology of Food and Animal Feedingstuffs-Protoc ol for the Validation of Alternative Methods, International Organisation fo r Standardisation, Geneva.], the precision data being calculated using robu st estimates. Overall values for repeatability (r) of EN ISO 11290-2 when used with food test materials were r = log(10) 0.58 (expressed as an absolute difference b etween log(10)-transformed test results) or r = 3.8 (expressed as an absolu te ratio between test results on the normal scale). For the reference mater ials (capsules containing approximately 5000 cfu), r = log(10) 0.34 (expres sed as an absolute difference between log(10)-transformed test results) or r = 2.2 (expressed as an, Absolute ratio between test results on the normal scale). Overall values for reproducibility (R) of EN ISO 11290-2 when used with foo d test materials were R = log(10) 0.81 (expressed as a difference between l og(10)-transformed test results) or R = 6.5 (expressed as an absolute ratio between test results on the normal scale). For the reference materials, R = log(10) 0.51 (expressed as a difference between log(10)-transformed test results) or R = 3.2 (expressed as an absolute, ratio between test results o n the normal scale). Further studies have been initiated by ISO TC34/SC9 to try to enhance the i solation of L monocytogenes from foods and improve the confirmation procedu res. (C) 2001 Elsevier Science B.V. All rights reserved.