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

The European and International Standard method for the enumeration of Listeria monocytogenes, described in EN ISO 11290 Part 2: 1998 [EN ISO 11290-2 Microbiology of Food and Animal Feedingstuffs-Horizontal Method for the Detection and Enumeration of L. monocytogenes: Part 2. Enumeration; International Organisation for Standardisation, Geneva.] was validated by order of the European Commission (Standards, Measurement and Testing Fourth Framework Programme Project SMT4-CT96-2098). The objective was to determine the precision of the method in terms of repeatability (r) and reproducibility (R) using three different food types inoculated with various levels of L. monocytogenes and a typical background flora. The results are intended for publication in the associated standards. Cheese, meat, dried egg powder and reference materials were examined by 21 laboratories in 16 countries in Europe. Each participant received eight test materials per food type: blind duplicates at four inoculum levels (0, 10(2), 10(3), 10(4) cfu/g). In addition, two reference materials containing L. monocytogenes were included in the study. All test materials were subjected to stringent homogeneity and stability testing before being used in the collaborative trial. Participants were required to use only PALCAM agar for enumeration of L. monocytogenes, as prescribed 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-Protocol for the Validation of Alternative Methods, International Organisation for Standardisation, Geneva.], the precision data being calculated using robust estimates. Overall values for repeatability (r) of EN ISO 11290-2 when used with food test materials were r = log10 0.58 (expressed as an absolute difference between log10-transformed test results) or r = 3.8 (expressed as an absolute ratio between test results on the normal scale). For the reference materials (capsules containing approximately 5000 cfu), r = log10 0.34 (expressed as an absolute difference between log10-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 food test materials were R = log10 0.81 (expressed as a difference between log10-transformed test results) or R = 6.5 (expressed as an absolute ratio between test results on the normal scale). For the reference materials, R = log10 0.51 (expressed as a difference between log10-transformed test results) or R = 3.2 (expressed as an absolute ratio between test results on the normal scale). Further studies have been initiated by ISO TC34/SC9 to try to enhance the isolation of L. monocytogenes from foods and improve the confirmation procedures.