Validation of EN ISO standard methods 6888 Part 1 and Part 2: 1999--enumeration of coagulase-positive staphylococci in foods.

ABSTRACT

The methods of European and International Organisations for Standardization for the enumeration of coagulase-positive staphylococci (CPS, Staphylococcus aureus and other species) described in EN ISO 6888 Part 1 and Part 2 1999 were validated by order of the European Commission (Standards, Measurement and Testing Fourth Framework Programme Project SMT4-CT96-2098). EN ISO 6888-1 prescribes the use of Baird-Parker (BP) agar whereas EN ISO 6888-2 prescribes the use of Rabbit Plasma Fibrinogen Agar (RPFA). The objective was to determine the precision of each method in terms of repeatability (r) and reproducibility (R) using three different food types inoculated with various levels of S. aureus and a typical background flora. The results are intended for publication in the associated standards. Cheese, meat and dried egg powder were examined by 24 laboratories from 16 countries in Europe. Each participant received eight test materials per food type blind duplicates at four inoculum levels (0, 10(3), 10(4) to 10(5), 10(5) to 10(6) cfu/g). In addition, two reference materials (RM) (capsules containing milk powder inoculated with S. aureus) were included in the study. All test materials were subjected to stringent homogeneity and stability testing before being used in the collaborative trial. Two statistical methods were used to calculate the precision parameters. Draft EN ISO 16140 2000 method appeared more appropriate to the case of microbiological data than ISO 5725-2 1994 method and was retained to calculate the precision data. Concerning EN ISO 6888-1, overall values for repeatability (r) when used with food test materials was r=log(10) 0.28 (expressed as an absolute difference between log(10)-transformed test results). For the reference materials, r=log(10) 0.19. Overall values for reproducibility (R) when used with food test materials were R=log(10) 0.43. For the reference materials, R=log(10) 0.39. Concerning EN ISO 6888-2, overall values for repeatability (r) when used with food test materials were r=log(10) 0.22. For the reference materials, r=log(10) 0.17. Overall values for reproducibility (R) when used with food test materials were R=log(10) 0.33. For the reference materials, R=log(10) 0.31. These results were presented to the ISO technical committee and to the Comité Européen de Normalisation (CEN). Both committees agreed to incorporate the precision data obtained with food materials as two amendments to EN ISO 6888-1 and -2, and to give an equal status to each part of the standard.