Purpose: Comparison of Compact Dry X-SA against the reference method (EN ISO 6888-1:1999) for enumeration of Staphylococcus aureususing validation protocol ISO 16140.
Methods: Compact Dry X-SA was used to enumerate numbers of Staphylococcus aureus present in 1 ml samples of diluted foods. Inoculated X-SA plates were incubated at 37°C for 24 h and typical blue colonies counted. No confirmation tests were required for this method. The standard ISO method used 0.1 ml spread plates on Baird Parker Agar incubated at 37°C for 24 and 48 hrs. Confirmation using rabbit plasma with EDTA was required. For the expert laboratory study, five inoculation levels in five food types were analyzed by both methods; foods tested were meat products, fish and seafoods, dairy products, bakery products and pasta. The interlaboratory study involved 10 laboratories in 5 countries. The results were analyzed using the principles of EN ISO 16140.
Results: The correlation coefficient (R2) for all foods was 0.99 and the selectivity (inclusivity/exclusivity) of X-SA was equal to the reference method. Specific AOAC RI analysis (ANOVA, lot-to-lot stability and ruggedness) showed no significant differences between the methods and the interlaboratory study revealed no differences in reproducibility and repeatability between the two methods. Compact Dry X-SA was therefore found to be equivalent to the standard ISO method for enumeration of Staphylococcus aureus in all foods.
Significance: Compact Dry gave comparable results to the reference method in a shorter time period without the necessity for confirmation steps.