Collaborative study on the precision of the enumeration of Staphylococcus aureus in food

In order to compare the precision of the enumeration of Staphylococcus aureus in food with egg yolk tellurite glycine pyruvate agar (ETGP-agar) and with rabbit plasma fibrinogen agar (RPF-agar), a collaborative study was carried out in which 23 laboratories participated. In this study each laboratory examined an identical series of 10 samples of ice cream, artificially contaminated with S. aureus and other Gram-positive cocci at roughly 10(4) per ml. The variations between the participating laboratories were smaller with RPF-agar than with ETGP-agar. The average values of the standard deviations were 0.10 and 0.13 log-units respectively. Laboratory specific errors were limited to 0.15 log-units with both media. For RPF-agar repeatability (r) was 0.26 (in log-units) and reproducibility (R) was 0.31 and for ETGP-agar r was 0.37 and R was 0.39. It is concluded that variations in S. aureus counts are limited and reproducibility is improved by using RPF-agar.