Certification of a Reference Material with Salmonella Enteritidis (NCTC 12694) at a Level of 5 Colony Forming Units on Nutrient Agar and 4 Colony Forming Units on Xylose Lysine Deoxycholate Agar, IRMM-352

Abstract

This report describes the certification of a reference material (IRMM-352) of Salmonella Enteritidis. Certified Reference Materials (CRMs) for microbiological analysis are indispensable tools for development and validation of detection methods and for the implementation and support of internal and external quality control in the area of microbiological analysis. Each vial contains one material sphere of S. enteritidis. The homogeneity and stability (at 4 °C, -20 °C and -70 °C) of the batch was assessed by monitoring colony forming units (cfu) on nutrient agar (NA) and xylose lysine deoxycholate (XLD) agar of selected vials by colony counting. The material is not stable at 4 °C byt no instability was detected when stored at -20 °C for up to 12 months and at -70 °C for up to 54 months. The batch was characterised by six laboratories to determine a certified value of cfu per vial on NA and XLD agar. A quite considerable performance difference of both agars was observed. The certified value is 5 cfu on NA with an expanded uncertainty of 2 and 4 cfy on XLD agar with an expanded incertainty of 2 using a coverage factor k = 2, corresponding to a level of confidence of about 95 %. DNA sequence analysis of the coding region for the sefA gene identified the material as S. enteritidis.