Abstract
Staphylococcal enterotoxins (SEs) account for a substantial number of food-poisoning outbreaks. European legislation (Commission Regulation 1441/2007) stipulates the reference procedure for SE analysis in milk and dairy products, which is based on extraction, dialysis concentration and immunochemical detection using one of two approved assays (VIDAS® SET2, Ridascreen® SET Total). However, certified reference materials (CRMs) are lacking to support laboratories in performing reliable detection of Staphylococcus aureus enterotoxin A (SEA) in relevant matrices at sub-nanogram per gram levels. The certification of a set of three reference materials (blank and two SEA-containing materials) for testing of the presence/absence of SEA in cheese is described. The reference procedure was applied in an intercomparison with 15 laboratories, and results were reported in a qualitative manner (presence or absence of SEA in the sample). No false-negative or false-positive results were obtained. The certified values were stated as diagnostic specificity (blank material) or diagnostic sensitivity (SEA-containing materials) and were 100 % in all cases. Stability studies demonstrated suitable material stability when stored cooled or frozen. An in-house study on the recovery of SEA in the cheese materials using a double-sandwich enzyme-linked immunosorbent assay (ELISA) revealed comparable recovery values of around 45 % at the two spiking levels and in both the SEA-containing CRMs as well as blank CRM freshly spiked prior to analysis. The values were also comparable over time and among different analysts. The materials provide valuable support to laboratories for method validation and method performance verification and will increase the reliability of measuring SEA in cheese.Electronic supplementary materialThe online version of this article (doi:10.1007/s00216-016-9642-5) contains supplementary material, which is available to authorized users.
Highlights
Staphylococcal enterotoxins (SEs) are released into food by certain strains of coagulase-positive Staphylococci (CPS), typically Staphylococcus aureus
One blank and two Staphylococcus aureus enterotoxin A (SEA)-containing cheese materials could be successfully certified for testing of the presence/absence of SEA
The presence/absence certification is expressed in either diagnostic specificity or diagnostic sensitivity (SEA-containing materials) a As defined in the formula depicted in the text and in [18] b As determined using the European screening method (ESM) with the VIDAS SET2 detection step and the Ridascreen SET Total detection step
Summary
Staphylococcal enterotoxins (SEs) are released into food by certain strains of coagulase-positive Staphylococci (CPS), typically Staphylococcus aureus. These proteinaceous enterotoxins have been causing a significant number of foodpoisoning outbreaks and illnesses. The European food safety authority (EFSA) reported that SEs accounted for 393 of the 843 documented outbreaks related to bacterial toxins in 2014 [1, 2]. 23 structurally related SE proteins with molecular masses of 22–28 kDa, documented stability to changing temperature and pH and resistivity to proteolytic digestion have been described in the literature [3, 6,7,8]. SEA is a single-chain 233 amino acid containing 27 kDa protein [9] and is the SE serotype most frequently involved in staphylococcal food poisoning (ca. 80 %)
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