Classical enterotoxins of coagulase-positive Staphylococcus aureus isolates from raw milk and products for raw milk cheese production in Ireland

Abstract

Toxin-producing Staphylococcus aureus can be present in raw milk and therefore in cheese made from raw milk. To determine the number and type of toxin producers in raw milk used for raw milk cheese production in Ireland, 117 samples of raw milk and related products from five raw milk suppliers, to four raw milk cheesemakers in the South of Ireland, were analysed for coagulase positive S. aureus. Enumeration, using ISO 688-2 and plating on Baird Parker Rabbit Plasma Fibrinogen selective agar showed samples were within limits set by EC regulations. Isolates (151 from 81 positive samples) were characterised for production of staphylococcal enterotoxins (SEs) SEA, SEB, SEC and SED by reverse passive latex agglutination (SET-RPLA) and by multiplex polymerase chain reaction for the sea, seb, sec, sed and see genes. The results showed 83.2% of the isolates did not contain the se genes or the toxin producing capability tested for. From only one supplier, 26 isolates contained the sec gene and produced SEC. Within these 26 isolates, there were only two PFGE types. One SEC-producing isolate showed no toxin production when grown in sterile 10% reconstituted skim milk at 10 °C and 12 °C for 96 and 74 h, respectively. Low concentrations of SEC were produced at 14 °C and 16 °C after 74 and 55 h, respectively. The results of this survey indicate that milk used for raw milk cheese production in Ireland poses a limited risk to public health, although further studies on occurrence of toxin producing S. aureus should be undertaken.