Modelling production of S. aureus enterotoxin Cbovine in milk, and its production during cheesemaking

Abstract

Staphylococcal enterotoxin type C (SEC) is one of the classical enterotoxins implicated in staphylococcal food poisoning. Staphylococcus aureus strains isolated from bovine unpasteurised milk and associated cheese samples produced the SECbovine sub-type. The objectives of this study were to determine the cell numbers required for SECbovine production in milk and to determine if SECbovine was produced by S. aureus during cheesemaking. To predict the point at which toxin production begins, SEC production was modelled against cell numbers of S. aureus (at different controlled pH/temperature combinations) in sterile reconstituted milk using a biphasic model fitted using the Solver routine in Excel. Under the conditions tested, the average cell number required for SECbovine production was log 8.43 ± 0.39 in sterile reconstituted milk. A semi-soft cheese was manufactured using milk inoculated with either washed (to remove any pre-formed toxin) or unwashed (containing toxin) S. aureus at 105 and 107 cfu.mL−1. An enzyme-linked immunosorbent assay was used to quantify toxin production and Baird-Parker agar to quantify S. aureus. Cheese made with washed and unwashed cells showed a one log increase in S. aureus cell numbers during cheesemaking. In cheese made with washed cells, no SEC was detected. With unwashed cells, the SEC concentration remained constant throughout cheesemaking. The results show that SECbovine is not formed in milk or cheese at <108 cfu.mL−1, and therefore, the risk-associated with SECbovine in cheese is low.