Diversity of Staphylococcus aureus enterotoxin types within single samples of raw milk and raw milk products

Aim: To find out if testing of up to 10 Staphylococcus aureus isolates from each sample from raw milk and raw milk products for staphylococcal enterotoxin (SE) might increase the chances of identifying potential sources of food intoxication. Methods and Results: Altogether 386 S. aureus isolates were tested for the presence of SE by reversed passive latex agglutination (SET-RPLA), and SE genes (se) by a multiplex polymerase chain reaction (PCR). In 18 of 34 (53%) S. aureus positive samples a mixture of SE and/or se positive and negative isolates were identified. Multiplex PCR increased the number of potential SE producing strains, i.e. isolates that harboured se, with 51% among the product and 48% among the raw bovine milk isolates. Examination by pulsed-field gel electrophoresis mostly confirmed clonal similarity among isolates sharing SE/se profile, but did not further differentiate between them. Conclusions: Isolates of S. aureus collected from one sample may show great diversity in SE production and different plating media seem to suppress or favour different strains of S. aureus. Significance and Impact of the Study: Several isolates of S. aureus from each sample should be tested for enterotoxin production in cases with typical SE intoxication symptoms with methods that are able to reveal new SE/se.