Determination of staphylococcal enterotoxin A in cheddar cheese produced without starter activity.

Three variants of the chloramine-T radioiodination method were used to iodinate staphylococcal enterotoxin A with 125I. Only one method consistently produced usable labels for radioimmunoassay. The iodine incorporation was 55 to 76%; the specific activity was 3.5 to 5.5 muCi/microgram of enterotoxin, and the label was extremely stable on storage at -20 degrees C. Determinations of the enterotoxin in extracts of cheddar cheese produced without starter activity were carried out with the radioimmunoassay system and protein A as antibody immunoadsorbent. The assay buffer used in this system significantly influenced the detected levels of enterotoxin in the cheese extracts. Phosphate buffer, but not tris(hydroxymethyl)aminomethane (Tris) buffer, caused gelling of cheese extract proteins, thus resulting in an incomplete separation of free from antibody-bound 125I enterotoxin. When Tris buffer was used, the results indicated a high degree of accuracy and precision for this radioimmunoassay. The lowest detectable enterotoxin concentration in cheese extract was 0.5 ng/ml.