Variation in Counts, Enterotoxin Levels and TNase in Swiss-Type Cheese Contaminated with Staphylococcus aureus.

In 1977, a number of staphylococcal intoxications occurred as a result of several lots of Swiss-type cheese being contaminated with Staphylococcus aureus . Blocks of cheese representing 59 of the recalled lots manufactured between January 5 and April 25, 1977, were examined for S. aureus counts and for staphylococcal thermonuclease (TNase), enterotoxin and pH. Counts ranged from <25 to 10 S. aureus /g, with most blocks containing between 10 and 10/g. Enterotoxin B was present in 72.6% of 186 samples and TNase in 52.5% of 122 samples tested. The pH was generally between 5.5 and 5.7 with some samples as high as 6.2; normal pH for Swiss-type cheese is 5.7. Variations in levels of these parameters occurred within lots and even within blocks, as well as between lots. Within blocks there seemed to be differences between samples at the center and the periphery. The activity and degree of contamination of the starter cultures, temperatures of cooking and ripening and the speed of salt penetration could be factors causing these variations. TNase was not detectable in all samples containing enterotoxin, especially those with <0.5 μg/100 g. Therefore, cheese suspected of being a health hazard, such as being manufactured under unsanitary conditions or suspected of being contaminated with S. aureus , should be sampled at the center and periphery of blocks, if these are available, and analyzed for S. aureus counts, TNase and enterotoxin.