Production of Enterotoxin by Yersinia Species Isolated from Milk

A total of 70 Yersinia strains, comprising 40 Y. enterocolitica , 21 Y. intermedia , 8 Y. frederiksenii , and 1 Y. aldovae , isolated from Northern Ireland milk supplies, were tested for their ability to produce a heat-stable enterotoxin (YeST) when grown in a broth medium (incubated at 25°C for 48 h). Only Y. enterocolitica strains produced detectable levels of YeST. Of these, enterotoxin was detected with 76.9% and 55.5% of strains identified as biotypes 1 and 2, respectively, and YeST was produced by all serotypes tested. Enterotoxin was detected when a single Y. enterocolitica strain was grown at 25°C for 48 h in culture media containing either lactose, butterfat, or milk proteins at levels approximating those found in milk. Also, YeST was detected when this strain was grown in skimmed and full-fat UHT milks. When grown at 4°C for up to 7 d, however, enterotoxin was not detected in any of these media.