Comparison of Different Assays for Definition of Heat-stable Enterotoxigenicity of Escherichia Coli Porcine Strains

Ninety-one Escherichia coli strains isolated from porcine neonatal diarrhea, representing 28 O-groups and rough and non-O-groupable strains, were examined for enterotoxigenicity (heat stable [ST] or heat labile [LT]) by using bacterial suspensions in intestinal loop tests in 3- to 7-week-old piglets and culture supernatant fluids in the Y1 adrenal cell test, the 18-h rabbit intestinal loop test, and the infant mouse test. Eleven strains in O-groups 101, 138, 147, and 149 were positive in all four assay systems and were designated ST + LT. Fourteen strains within O-groups 8, 9, 20, 64, 141, and 149 and non-O-groupable were positive only in the 3- to 7-week-old piglet loop test and the infant mouse test and were designated ST pig + mouse. Sixteen strains distributed among O-groups 8, 16, 32, 50, 51, 98, 115, 141, 149, and 157 were positive only in the piglet intestinal loop test and were designated ST pig. Three strains of O-groups 8, 9, and 140 were positive only in the infant mouse assay and were designated ST mouse. Two strains of O-group 149 were positive in all tests except the infant mouse test and were designated LT. A total of 42 strains were negative in all four tests (Ent(-)), and 3 strains could not be categorized by the enterotoxigenicity criteria used. All K88-positive isolates, 17 strains of O-groups 8, 32, 147, and 149, were positive in at least one enterotoxigenicity test. ST pig and ST mouse strains gave positive intestinal loop tests as bacterial suspensions in 4- to 10-day-old piglets. A 6-h piglet intestinal loop test performed with heat-inactivated culture supernatants was preferable to an 18- to 20-h test for determination of ST production by strains of diverse O-groups. ST production by the two strains designated LT was detected by the 6-h test. The infant mouse test, although highly reproducible and convenient, appears to possess considerable limitations in routine screening of E. coli of porcine origin for ST production.