Prevalence of Pilus Antigens, Enterotoxin Types, and Enteropathogenicity Among K88-Negative Enterotoxigenic Escherichia coli from Neonatal Pigs

Enterotoxigenic Escherichia coli (ETEC) that were isolated from neonatal pigs and that did not react in preliminary tests for pilus antigen K88 were subjected to additional tests for K88 and for pilus antigens K99 and 987P. Four such isolates produced K88, 9 isolates produced K99, 55 isolates produced 987P, and the remaining 43 isolates produced none of the three pilus antigens (3P⁻). Immunofluorescence tests of ileal sections from pigs were more sensitive for 987P detection than was serum agglutination of bacteria grown from the ileum. Most ETEC that produced K88, K99, or 987P were enteropathogenic (adhered to ileal villi, colonized intensively, and caused profuse diarrhea) when given to neonatal pigs. In contrast, only 3 of the 43 ETEC that produced none of the pilus antigens were enteropathogenic. The isolates were also tested for the type of enterotoxin produced. The K88⁺ isolates all produced heat-labile enterotoxin (LT) detectable in cultured adrenal cells (i.e., were LT⁺). None of the 987P⁺, K99⁺, or enterpathogenic 3P⁻ isolates produced LT. However (except for a single K99⁺ isolate), they all produced heat-stable enterotoxin detectable in infant mice (STa⁺). Sixteen isolates produced neither LT nor STa but did produce enterotoxin detectable in ligated intestinal loops of pigs (STb). Most of these LT⁻ STa⁻ STb⁺ isolates were also K88⁻, K99⁻, and 987P⁻ and non-enteropathogenic. One of them was K99⁺ and enteropathogenic. Our conclusions are as follows. (i) Most enteropathogenic ETEC from neonatal pigs produce either K88, 987P, or K99; however, there are some that produce none of the three antigens. (ii) Immunofluorescence tests for pilus antigens produced in vivo are recommended for the diagnosis of ETEC infections. (iii) Reports of LT⁻ STa⁻ STb⁺ swine ETEC are confirmed; furthermore, such isolates can be enteropathogenic.