RESEARCH ARTICLE Relationship between Heat-Labile Enterotoxin Secretion Capacity and Virulence in Wild Type Porcine-Origin Enterotoxigenic Escherichia coli Strains

Heat-labile enterotoxin (LT) is an important virulence factor secreted by some strains of entero-toxigenic Escherichia coli (ETEC). The prototypic human-origin strain H10407 secretes LT via a type II secretion system (T2SS).We sought to determine the relationship between the capacity to secrete LT and virulence in porcine-origin wild type (WT) ETEC strains. SixteenWTETEC strains isolated from cases of severe diarrheal disease were analyzed by GM1ganglioside en-zyme-linked immunosorbent assay tomeasure LT concentrations in culture supernatants. All strains had detectable LT in supernatants by 2 h of culture and 1 strain, which was particularly vir-ulent in gnotobiotic piglets (3030-2), had the highest LT secretion level all porcine-originWT strains tested (P<0.05). The level of LT secretion (concentration in supernatants at 6-h culture) explained 92 % of the variation in time-to-a-moribund-condition (R2 = 0.92, P<0.0001) in gnotobi-otic piglets inoculated with either strain 3030-2, or an ETEC strain of lesser virulence (2534-86), or a non-enterotoxigenicWT strain (G58-1). All 16 porcine ETEC strains were positive by PCR analysis for the T2SS genes, gspD and gspK, and bioinformatic analysis of 4 porcine-origin strains for which complete genomic sequences were available revealed a T2SSwith a high de