Significance of LT and STb Enterotoxins in the Virulence of F4+ Escherichia coli in Pigs
Document Type Article
A DISSERTATION Presented to the Faculty of The Graduate College at the University of Nebraska In Partial Fulfillment of Requirements For the Degree of Doctor of Philosophy
Major: Integrative Biomedical Sciences Under the Supervision of Professor Rodney A. Moxley
Lincoln, Nebraska; August, 2008
Copyright © 2008 Joseph Erume
The most common and severe enterotoxigenic Escherichia coli (ETEC) infections in swine are caused by strains expressing F4+ fimbria, heat-labile enterotoxin (LT), heat-stable enterotoxin-b (STb), and having astA gene.. Information is lacking regarding the relative contribution of these enterotoxins to virulence and whether pig F4 receptor status is important for the toxins to elicit disease. To address these concerns, 9-day-old F4ac receptor-positive gnotobiotic piglets were challenged with isogenic F4ac+ strains: (1) NalR EAST1+, LT+, STb+ parent; (2) STb- (ΔestB) mutant; (3) STb complemented mutant; (4) STb- LT- (ΔeltAB ΔestB) mutant; and (5) STb- LT- mutant complemented with LT. Additionally, 6-8 week-old weaned homozygous and heterozygous F4+ab/ac susceptible pigs (based on mucin 4 gene polymorphism), were challenged with these strains to determine if swine F4 receptor status is important in elicitation of LT and STb effect. Analysis of post-inoculation percent weight change per hour and serum bicarbonate among gnotobiotic piglets revealed that deletion of estB did not significantly reduce virulence compared with that of the parent, and complementation with estB did not significantly increase virulence compared with that of the ΔestB mutant. However, deletion of eltAB in the STb- mutant resulted in complete abrogation of virulence and LT complementation restored the virulence of STb- LT- mutant. Contrary to our previous studies, there was no evidence that LT enhanced F4+ ETEC colonization of the small intestine. Among weaned pigs, STb- LT- mutant complemented with LT stimulated significantly greater loop fluid production in homozygous F4ab/ac susceptible pigs compared to heterozygous individuals (P = 0.0066). In contrast, STb-expressing strains stimulated marked fluid responses irrespective of pig F4ab/ac susceptibility. In summary these data demonstrate that LT is a more significant contributor to F4+ ETEC virulence in young (i.e. < 2 weeks old) F4acR+ piglets than is STb and they further suggest that the opposite may be true in older pigs. Data also suggest that the F4ab/ac receptor status of pigs is important for elicitation of LT- but not STb-response.