Increasing dietary tryptophan in conjunction with decreasing other large neutral amino acids increases weight gain and feed intake in weaner pigs regardless of experimental infection with enterotoxigenic Escherichia coli.

Dietary tryptophan (Trp) is a precursor for serotonin, a neuromediator involved in stress responses. Tryptophan competes with other large neutral amino acids (LNAA: tyrosine, isoleucine, leucine, valine, and phenylalanine) to cross the blood-brain barrier; therefore, the regulation of circulating LNAA can influence Trp availability in the cortex and serotonin biosynthesis. The hypothesis examined in this study was that increased supplementation of dietary Trp and a reduction in LNAA for weaned pigs experimentally infected with enterotoxigenic Escherichia coli (ETEC; F4) will increase Trp availability in plasma and reduce indices of the stress response, which will translate to reduced production losses. At 21 ± 3 d of age (mean ± SEM), 96 male pigs (Large White × Landrace) weighing 6.3 ± 0.98 kg (mean ± SEM) were individually penned and allocated to a 4 × 2 factorial arrangement of treatments, with respective factors being 1) four dietary standardized ileal digestible (SID) Trp and LNAA contents, being HTrpHLNAA (Low Trp-High LNAA; 0.24% SID Trp: 5.4% SID LNAA), HTrpHLNAA (Low Trp-Low LNAA; 0.24% SID Trp: 4.6% SID LNAA), HTrpHLNAA (High Trp-High LNAA; 0.34% SID Trp: 5.4% SID LNAA), and HTrpHLNAA (High Trp-Low LNAA; 0.34% SID Trp: 4.6% SID LNAA), and 2) without/with ETEC infection. Pigs were orally infected with 0.8 mL (3.6 × 109 CFU/mL) ETEC at days 7 and 8 after weaning. Pigs fed diets high in Trp irrespective of the level of LNAA (HTrpHLNAA and HTrpLLNAA) had higher plasma Trp concentrations (P < 0.001) and a Trp:LNAA ratio (P < 0.001) before infection and 6 d after infection. Following infection, noninfected pigs had higher plasma Trp (P = 0.03) and a Trp:LNAA ratio (P = 0.004) compared with pigs infected with ETEC. Plasma cortisol levels after infection were higher in ETEC-infected pigs (P = 0.05) and altering dietary Trp and LNAA concentrations did not influence (P > 0.05) plasma cortisol. Pigs fed diet HTrpLLNAA had higher serum serotonin levels 24 h after infection (P = 0.02) compared with pigs fed diets LTrpLLNAA and HTrpHLNAA. Similarly, pigs fed diet HTrpLLNAA had a higher (P = 0.02) average daily gain during the 3-wk study. Overall, average daily feed intake tended to be higher in pigs fed an HTrpLLNAA diet compared with the other diets (P = 0.08). These results suggest that the increased supplementation of dietary Trp with reduced LNAA increased circulating Trp levels that, in turn, likely caused higher serum serotonin levels, irrespective of infection with ETEC, and improved aspects of post-weaning performance.

Technical note: novel delivery methods for an enterotoxigenic Escherichia coli infection model in MUC4-locus sequenced weaner pigs1.

An infection model with enterotoxigenic Escherichia coli (ETEC) harboring the F4 fimbriae can be used to assess the impacts that various challenges associated with weaning (e.g., dietary, psychological, environmental) have on the expression of postweaning diarrhea. The objective of this study was to develop a novel inoculation method for administering an ETEC culture that would induce a higher proportion of ETEC-F4 diarrhea, in pigs that genetically showed ETEC-F4 susceptibility or resistance. The study was designed as a factorial arrangement of treatments with the factors being 1) partially susceptible or resistant to ETEC-F4 based on genetic testing, and 2) 4 challenge treatments, being a) a conventional liquid broth method using a drenching gun [Positive control (PC)], b) a Syringe method, c) a Capsule method, and d) Negative control [pigs not challenged (NC)]. At 21 ± 3 d of age (mean ± SEM), 48 male castrate pigs (Large White × Landrace) weighing approximately 7.0 ± 1.18 kg were allocated to 4 treatment groups in 2 replicate pens (6 pigs per pen). Initial ETEC-F4 susceptibility was based on a DNA marker test and each treatment group had 9 partially susceptible and 3 resistant pigs. On days 7 and 8 after weaning, pigs were challenged with ETEC (serotype O149:K88; toxins LT1, ST1, ST2, and EAST). On each inoculation day the PC pigs were orally dosed with 9 mL 7.12 × 109 colony-forming unit (CFU), the Syringe pigs with 0.8 mL 6.72 × 109 CFU, the Capsule pigs were orally administered 2 capsules containing 0.8 mL 3.28 × 109 CFU, and the NC pigs 1 mL of phosphate-buffered saline (PBS) solution. Approximately 72 h after infection, 44, 22, 78, and 0% of partially susceptible pigs in the PC, the Syringe, the Capsule, and the NC group had developed ETEC-F4 diarrhea (P = 0.007). Partially susceptible pigs had a higher diarrhea index (DI) compared to resistant pigs (31.5 vs. 4.8, P < 0.001). The NC group had a lower DI compared to the PC and Capsule pigs (3.9, 38.1, and 40.3, respectively, P < 0.005). Following infection, genetically resistant pigs in the Capsule group had a DI of zero and the partially susceptible pigs had a DI of 55.6 (P = 0.014). This study showed that genetically screening pigs and using a Capsule to deliver ETEC-F4 can increase cases of diarrhea and the efficiency of the challenge model. Taken together, these methods have the potential to reduce the number of pigs needed in future experimental infection studies.