Understanding the Performance Enhancing Mode of Action of in-Feed Sub-Therapeutic Antibiotics in Nursery Pigs

In the face of the Veterinary Feed Directive, alternatives to feeding growth-promoting sub-therapeutic antibiotics (AGPs) to nursery pigs are needed. However, the mechanism of action by which AGPs work to enhance pig performance is not fully understood. Thus, the objective of this study was to determine the mechanisms of action by which AGPs increase nursery pig performance. Over two replicates, a total of 24 weaned pigs (6.75 ± 0.75 kg BW) were randomly allotted to either control (CON, n = 12) or sub-therapeutic antibiotic (AB, n = 12) treatments and housed individually. A 2-phase corn-soybean-based nursery diet was fed, with the AB diets containing 40 g/ton feed-grade chlortetracycline. Individual pig ADG, ADFI, and G:F were calculated weekly for 5 weeks. Thereafter, all pigs were euthanized and necropsied for tissue collection. Protein from the ileum, colon, skeletal muscle, and liver was extracted and digested in trypsinogen for proteomic evaluation via liquid chromatography-tandem mass spectrometry (LC-MS/MS). Peptide analysis, protein identification, and protein quantification were performed to capture tissue specific changes in the proteomic profile. Performance data was analyzed using the MIXED procedure in SAS with fixed effect of dietary treatment and random effect of replicate. Protein abundances were tabulated using label-free methods and analyzed for statistical significance between dietary treatments via a Student’s two-tailed t-test. The overall performance data indicated that AB pigs had increased ADG (0.43 vs. 0.32 kg/d, P = 0.001) and ADFI (0.51 vs. 0.37 kg/d, P = 0.002) compared with CON pigs; however, G:F was not different as a result of dietary treatment (0.85 vs. 0.88, P = 0.617). Mass spectroscopy analysis found 90 liver, 20 colon, 12 ileum, and 2 skeletal muscle proteins of higher abundance (P < 0.05) in AB compared with CON pigs. Alternatively, there were 2 liver, 13 colon, 7 ileum, and 14 skeletal muscle proteins of lower abundance (P < 0.05) in AB vs. CON pigs. Proteins of increased abundance in AB pigs included several proteins involved in ATP generation through glycolysis and oxidative phosphorylation, protein synthesis and folding, and cell proliferation. These data indicate that AGPs act to increase feed intake and increase abundance of proteins important in metabolism and growth pathways, thereby allowing for increased whole-body tissue accretion of nursery pigs.