The longevity of the antimicrobial response in rainbow trout (Oncorhynchus mykiss) fed a peptidoglycan (PG) supplemented diet

• We have studied AMP expression in skin of rainbow trout after feeding of a peptidoglycan (PG) enriched diet.
• Continuous PG treatment maintained high levels of AMP expression in skin, although levels decreased with time on the diets.
• Withdrawal of PG diets resulted in a fall in AMP expression but omDB-3/-4 remained elevated compared to fish on a control diet.
• These results confirm that orally administered PG enhances trout innate immune defences at mucosal surfaces.

This study builds upon previous work studying antimicrobial peptide (AMP) gene expression in rainbow trout (Oncorhynchus mykiss) fed a peptidoglycan (PG) enriched diet. The aims here were 1) to evaluate how long AMP expression is elevated in skin with continuous feeding of fish with the PG enriched diet for 21 or 28 days, and 2) to assess the impact of stopping PG feeding at day 14 when sampled at day 21 or 28. The rainbow trout were divided into 6 groups, with two fed a control commercial diet for the duration of the experiment and the other four given the same diet enriched with 10 mg PG/Kg for 14 days (PG 1–14) or continuously (PG continuous), the former reverting back to the commercial diet at day 14. No mortalities occurred during the study and there were no significant differences in growth among the fish in the different diet groups.The expression of six AMP genes was studied by real-time PCR in the skin, since these genes were shown to be induced in response to the PG enriched diets in a previous experiment. We show that continuous PG treatment for 21 or 28 days maintained high levels of AMP expression, although in general the levels decreased with time on the diets. Withdrawal of the PG diets at day 14 resulted in a fall in expression level especially apparent with omCATH-1, omCATH-2 and omLEAP-2a, but with omDB-3 and omDB-4 remaining at elevated levels (x10) in comparison to fish given a control diet. These results confirm that orally administered PG clearly enhances the trout innate immune system and could be used as a means to boost fish defences. Future studies should be conducted to verify the impact on survival after pathogen challenge in trout fed PG enriched diets under these regimes.