Abstract
Nutrient digestibility, growth, and mucosal barrier status of fish skin, gills, and distal intestine were studied in Atlantic salmon fed feeds based on marine or plant-derived ingredients. The barrier status was assessed by considering the expression of four mucin genes, five genes that encode antimicrobial proteins, distal intestine micromorphology, and design-based stereology of the midgut epithelium. In addition, the head kidney leukocytes were examined using flow cytometry; to understand the differences in their counts and function. Five experimental feeds containing the main components i) fishmeal and fish oil (BG1), ii) soybean meal (BG2; to induce enteritis), iii) fishmeal as the main protein source and rapeseed oil as the main lipid source (BG3), iv) a mix of plant protein concentrates as the protein sources and fish oil as the lipid source (BG4), and v) plant and marine ingredients in the ratio 70:30 (BG5) were produced for the study. Atlantic salmon with initial weight 72.7 ± 1.2 g was offered the experimental feeds for 65 days. The results revealed that the weights of all fish groups doubled, except for fish fed BG2. Fish fed the BG2 diet had lower blood cholesterol concentration, developed enteritis, had lower expression of muc2 in the distal intestine, and had a compromised barrier status in the intestine. Expression of both the mucin genes and genes that encode antimicrobial peptides were tissue-specific and some were significantly affected by diet. The fish fed BG1 and BG3 had more head kidney lymphocyte-like cells compared to BG5-fed fish, and the phagocytic activity of macrophage-like cells from the head kidney was the highest in fish fed BG1. The intestinal micromorphology and the mucosal mapping suggest two different ways by which plant-based diets can alter the gut barrier status; by either reducing the mucous cell sizes, volumetric densities and barrier status (as noted for BG2) or increasing volumetric density of mucous cells (as observed for BG4 and BG5). The results of the compromised intestinal barrier in fish fed plant ingredients should be further confirmed through transcriptomic and immunohistochemical studies to refine ingredient composition for sustainable and acceptable healthy diets.
Highlights
Mucosal surfaces of fishes, the skin, gills, and gastrointestinal tract, are important barriers that protect the host from pathogens and infections
The dry matter (DM) digestibility was significantly lower in BG4-fed (59%) fish compared to BG2 (66%) and BG3 (68%), while no differences were noted among fish fed BG1, BG2, BG3, and BG5
Plant protein concentrates were chosen to evade the negative effect of carbohydrate and antinutritional factors in plant ingredients on fish health, as noted by other researchers [30,31,32,33]
Summary
The skin, gills, and gastrointestinal tract, are important barriers that protect the host from pathogens and infections. The barriers include a mucosal epithelium which is covered by mucus and a wide range of components such as antimicrobial peptides that inhibit the entry of pathogens [1, 2]. The mucin glycosylation itself plays a key role in disease resistance in fish [5] and is affected both by the origin and size of Atlantic salmon [6]. Antimicrobial peptides (AMPs) are important components of the innate immune system in fish [2]. The innate immune system plays a key role in keeping fish healthy in intensive aquaculture systems, especially the components at the semipermeable mucosal epithelia in the gut [9,10,11]. Little information is available as to how the intensive production systems and use of modern diets affect the gut barrier function of fishes
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