Precision biotic alters the effects of mixed <i>Eimeria</i> infection on gut health and microbiota in broiler chickens fed wheat-maize-soybean meal diets

1. This 42 d study further examined the physiological mechanisms underlying responses to a precision biotic (PB) on growth performance and ileal amino acid digestibility. It focussed on fermentation profile, caecal soluble protein levels, gut integrity, immune modulation, intestinal lesion scores and antimicrobial resistance gene (AMR) abundance in <i>Eimeria</i> challenged broiler chickens.2. In total, 1012 one-d-old male chicks were assigned to one of six treatments, each with a minimum of seven replicates in a randomised incomplete block design. The treatments were: control (C) maize-soya (SBM) based diet, no challenge (C-NCH); or with challenge, without PB (C-CH-0); wheat-based (W) diet plus challenge, with 0, 300, 600, or 900 mg/kg PB (W-CH-0, W-CH-300, W-CH-600, W-CH-900). On d 12, except for C-NCH, all birds were inoculated with mixed <i>Eimeria</i> spp. oocysts through feed to induce enteric stress. Caecal contents, jejunal and splenic tissues were collected on d 20 and 42 to determine caecal SCFA profiles, soluble protein concentrations, relative abundance of antimicrobial resistant genes (AMR) genes, mRNA expression of gut integrity (<i>CLDN1, OCLDN</i> and <i>JAM2</i>) and immune-related genes (<i>IL-6, IL-10, IL-1 β, IFN-γ, TGF-β1</i> and <i>iNOS</i>), respectively.3. On d 42, caecal isobutyrate, isovalerate and total branched-chain fatty acids (BCFA) were lower (<i>p</i> < 0.05) in W-CH-0 than in C-CH-0. No significant treatment effects were observed for caeca soluble protein concentrations. On d 20, PB linearly increased (<i>p</i> < 0.05) the expression of <i>OCLDN</i> gene, while <i>IL-1β</i> expression decreased quadratically (<i>p</i> < 0.05). However, <i>IFN-γ</i> expression was lower (<i>p</i> < 0.05) in challenged birds on d 42. Relative abundance of AMR genes (<i>sul2, tetM, strB and blaCTX-M</i>) showed no significant treatment effects.4. Overall, decreased caecal BCFA suggested lower protein fermentation with possible modulation of caeca microbiota. The PB supplementation increased OCLDN expression, possibly positively affecting gut integrity in the challenged birds. This could have accounted for the beneficial effect of PB on the growth performance in challenged broilers.