The early life stages of fish are critical for determining the overall productivity and sustainability of aquaculture operations. This study investigated the effects of dietary autolyzed brewer's yeast (ABY) on the growth performance and intestinal health of Nile tilapia fry (Oreochromis niloticus). A 35‐day feeding trial with 480 Nile tilapia fry across 12 experimental tanks tested the effects of diets supplemented with 1, 2, and 4 g/kg of ABY, with a control diet for comparison. The results indicated that the diet which had 1 g/kg ABY supplementation (ABY1) significantly enhanced the weight gain and specific growth rate (SGR) while reducing feed conversion ratio (FCR) compared to the control diet. Intestinal gene expression analysis showed significant upregulation of immunoregulatory genes (tlr2, il10, il1β, and igm), and a tight junction gene (occludin) in the ABY1 group. Conversely, tnfα expression was significantly downregulated in the ABY1 and ABY2 groups, indicating a potential anti‐inflammatory effect. Histological analysis of the mid‐intestine revealed healthy morphology with no significant differences in mucosal fold length, muscularis thickness, lamina propria width, and goblet cell count among the treatment groups. The findings demonstrate that supplementing diets with ABY at 1 g/kg level enhances growth performance and modulates the intestinal immune response of Nile tilapia fry, without compromising intestinal integrity. This study offers valuable insights into using ABY as a functional feed additive to enhance Nile tilapia health and development, as well as to promote aquaculture sustainability.

Yeast-based feed additives have emerged as promising functional feed additives (FFAs) to promote sustainable aquaculture development through enhanced gut health and immune modulation in fish. The present study evaluated the impact of autolysed brewer’s yeast (ABY) and soluble dried yeast extract (SDYE) in improving the intestinal and skin histology, immune response, and intestinal microbiome of Atlantic salmon parr (Salmo salar) over a 9-week feeding trial. Three experimental diets were produced: a control diet, a diet supplemented with ABY at 2.5 g/kg, and a diet supplemented with SDYE at 2.5 g/kg. These diets were administered to triplicate tanks of Atlantic salmon. The yeast-supplemented diets, especially ABY, improved intestinal mucosal fold length, lamina propria width, microvilli density, and intestinal goblet cell counts, as well as skin goblet cell counts. The yeast additives had no detrimental effects on the fish haematology, with no significant differences in haemoglobin concentration, red blood cell counts, and white blood cell counts among the treatment groups. Gene expression analysis revealed upregulation of il-1β and muc-2 in fish fed the ABY diet, indicating enhanced immune function and potentially mucosal protection. Intestinal microbiota analysis revealed Firmicutes as the most dominant phylum in all groups, followed by Actinobacteriota. Distinct bacterial community shifts were observed between the treatment groups, with a significant increase in the relative abundance of taxa such as Staphylococcus in yeast-supplemented diets and a significant decrease in Streptococcus and Weissella. Collectively, these findings suggest that the yeast additives, especially ABY, enhance gut health and immune function without compromising growth performance.

The host microbiome is a promising source of probiotics for aquaculture species including Nile tilapia. In this study, the probiotic potential of autochthonous bacterial isolates from Nile tilapia and carp mid-intestines were screened in vitro. Two isolates (C61 and T70), closely related to Bacillus subtilis, exhibited antagonistic activity against multiple pathogen species and demonstrated multiple digestive enzyme activities. Their antagonistic activity in Aeromonas hydrophila assays remained even under simulated intestinal juice (SIJ) exposure. Subsequently, C61 (PT1) and T70 (PT2) were added to experimental diets at log 7 CFU/g of diet, and fed to Nile tilapia (5.32 ± 0.12 g) for 40 days. There were no significant differences observed in the growth performance across treatments. Despite limited Bacillus intestinal recovery levels, 16S rRNA gene metabarcoding revealed subtle shifts in the intestinal microbial community composition of the probiotic-fed groups. In addition, the PT1 group showed significantly longer mucosal fold length, elevated intestinal and skin goblet cell levels, and higher skin goblet cell coverage compared to the control. These results indicate the potential benefits of the isolates as functional feed additives for enhancing the mucosal health of Nile tilapia, but their benefits were likely achieved through transient activity given the low level of Bacillus recovery in the intestine.

Soybean meal (SBM) is commonly used in aquafeeds due to its wide availability, reasonable protein content, and cost‐effectiveness. However, high SBM inclusion levels in the diets of carnivorous fish, such as Atlantic salmon ( Salmo salar ), can cause soybean meal‐induced enteritis (SBMIE), resulting in compromised gut health, reduced nutrient absorption, and impaired growth. An 8‐week study was conducted to evaluate the potential of brewers’ yeast‐derived functional feed additives (FFAs), specifically yeast cell wall β ‐glucans (P β G [purified β ‐glucan]) and yeast cytosolic extracts (YEs), to mitigate the adverse effects of SBMIE in Atlantic salmon parr. Fish were fed diets containing 30% SBM (30‐SBM) with either 0.02% β ‐glucan (30‐SBM+P β G) or YE at 1% (30‐SBM+YE1) and 2.5% (30‐SBM+YE2.5) inclusion levels and compared against a control diet without SBM (0‐SBM). The study assessed growth performance, haematological parameters, distal intestinal morphology, and the distal intestinal gene expression levels of enteritis biomarkers ( casp3b , pcna , and hsp70 ). The results showed that P β G and 1% YE supplementation significantly reduced the severity of SBMIE, with improvements in intestinal morphology, including reduced intraepithelial leukocytes (IELs) levels and goblet cell hyperplasia. Intestinal gene expression levels of casp3b and pcna were significantly downregulated in the P β G and YE fed fish relative to the 30‐SBM fed fish, indicating reduced apoptosis and more controlled cell proliferation. However, the effects of 2.5% YE supplementation were less pronounced, indicating a dose‐dependent response. These findings demonstrate that both P β G and YE from 100% Saccharomyces cerevisiae can alleviate SBMIE in juvenile Atlantic salmon by supporting gut health and modulating cellular recovery processes.