Effects of dietary supplementation with heat-killed Lactobacillus acidophilus on growth performance, digestive enzyme activity, antioxidant capacity, and inflammatory response of juvenile large yellow croaker (Larimichthys crocea)

A ten-week culture trial in juvenile large yellow croaker (Larimichthys crocea) (10.80 ± 0.10 g) was conducted to assess the impact of supplementing heat-killed Lactobacillus acidophilus (HLA) on growth performance, intestinal digestive enzyme activity, antioxidant capacity and inflammatory response. Five iso-nitrogenous (42 % crude protein) and iso-lipidic (12 % crude lipid) experimental feeds with different levels of HLA (0.0 %, 0.1 %, 0.2 %, 0.4 %, or 0.8 %) were prepared. They were named FO (control group), HLA0.1, HLA0.2, HLA0.4 and HLA0.8, respectively. The results indicated that HLA addition had no impact on survival (P > 0.05). In this experiment, the final body weight, weight gain rate and specific growth rate showed a quadratic regression trend, initially increasing and subsequently decreasing with the increasing in HLA levels, and attained the peak value at 0.2 % HLA supplemental level (P < 0.05). In contrast to the control group, in terms of digestive ability, amylase, lipase and trypsin exhibited a notable linear and quadratic pattern, demonstrating a substantial increase when 0.1% 0.2 % HLA was added in the diets (P < 0.05). Notably, elevated levels of catalase (CAT) activity, superoxide dismutase (SOD) activity, and total antioxidant capacity (T-AOC) were observed in the liver when adding 0.1%-0.2 % HLA, and the level of malondialdehyde (MDA) was significantly decreased and the liver exhibited a notable upregulation in the mRNA expression levels of nrf2, cat, sod2, and sod3 (P < 0.05). Additionally, the mRNA levels of genes associated with tight junctions in the intestines (zo-1, zo-2 and occludin) exhibited a significant upregulation when 0.2 % HLA was added in the feed (P < 0.05). Furthermore, the levels of mRNA expression for proinflammatory genes in the intestines including tnf-α, il-1β, il-6 and il-8 exhibited a quadratic regression trend, characterized by an initial decline followed by subsequent growth (P < 0.05). Meanwhile, the levels of mRNA expression for genes linked to anti-inflammatory responses in the intestines (including il-10, tgf-β, and arg1) exhibited a quadratic regression pattern, initially increasing and subsequently decreasing (P < 0.05). Compare with the control group, the levels of tnf-α, il-1β and il-8 expression were notably downregulated in all HLA addition groups (P < 0.05). When 0.2 % HLA was added, the expression levels of il-10, tgf-β and arg1 in the intestinal tract were markedly increased (P < 0.05). Overall, the supplementation of 0.2 % HLA in the feed has been shown to enhance the growth performance. The enhancement was attributed to HLA's capacity to improve antioxidant function, intestinal barrier integrity, and mitigate inflammatory responses. This research offers a scientific foundation for the utilization of HLA in aquaculture.