The present study aimed to explore the effects of
phytic acid (PA) on the antimicrobial activity and inflammatory response in three immune organs (head kidney, spleen and skin) of on-growing grass carp (Ctenopharyngodon idella). To achieve this goal, we first conducted a 60-day growth trial by feeding fish with graded levels of PA (0, 0.8, 1.6, 2.4, 3.2 and 4.0%). Then, the fish were challenged with Aeromonas hydrophila for 6 days. Compared with the control group, the following results were obtained regarding supplementation with certain levels of PA in the diet. (1) There was an increase in skin haemorrhage and lesion morbidity in fish. (2) There was a decrease in activities or contents of
immune factors, including
lysozyme (LZ),
complement 3 (C3), C4 and
immunoglobulin M (
IgM), and there was downregulation of gene expression levels of
hepcidin,
liver-expressed antimicrobial peptide 2A (LEAP-2A), LEAP-2B, and β-defensin-1 in immune organs. (3) There was upregulation in the gene expression of the following pro-inflammatory
cytokines: tumour
necrosis factor α (TNF-α),
interleukin 1β (IL-1β) (except in the spleen),
interferon γ2 (IFN-γ2),
IL-6 (except in the spleen),
IL-8,
IL-12p40,
IL-15 and
IL-17D. These changes were partly related to the
nuclear factor kappa B (NF-κB) signalling pathway, but downregulation of
mRNA levels of anti-inflammatory
cytokines (
transforming growth factor β1 (TGF-β1), TGF-β2, IL-413/A, IL-413/B, IL-10 (except in the skin) and IL-11) occurred in a manner partially related to the target of
rapamycin (TOR) signalling pathway. Finally, based on the broken-line analysis of skin haemorrhage and lesion morbidity and
IgM content in the head kidney, the maximum tolerance levels of PA for on-growing grass carp (120.56-452.00 g) were estimated to be 1.79 and 1.31% of the diet, respectively.