Postbiotics as Regulators of Gene Expression and Enzymatic Activity in Fish and Shellfish: Implications for Growth, Immunity, and Reproduction

Postbiotics, a novel class of non-viable probiotic-derived feed additives, are emerging as a promising, sustainable alternative to antibiotics and live probiotics in aquaculture. Comprising inactivated microbial cells, cell wall components, and microbial metabolites (e.g., cell-free supernatants, short-chain fatty acids, and bacteriocins), postbiotics have been shown in laboratory and animal studies to enhance growth, modulate immune responses, improve gut health and microbiome balance, and increase disease resistance in fish and shellfish. Compared with live microbes, postbiotics offer greater physicochemical stability and lower risk of transferring antimicrobial-resistance genes, which improves their suitability for farm applications. Nevertheless, significant limitations remain: lack of standardized production and dosing protocols, incomplete understanding of cellular and systemic mechanisms of action, scarce species- and life-stage-specific efficacy data, and a shortage of long-term environmental and field evaluations. This review synthesizes current preparation methods, active components, and evidence of biological activity for postbiotics, identifies key research gaps, and highlights priorities such as diversified formulations, exploration of synergistic combinations with other feed additives, standardization of experimental approaches, and expanded field trials to bridge laboratory findings with practical aquaculture use.