Enhancement of Disease Resistance in Pengze Crucian Carp (Carassius auratus var. Pengze) by Carvacrol Through Modulation of Intestinal Microbiota and Serum Metabolism

<b>Objectives:</b> This study aimed to investigate the regulatory effects of dietary carvacrol on intestinal micro biota composition, serum metabolic profiles, and their association with increased resistance to <i>Aeromonas hydrophila</i> in Pengze crucian carp. <b>Methods:</b> Juvenile fish (5.63 ± 0.35 g) were randomly allocated into two experimental groups: a control group (CK) fed a basal diet and a treatment group (CA) supplemented with 600 mg/kg microencapsulated carvacrol. Following an 8-week feeding trial, nine specimens per group were sampled for venous blood and intestinal tract collection. Remaining individuals were subjected to a 12-h <i>A. hydrophila</i> challenge prior to identical sample collection. <b>Results:</b> Key findings revealed that carvacrol supplementation induced significant microbial modulations, notably reducing Firmicutes abundance while enhancing <i>Cetobacterium</i> populations by 33.25% compared to controls. Post-challenge analysis demonstrated marked declines in intestinal microbial diversity indices (Observed ASV, Chao1, ACE, and PD whole tree) in the CK group, whereas the CA group maintained stable microbial diversity. Pathogenic genera including <i>Aeromonas</i>, <i>Shewanella</i>, and <i>Vibrio</i> showed significant proliferation in challenged controls, contrasting with maintained microbial homeostasis in carvacrol-fed specimens. Serum metabolomic profiling identified the most significantly altered metabolic pathways associated with carvacrol administration: glycerophospholipid metabolism, linoleic acid metabolism, arachidonic acid metabolism, α-linolenic acid metabolism, GPI-anchor biosynthesis, and autophagy-animal pathways. <b>Conclusions:</b> Our results demonstrate that dietary carvacrol may reinforce intestinal microbial barrier function by optimizing beneficial microbial composition and reducing the proportion of pathogens, and modulate immune-related metabolic pathways critical for host defense, which might be involved in enhanced disease resistance.