Chun Liu

Aeromonas schubertii is an emerging fish pathogen that causes visceral nodules and high mortality in snakehead (Channa argus), but the molecular basis of its virulence remains poorly understood. Catalase detoxifies host-derived hydrogen peroxide and may be critical for pathogenesis. In this study, we constructed an isogenic catalase-deficient mutant (Δcat) of A. schubertii and its complemented strain (C-cat) to evaluate the contribution of catalase to pathogenesis. The results showed that deletion of catalase did not affect growth or colony morphology but significantly reduced catalase activity, decreased hydrogen-peroxide tolerance more than 100-fold, and impaired survival at mildly acidic pH 6. In RAW264.7 macrophages, Δcat showed more than twofold lower intracellular survival after 6h, but this effect was partially mitigated in C-cat. During immersion challenge of juvenile snakehead, cumulative mortalities at 14 days were 100% for wild-type A. schubertii, 0% for Δcat, and 50% for C-cat. These findings demonstrated that catalase-mediated catalase activity was indispensable for oxidative stress resistance, intracellular persistence, and full virulence of A. schubertii, and highlighted catalase as a potential prophylactic target in aquaculture disease control.

Vibrio infections cause enteritis in grouper fish, leading to high mortality and stunted growth, which is a major challenge for aquaculture. Oligochitosans, marine prebiotics with bioactive properties, have proven their potential for growth promotion and immune regulation. However, the impacts of Vibrio harveyi on the gut microbiome of grouper fish and the potential of oligochitosans to modulate these effects remain poorly understood. This study investigates the influence of oligochitosan on the gut morphology, microbiota and metabolic patterns of hybrid grouper following low‐dose infection with V. harveyi . After 8 weeks of feeding, infected groupers exhibited histopathological changes in the intestine that were mitigated by oligochitosan, as evidenced by improved villus height and intestinal wall thickness. Microbiome sequencing revealed that oligochitosan enhanced α‐diversity and shifted bacterial communities, particularly increasing the abundance of Fusobacteriota and Actinobacteriota . Furthermore, the prevalence of Shewanella and Vibrio , which were more abundant in infected groupers, was reduced upon oligochitosan treatment. Metabolomic analysis indicated that lipid metabolism pathways were significantly altered in response to infection and oligochitosan intervention. Overall, oligochitosan modulates the gut microbiome and metabolite profiles, potentially protecting against V. harveyi ‐induced intestinal dysbiosis and metabolic disorders in hybrid groupers.

The American bullfrog (Aquarana catesbeiana) is both an economically important aquaculture species and a globally distributed invasive organism with high environmental adaptability. In this study, we present a high-quality chromosome-level genome assembly for the species, comprising 13 chromosomes with a total length of 6.32 Gb and a scaffold N50 of 691.8 Mb. Genome completeness was evaluated at 95.5% using BUSCO and 99.9% using Merqury. Repetitive sequences accounted for 79.51% of the genome. Through a combination of RNA-seq, Ab initio and homology-based gene prediction, we identified 32,382 protein-coding genes, with 98.96% of these genes functionally annotated. This chromosome-level assembly provides an important resource for future studies on the evolution, functional genomics and molecular breeding of the American bullfrog.