The rainbow trout (Oncorhynchus mykiss) is an important commercial fish. Studies have shown that probiotics can promote the healthy growth of fish. In this study, we assessed the impact of an endogenous Bacillus subtilis strain (RT-BS07) on the growth, survival and immunological response of juvenile rainbow trout. Additionally, the morphology of rainbow trout’s intestinal tissue was measured. Control (CT) and experimental groups (B) were fed with conventional and probiotic-supplemented diets (1.0 × 10⁸ CFU per gram) for 42 days, respectively. Following feeding, a challenge experiment was carried out with Aeromonas hydrophila at 1.0 × 10⁷ CFU/mL to determine the experimental fish’s tolerance to the bacteria. The results showed that the weight gain rate (WGR) in group B was 118.2%, which was significantly higher than that in group CT (54.86%) after 42 days. The height of intestinal villi and the number of goblet cells increased in group B, thus increasing the resistance of rainbow trout to pathogenic bacteria. The alkaline phosphatase (AKP) activity (U/mg prot) in the liver, spleen and intestinal tissues of group B was significantly higher than that in the CT group. The expression of interleukin-1β (IL-1β) in the intestinal tissue of group B was 6-fold higher than that in group CT. Furthermore, the expression of growth hormone-2 (GH-2) in the spleen was 3-fold higher than that in group CT. After 12 days of A. hydrophila injection, the survival rate was 40% and 70% in group CT and group B, respectively. Collectively, the present study demonstrated that the addition of endogenous B. subtilis strain (1.0 × 10⁸ CFU per gram) to the feed can effectively promote the growth of rainbow trout and enhance immunity, which is beneficial to rainbow trout culture.

In 2023, Rana dybowskii exhibiting characteristic skin ulcers were found on a farm in northeastern China. Subsequently, two dominant bacteria, Aeromonas hydrophila Rd001 and Acinetobacter johnsonii Rd002, were isolated from naturally infected R. dybowskii. Experimental infection confirmed that Rd001 was the primary pathogen responsible for the disease in R. dybowskii, with a mean lethal dose (LD₅₀) of 6.25 × 10² CFU/g. The virulence genotype of Rd001 was identified as ser+/aha+/lip+/nuc+/hlyA+/aer+/alt+/ast+/act+. Antimicrobial susceptibility testing indicated that Rd001 was sensitive to enrofloxacin, flumequine, and neomycin. MLST analysis showed that Rd001 belonged to a new sequence type of A. hydrophila, named ST2378. This study offered the first comprehensive investigation into the pathogenicity, virulence genotypes, antimicrobial resistance, and genetic traits of A. hydrophila isolated from R. dybowskii, providing a theoretical foundation for preventing and controlling A. hydrophila infections.

Background: As a broad-spectrum fluoroquinolone, enrofloxacin (ENR) is commonly employed to manage bacterial infections in aquatic species. Nevertheless, there have been no documented pharmacokinetic and residue studies conducted on Dybowski’s frog (Rana dybowskii). Therefore, the objective of our study was to characterize the pharmacokinetics (PK) of ENR and its metabolite ciprofloxacin (CIP) in R. dybowskii, establish withdrawal times, and evaluate the physiological effects associated with ENR administration. Methods: Adult Rana dybowskii (120 individuals; 60 males and 60 females) were sex-separated and acclimated in four tanks. Prior to dosing, three males and three females were randomly selected as untreated controls (without ENR administration). Following the oral gavage of ENR (10 mg/kg), blood, liver, and kidney tissues were collected at 0.25, 0.5, 1, 1.5, 2, 4, 6, 8, 12, 24, 36, 48, and 72 h (n = 6) for pharmacokinetic analysis. Muscle and oviduct tissues were additionally sampled at 1, 3, 7, 15, and 30 days post-dose (n = 6) for ENR content determination. Serum/tissue ENR concentrations were measured via Liquid Chromatography–Tandem Mass Spectrometry (LC-MS/MS) and analyzed using a non-compartmental model (WinNonLin 6.1 software) to calculate PK parameters including peak time (T_max), peak concentration (C_max), and area under the curve (AUC_0−t). In studying the physiology effects of ENR administration, biochemical enzyme activities and gene expressions in the liver and intestine were assessed post-ENR administration. Results: ENR demonstrated rapid absorption and extensive distribution in R. dybowskii. The withdrawal periods were determined to be over 33 days for females and 34 days for males in R. dybowskii. Following ENR administration, there was an increase in immune enzymes (AKP (alkaline phosphatase) and ACP (acid phosphatase)) as well as glycolytic enzymes (HK (hexokinase), PK (pyruvate kinase), PFK (phosphofructokinase)). Antioxidant enzyme levels, specifically SOD (superoxide dismutase) and CAT (catalase), peaked at 1.5 h post-ENR administration but subsequently declined by the 8 h mark. Additionally, following ENR treatment, IGF1, PI3K, and Akt exhibited up-regulation, whereas Keap1 and GYS1 showed down-regulation. Conclusions: The administration of ENR at a dosage of 10 mg/kg significantly enhances the activities of AKP and ACP, promotes glycolysis, and activates the Keap1/Nrf2 and PI3K-Akt signaling pathways in R. dybowskii. These findings establish a foundation for the rational application of ENR and the determination of withdrawal times in frog aquaculture.

The presence of intermuscular bones severely affects the edibility and value-added processing of crucian carp (Carassius auratus), becoming a constraint to the high-quality development of its industry. Our previous study identified bmp6 as the key osteogenic regulator and successfully developed a new crucian carp strain lacking intermuscular bones (WUCI) using CRISPR/Cas9 technology. To accelerate its industrialization, we comprehensively assessed WUCI’s growth performance, hematological parameters, antioxidant capacity, innate immunity, and disease resistance. The results demonstrated that the WUCI exhibited significant growth performance compared to the wild-type crucian carp (WT), with significantly higher weight gain (WG) and specific growth rate (SGR) (p < 0.05) from one month to four months of age. The α-amylase (α-AL) activity of the liver and intestines of WUCI was significantly higher than that of WT. WUCI also displayed enhanced intestinal antioxidant capacity, with superoxide dismutase (SOD) and catalase (CAT) activities significantly higher than those in WT (p < 0.05). The malondialdehyde (MDA) content in the spleen of WUCI was significantly lower than that of WT (p < 0.05); no differences were observed in the liver and intestines (p > 0.05). Additionally, hepatic acid phosphatase (ACP) activity in WUCI was significantly higher than that in WT (p < 0.05). In contrast, splenic ACP and intestinal alkaline phosphatase (ALP) activities were significantly lower than those in WT (p < 0.05). Notably, the iron concentration in the serum was significantly higher in WUCI than in the WT (p < 0.05). Meanwhile, WUCI exhibited significantly lower a expression of hepcidin, TF, and TFR1 mRNA in the liver compared to WT (p < 0.05), while FPN mRNA expression was significantly higher (p < 0.05). Routine blood tests revealed significantly lower WBC in WUCI compared to that of WT (p < 0.05). Following an Aeromonas hydrophila challenge, WT demonstrated a rapid transcriptional induction of pro-inflammatory cytokines (TNF-α, IL-1β, IL-6) and immunoregulatory mediators (IL-10, TGF-β), with mRNA levels reaching maximal expression at 24 h post-infection (hpi) followed by progressive attenuation. In contrast, WUCI exhibited a delayed immune activation profile characterized by the peak expression of TNF-α, IL-1β, IL-6, and IL-10 transcripts after 72 hpi, with the maximum transcript abundance remaining lower than corresponding peak values observed in WT at 24 hpi. Finally, we observed that the mortality rate of WUCI was slightly higher post A. hydrophila infection when compared to WT, but was not significant (p > 0.05). In conclusion, this study provides a comprehensive evaluation of WUCI, revealing its distinct growth advantages, physiological adaptations, and immune function, presenting its potential for aquaculture breeding applications.

Cas9 with specialized temperature adaptations are essential for broadening the application of CRISPR-based genome editing across diverse biological contexts. Although Cas9 orthologs from thermophilic and mesophilic organisms have been characterized for high- and moderate-temperature applications, cold-active variants remain largely unexplored, limiting genome engineering in low-temperature systems such as aquaculture species. Here, we report the functional characterization of Fp2Cas9, a cold-adapted Type II-C nuclease from Flavobacterium psychrophilum. In vitro assays showed that Fp2Cas9 efficiently cleaves double-stranded DNA with a refined PAM requirement of 5′-SNAAAG-3′, and that its engineered sgRNA scaffold (sgRNA-V2) supports programmable DNA targeting. Notably, Fp2Cas9 retains 75% cleavage efficiency at 5 °C, approximately 2.5-fold higher than SpCas9 under the same conditions, but shows a marked reduction in activity at 35 °C. In vivo, a nuclear-localized variant (2NLS-Fp2Cas9) mediated efficient mutagenesis of the zebrafish slc45a2 gene, yielding ~60% indel frequencies and pigmentation-deficient phenotypes in ~43% of injected embryos. Collectively, these findings establish Fp2Cas9 as a cold-adapted Cas9 with reliable activity at low temperatures. This work adds a valuable tool to the CRISPR-Cas9 toolkit and may facilitate genome editing in cold-water organisms and other low-temperature systems.

Aeromonas salmonicida, the causative agent of furunculosis, poses a serious threat to salmonid aquaculture, but its pathogenicity in rare cold-water fish species remains insufficiently characterized. This study reports the first isolation and characterization of a pathogenic A. salmonicida strain (Ag001) from diseased Thymallus arcticus grubei in China. Multilocus sequence typing (MLST), together with 16S rRNA, gyrB, and vapA gene analyses and phylogenetic characterization, identified the isolate as A. salmonicida subsp. salmonicida ST2. In intraperitoneal challenge assays using six bacterial dose groups and PBS controls, Ag001 showed high virulence toward T. arcticus grubei [LD50: 3.08 × 10³ CFU/mL (95% CI: 9.42 × 10²–1.01 × 10⁴ CFU/mL), equivalent to 7.16 × 10¹ CFU/g (95% CI: 2.19 × 10¹–2.34 × 10² CFU/g)] and Oncorhynchus mykiss [LD50: 3.16 × 10⁴ CFU/mL (95% CI: 1.19 × 10⁴–8.38 × 10⁴ CFU/mL), equivalent to 3.46 × 10¹ CFU/g (95% CI: 1.31 × 10¹–9.17 × 10¹ CFU/g)]. Comparative histopathological examination of spleen, liver, and kidney tissues from diseased and healthy fish revealed splenic necrosis, hepatic degeneration, and renal tubular damage in diseased fish. Genomic analysis identified complete T2SS and T3SS secretion systems, along with eight virulence-associated genes including aerA, hlyA, and vapA, suggesting that secretion systems and virulence factors may contribute to pathogenicity. Antimicrobial susceptibility testing showed susceptibility to enrofloxacin, neomycin, and doxycycline, but resistance to several other tested agents. These findings provide information on the host range, pathogenicity, genomic virulence features, and potential treatment options for A. salmonicida ST2 in cold-water aquaculture systems.