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To gain insights into the prevalence and antimicrobial resistance patterns of major bacterial pathogens affecting largemouth bass (Micropterus salmoides) in the Pearl River Delta (PRD) region, Guangdong, China, a study was conducted from August 2021 to July 2022. During this period, bacteria were isolated and identified from the internal organs of diseased largemouth bass within the PRD region. The antimicrobial resistance patterns of 11 antibiotics approved for use in aquaculture in China were analyzed in 80 strains of Edwardsiella piscicida using the microbroth dilution method. The results showed that 151 bacterial isolates were obtained from 532 samples, with E. piscicida (17.29%, 92/532), Aeromonas veronii (4.70%, 25/532), and Nocardia seriolae (2.26%, 12/532) being the main pathogens. Notably, E. piscicida accounted for the highest proportion of all isolated bacteria, reaching 60.92% (92/151), and mainly occurred from November to April, accounting for 68.48% (63/92) of the cases. The symptoms in largemouth bass infected with E. piscicida included ascites, enteritis, and hemorrhaging of tissues and organs. The drug sensitivity results showed that the resistance rates of all E. piscicida strains to ciprofloxacin, all sulfonamides, thiamphenicol, florfenicol, enrofloxacin, doxycycline, flumequine, and neomycin were 96.25%, 60–63%, 56.25%, 43.75%, 40%, 32.5%, 16.25%, and 1.25%, respectively. In addition, 76.25% (61/80) of these strains demonstrated resistance to more than two types of antibiotics. Cluster analysis revealed 23 antibiotic types (A–W) among the 80 isolates, which were clustered into two groups. Therefore, tailored antibiotic treatment based on regional antimicrobial resistance patterns is essential for effective disease management. The findings indicate that in the event of an Edwardsiella infection in largemouth bass, neomycin, doxycycline, and flumequine are viable treatment options. Alternatively, one may choose drugs that are effective as determined by clinical drug sensitivity testing. [ABSTRACT FROM AUTHOR]

The purpose of this study was to investigate whether a defensin‐like antimicrobial peptide (C‐13326 peptide) identified in Hermetia illucens could possess protective effect against multidrug‐resistant Aeromonas schubertii in hybrid snakehead (Channa maculate ♀ × Channa argus ♂). The cDNA of C‐13326 peptide comprised 243 nucleotides encoding 80 amino acids, with six conserved cysteine residues and the classical CSαβ structure. The recombinant expression plasmid pPIC9K‐C‐13326 was constructed and transformed into GS115 Pichia pastoris, and the C‐13326 peptide was expressed by induction with 1% methanol. The crude extract of C‐13326 peptide was precipitated by ammonium sulfate, assayed by Braford method, detected by tricine‐SDS‐PAGE, evaluated by BandScan software and identified by liquid chromatography‐mass spectrometry. The C‐13326 peptide was shown to have inhibitory activity against the growth of multidrug‐resistant A. schubertii DM210910 by using the minimum growth inhibitory concentration and Oxford cup method. In addition, scanning electron microscopy analysis suggested that C‐13326 peptide inhibited the growth of A. schubertii DM210910 by damaging the bacterial cell membrane. To explore the role of peptide C‐13326 in vivo, hybrid snakehead was fed with peptide C‐13326 as feed additives for 7 days. The results revealed that C‐13326 peptide could significantly down‐regulate the expression levels of IL‐1β, IL‐8, IL‐12 and TNF‐α (p <.05), and significantly improved the survival rate of hybrid snakehead after challenging with A. schubertii DM210910. Therefore, the C‐13326 peptide is a promising antimicrobial agent for A. schubertii treatment in aquaculture. [ABSTRACT FROM AUTHOR]

Edwardsiella piscicida, an infectious bacterium, causes great economic losses to the aquaculture industry. Immersion bath which is the closest way to how the fish infect bacterial pathogens in the natural environment is an effective route of artificial infection. In this study, the dynamic process of E. piscicida infection, in the spotted sea bass (Lateolabrax maculatus) was evaluated via the immersion bath. The results showed that soaking the spotted sea bass with 3 × 106 CFU mL−1E. piscicida for 30 min could artificially induce edwardsiellosis. The higher culture temperature (28.5 ± 0.5°C) or the longer bath time (30 min) would lead to higher mortality of fish. E.piscicida first invaded the gill, then entered the blood circulation to infect the spleen and kidney, where it is colonized, and gradually multiplied in the liver and brain. Meanwhile, the fluorescence in situ hybridization showed that the localization of E. piscicida in the gill and foregut after the immersion challenge proceeded from the exterior to the interior. The invasion of pathogens triggers the immune response of fish and causes tissue damage to the host. The quantitative real‐time PCR results displayed an increase in the relative expression level of immune genes (NK‐lysin, LZM, IgM and IgD). Otherwise, the most notable histopathological changes of the infected spotted sea bass were multifocal necrosis. Findings in this study broaden our understanding of the infection conditions of E. piscicida and its pathogenicity to the spotted sea bass. [ABSTRACT FROM AUTHOR]