Aeromonas hydrophila is a well-known bacterial pathogen associated with mass mortalities in aquaculture. Yet, few reports are available on whiteleg shrimp-pathogenic A. hydrophila. In the present study, a virulent isolate WS05 was confirmed as a causative agent of diseased freshwater-cultured whiteleg shrimp and showed a mean lethal dose (LD₅₀) value of 4.8 × 10⁴ CFU mL⁻¹. It was identified phenotypically and molecularly as an A. hydrophila strain, and exhibited susceptibility to several veterinary antibiotics extensively used in aquaculture, including cotrimoxazole, doxycycline, florfenicol, neomycin, and tetracycline. In view of the strongest inhibition zone of florfenicol against isolate WS05, the synergistic effect of the combinations of florfenicol and herb extracts was further evaluated, and the result indicated that Punica granatum extract was a potential synergist of florfenicol against isolate WS05 and the fractional inhibitory concentration index (FICI) for the florfenicol-P. granatum extract was calculated as 0.31. When combined with 7.81 mg mL⁻¹ P. granatum extract, the minimum inhibitory concentration (MIC) of florfenicol against isolate WS05 was reduced from 0.50 to 0.03 mg L⁻¹, and its activity against isolate WS05 was also enhanced with a significant reduction of ≥3.61 log in cell density after 24 h of treatment compared with that in the single drug treatment. In addition, the protective effect was potentiated by the combination of florfenicol and P. granatum extract, with a cumulative mortality of 36.66% (p < 0.05) and 33.33% (p < 0.05) lower than that in the single treatment with florfenicol and P. granatum extract after the challenge with isolate WS05 for seven days. As far as we know, this is the first study to describe whiteleg shrimp-pathogenic A. hydrophila and suggest P. granatum extract as a potential synergist of florfenicol against the A. hydrophila pathogen.

This study investigated the impacts of white spot syndrome virus (WSSV) on the gut microbiota of Penaeus monodon through a comparative microbiota analysis of infected and healthy shrimp using 16S rDNA sequencing. The WSSV-infected shrimp exhibited characteristic white spots, reduced feeding activity, and behavioral lethargy preceding 100% mortality. The comparative microbiota analysis revealed a significantly diminished α-diversity in the infected specimens, marked by phylum-level dominance shifts from Proteobacteria (72.68%) and Firmicutes (11.27%) in the controls to Cyanobacteria (75.51%) and Proteobacteria (15.63%) in the WSSV-infected shrimp. An LEfSe analysis identified Arthrospira_PCC-7345 and Halochromatium as significantly enriched taxa during infection, contrasting with depleted populations of Ruegeria, Marivita, Bacillus, and seven other genera. The distinct dysbiosis pattern characterized by the pathogen-favored taxa proliferation and commensal species suppression demonstrates WSSV-associated microbiota restructuring, potentially contributing to disease progression in farmed P. monodon. These findings establish intestinal microbial biomarkers for early WSSV detection in aquaculture systems.

Mycobacterium marinum is an opportunistic pathogen prevalent in aquatic environments, causing significant morbidity in fish, including Coho salmon (Oncorhynchus kisutch), a species increasingly cultured in Chinese salmonid aquaculture. This study investigated the immune response of Coho salmon to M. marinum infection and the bacterial proliferation dynamics in the liver and kidney. Transcriptome analysis revealed 5028 differentially expressed genes (DEGs) in the kidney and 3419 DEGs in the liver at 6 weeks post-infection. Gene Ontology and KEGG enrichment analysis highlighted pathways such as cytokine–cytokine receptor interaction, metabolic pathways, and Toll-like receptor signaling in the kidney, while the DEGs in the liver were enriched in metabolic pathways, immune system processes, and stress and defense responses. The temporal expression profiling of 15 immune-related genes, including acute-phase proteins (serum amyloid A-5 and hepcidin), cytokines (TNF-α, IL-1β, IL-17A), chemokines (CXCL13 and CCL19), pattern recognition receptors (Toll-like receptor 13), and other immune-related genes, showed significant upregulation against M. marinum infection, with stronger responses in the liver. Furthermore, it was found that there was a progressive proliferation of M. marinum in the infected liver and kidney from approximately 2.5 log₁₀ cfu/g at week 2 to about 6 log₁₀ cfu/g by 6 weeks, with a significantly higher load in the liver. These findings provide critical insights into the immune mechanisms of Coho salmon against M. marinum and the pathogen’s tissue-specific proliferation, offering a foundation for developing targeted control strategies against M. marinum in aquaculture.

Pseudomonas aeruginosa is a clinically significant bacterial pathogen that poses a serious threat to aquaculture. However, there is limited information on Massilia isolates against pathogenic P. aeruginosa in aquaculture. In this study, a potential predator, M. varians isolate P2-4, was isolated from aquaculture sediment using Chinese mitten crab Eriocheir sinensis -pathogenic P. aeruginosa as the prey bacterium, and its genomic features, antibacterial-related genes, virulence genes, toxic metabolites, pathogenicity, antibacterial spectrum, and in vitro and in vivo antibacterial effects against pathogenic P. aeruginosa in E. sinensis were further characterized. Isolate P2-4 possessed one chromosome and one plasmid, with a total of 75 tRNAs, 7 5S rRNAs, 7 16S rRNAs, 7 23S rRNAs, 34 sRNAs, 5,238 coding genes, 20 genomic islands, 1 prophage, 23 insertion sequences, and 102 repeat sequences. It harbored 19 antibacterial-related genes ( pilA , pilB , pilC , pilD , pilF , pilG , pilH , pilM , pilO , pilP , pilQ , pilS , pilR , pilT , mltA , mltB , mltC , mltD , and dacB ) associated with cellular motility and cell wall lysis. In addition, the isolate carried no virulence genes, was unable to produce hemolysin, hydrogen sulfide, nitrite, and ammonia, and was avirulent in E. sinensis with a 7-day acute LD ₅₀ value of above 5.0 × 10 ⁸ CFU/mL. Furthermore, the isolate exerted potential antibacterial activity against pathogenic Shewanella algae , Aeromonas caviae , A. hydrophila , and Photobacterium damselae , besides P. aeruginosa . It exhibited bacteriostatic rates of 99.35 to 99.99% against pathogenic P. aeruginosa at concentrations of 1.0 × 10 ³ to 1.0 × 10 ⁵ CFU/mL, and displayed relative percentage survivals of 42.31 to 73.08% against P. aeruginosa infection in E. sinensis at doses of 6.0 × 10 ³ to 6.0 × 10 ⁵ CFU/g diet. To our knowledge, this study, for the first time, demonstrates a M. varians strain as a potential biocontrol agent against pathogenic P. aeruginosa in aquaculture.