Molecular identification and pathogenicity study of Pseudomonas juntendi isolated from infected Nile tilapia

Bacterial pathogens are major threats to sustainable aquaculture, leading to huge economic losses and reducing fish health and productivity. In this study, isolation, identification and characterization of the causative agent of acute mortality (approximately 55%) in Nile tilapia (Oreochromis niloticus) was carried out. The isolate was identified as Pseudomonas juntendi based on multilocus sequence analysis (16S rRNA, gyrB, rpoB and rpoD), showing highest similarity to reference strains. Most of the biochemical features were in line with previously reported P. juntendi. Virulence factors of the bacterium included high swimming motility, efficient biofilm formation and β-hemolysis. Antibiogram assay indicated that the isolate was resistant to six types of antibiotics with a calculated MAR index of 0.25. Pathogenicity was confirmed experimentally after challenge of healthy Nile tilapia fingerlings, resulting in development of hemorrhagic lesions and mortality with 96 h median lethal dose (LD₅₀) of 5.9 × 10⁵ CFU/fish. Sublethal infection significantly altered host-response gene expression in liver and head kidney tissues, marked by upregulation of pro-inflammatory cytokines (TNF-α, IL-1β, IL-6 and IL-8), immune-related transcription factors (NF-κB and TLR-7), the chemotactic mediator CC-chemokine 1 and the antigen-presentation marker MHC-IIα. Histopathological examination revealed extensive tissue damage in the liver and head kidney with cellular degeneration, aggregation and structural disorganization, indicating systemic infection. The findings in this study clearly demonstrate for the first time that P. juntendi is a virulent pathogen in Nile tilapia, causing significant mortality, tissue damage and immune responses. This investigation contributes to our growing understanding of bacterial pathogens in tilapia farming.