Rapid qPCR detection of Pseudomonas plecoglossicida and antimicrobial peptide dynamics in infected Larimichthys polyactis.

Pseudomonas plecoglossicida has emerged as a serious threat to aquaculture, causing visceral white-nodules disease (VWND) in a range of commercially valuable fish, including the small yellow croaker (Larimichthys polyactis). In this study, a pathogenic strain of P. plecoglossicida, designated XSLPP-1, was isolated from diseased L. polyactis. Controlled challenge experiments revealed an exceptionally low 96 h median lethal concentration of 1 × 103 CFU/mL. A high accuracy quantitative PCR assay targeting the gyrB gene was developed (standard curve: y = 119538.14e-0.58x - 3.45, R2 = 0.99), enabling precise quantification of bacterial burden in infected tissues throughout the course of infection. Additionally, three antimicrobial peptides (AMPs), namely Lp_piscidin, Lp_hepcidin, and Lp_NK-lysin, were identified in L. polyactis, each exhibiting high sequence conservation with homologs in Sciaenidae species. Tissue-specific expression profiling showed significant upregulation of these AMPs in immune-related organs such as the liver, spleen, head kidney, and intestine following infection. Notably, Lp_hepcidin expression in the liver exhibited a significant negative correlation with bacterial loads, suggesting its potential as both a biomarker for disease resistance and a target for future therapeutic strategies. This study provides new insights into the pathogenic mechanisms of P. plecoglossicida and the host innate immune strategies. The integrated approach combining pathogen characterization, diagnostic development, and host immune response analysis offers promising strategies for the effective management of VWND in aquaculture systems.