Virulence profiling and integrated pathogenic risk assessment of bacterial isolates from freshwater-farmed Asian seabass (Lates calcarifer)

Abstract

Asian seabass (Lates calcarifer) is one of the most commercially important fish species because of its rapid growth and tolerance to a wide range of salinities. However, bacterial diseases remain a major constraint to sustainable production, and the virulence and antimicrobial resistance status of associated bacterial pathogens in freshwater farming systems remain poorly characterized. This study aimed to identify bacterial isolates from freshwater-farmed Asian seabass and evaluate their virulence potential and antibiotic resistance status for pathogenic risk assessment. Diseased fish were collected from freshwater farms across four Indian states, and the bacterial isolates were characterized using biochemical tests and 16S rRNA gene sequencing. Aeromonas (42%), Vibrio (20%) and Plesiomonas (18%) were the most predominant genera, with Aeromonas veronii comprising 22% of identified isolates. Gelatinase was the most prevalent trait (100% in Aeromonas, 90% in Vibrio), followed by DNase (90% and 70%), amylase (76% and 90%) and hemolytic activity (86% and 70%), respectively. PCR detected act (95%) and aerA (38%) genes in Aeromonas, hlyA and rtxA (90%) in Vibrio, while all Plesiomonas isolates carried astA and aphA. Mean virulence scores were highest for Vibrio (5.60) and Aeromonas (5.57). Multiple antibiotic resistance (MAR) index analysis revealed that 71.4% of Aeromonas isolates exceeded the 0.2, and several Aeromonas isolates exhibited high virulence scores and elevated MAR indices. The isolation of Vibrio cholerae from food fish highlights an additional ‘one health’ concern. These findings underscore the need for integrated pathogen monitoring, prudent antimicrobial use and robust biosecurity practices in freshwater Asian seabass aquaculture.