Characteristics of Aeromonas salmonicida, a rainbow trout pathogen, and early signatures of host immune response

Abstract

Rainbow trout is a key aquaculture species valued for its high nutritional content and adaptable immune system. However, intensive farming increases susceptibility to infectious diseases such as furunculosis, caused by Aeromonas salmonicida, a highly adaptable pathogen with significant genomic variability driven in part by plasmids. This variability complicates the study of its virulence and infection strategies. This study aimed to elucidate early infection mechanisms of A. salmonicida in rainbow trout using a systems biology approach, treating host and pathogen as a dynamic, interdependent system. Bacterial strains with similar genotypes but differing virulence were compared. Genomic analyses revealed that even minor sequence differences can significantly impact virulence, particularly among genes associated with infection processes. Transcriptomic analyses demonstrated that infection is governed by a complex network of interacting pathways on both the bacterial and host sides. Pathways, including arginine metabolism, iron acquisition, and flagellar regulation, contribute to infection. Both known and novel genes were implicated in early infection stages, with virulence-associated molecular patterns (VAMPs) playing a key role in immune recognition. Cellular analyses showed cytotoxic effects on erythrocytes and highlighted the importance of head kidney leukocytes in early defense. Myeloid cells were primarily responsible for pathogen recognition, while some lymphoid cells also contributed. Based on the conducted analyses, this study provides a comprehensive characterization of Aeromonas salmonicida virulence mechanisms and their dynamic interactions with the immune system of rainbow trout, highlighting the complexity of infection and adaptation processes that ultimately shape disease progression.