Strain-Dependent Thermoadaptation in the Fish Pathogen Aeromonas salmonicida subsp. salmonicida

Strains of Aeromonas salmonicida subsp. salmonicida, a major pathogen of salmonids, typically do not grow at temperatures above 30 °C. The effects of thermal stress on this bacterium have been extensively studied. Recently, we demonstrated that repeated exposure to cyclical thermal stress, reaching up to 37 °C, can induce permanent thermoadaptation in certain strains of this bacterium. Many aspects of this adaptation process remain poorly understood. We generated 88 thermoadapted strains of A. salmonicida subsp. salmonicida capable of sustained growth at 33 °C or higher demonstrating that prolonged heat exposure can shift a substantial proportion of psychrophilic strains toward a more mesophilic-like behavior. Although growth at 35 °C was still weaker than in naturally mesophilic A. salmonicida strains, some thermoadapted strains were able to grow up to 37 °C. North American strains harboring the genomic island AsaGEI1a, a known biomarker, exhibited a significantly reduced capacity for thermoadaptation, suggesting a possible genetic constraint, although genomic analyses indicate that AsaGEI1a itself is not directly responsible for this limitation. Genotyping and phenotypic analyses revealed that thermoadaptation is frequently associated with the loss of Type III secretion system and the A-layer, two key virulence factors. Only 7% of the thermoadapted strains retained both features. Overall, our findings suggest that thermoadaptation may represent a potential mechanism influencing the persistence of some psychrophilic A. salmonicida subsp. salmonicida strains in warming aquatic environments under climate change.