Transcriptome analysis during thermal-induced endogenous mutagenesis in a strict psychrotrophic Aeromonas salmonicida subp. salmonicida strain

Aeromonas salmonicida is one of the earliest identified fish pathogens and the causative agent of furunculosis. A. salmonicida can be cultivated at temperatures as high as 30 °C, but at temperatures over 24 °C, insertion sequence (IS) elements cause endogenous mutagenesis. Here, we used a phenotypical and transcriptomics analysis to study the molecular aspects of A. salmonicida endogenous mutagenesis. Endogenous mutagenesis became evident after 12 h post-thermal induction and reached the highest level after 24 h. RNA was extracted from A. salmonicida grown at 15 and 28 °C after 16 h. Differential gene expression analysis showed that 344 differential expressed genes were up-regulated and 364 were down-regulated at 28 °C. Several ISs were upregulated in the chromosome and virulence plasmid. Virulence genes, including the type 3 secretion system, A-layer, and melanin synthesis, were downregulated. Overall, our result indicates that A. salmonicida incubated at 28 °C was under moderate heat stress but under strong stress caused by ISs mutagenesis. These results align with the hypothesis that ISs constrain virulent A. salmonicida to a psychrotrophic lifestyle, potentially transitioning to an attenuated mesophilic lifestyle.