Catfish farming is one of the largest and most economically significant aquaculture sectors in the United States and is challenged by infectious diseases that affect its profitability. Among these, motile Aeromonas septicemia (MAS) is one of the most devastating bacterial diseases affecting the catfish industry. The disease is caused by virulent Aeromonas hydrophila, a Gram-negative, facultative anaerobic bacterium, and is a globally important aquatic pathogen. A. hydrophila persists across diverse aquatic environments, and its high adaptability adversely impacts aquaculture production while complicating disease management strategies.
To study how osmotic stress affects bacterial physiology at the genome level, we conducted comparative sequencing of A. hydrophila ML09-119 grown in medium (tryptic soy broth) at different final salt concentrations (0.5% and 4.5% NaCl). RNA was extracted from 48h cultures and samples were submitted to Novogene (Sacramento, CA) for sequencing. High-quality RNA sequencing data were obtained from our samples (RINe 8.1–8.9; mapping rate 96–99%). Our initial analyses showed a clear separation between the gene expression profiles in low- and high-salinity conditions, indicating that osmotic stress causes distinct transcriptional changes.
Differential expression analysis identified a total of 2,078 significantly regulated genes (p<0.05), with 900 genes upregulated under high salt stress and 1,178 under low salt conditions. Genes related to membrane transport, especially those encoding ABC transporters and transmembrane transporters, were significantly upregulated, suggesting improved ion regulation. Functional enrichment analyses highlighted that activation of sulfur and nitrogen metabolism pathways reflects metabolic regulation to support adaptation to high osmotic pressure. On the other hand, motility-associated genes, such as those involved in flagellar assembly, were downregulated. Additionally, genes involved in central energy metabolism, including oxidative phosphorylation and the tricarboxylic acid cycle, were significantly downregulated.
Together, these results demonstrate that A. hydrophila responds to high salinity stress by transcriptional reprogramming, favoring transport and stress-adaptive pathways over energy-demanding cellular functions. This study provides novel insight into environmental regulation of bacterial physiology with direct relevance to disease dynamics and health management in the aquaculture industry.