Transcriptome changes in response to temperature in the fish pathogen Photobacterium damselae subsp. damselae: Clues to understand the emergence of disease outbreaks at increased seawater temperatures

The marine bacterium Photobacterium damselae subsp. damselae (Pdd) is a generalist and facultative pathogen that causes disease in a wide range of marine animals including fish species of importance in aquaculture. Disease outbreaks in fish farms have been correlated with an increased water temperature during summer months. In this study, we have used RNA sequencing to analyze the transcriptome of Pdd RM-71 cultured at two different temperatures, which simulated temperature conditions experienced during free swimming lifestyle at mid latitudes in winter months (15°C) and during outbreaks in aquaculture in warm summer months (25°C). The enhanced bacterial growth of Pdd observed at 25°C in comparison to 15°C suggests that an elevated seawater temperature contributes to the build-up of a sufficient bacterial population to cause disease. In comparison to growth at 15°C, growth at 25°C resulted in the upregulation of genes involved in DNA synthesis, nutrient uptake, chemotaxis, flagellar motility, secretion systems and antimicrobial resistance. Plasmid-encoded virulence factors, which include a putative adhesin/invasin OmpU, a transferrin receptor and a serum resistance protein, were also upregulated. Transcription factor RpoS, genes involved in cold shock response, modulation of cell envelope and amino acid metabolism, as well as genes of yet unknown function were downregulated at 25°C. Notably, the gene encoding damselysin cytotoxin (Dly) was among the most highly transcribed genes at the two assayed temperatures, at levels comparable to the most highly expressed housekeeping genes. This study contributes to our understanding of the regulatory networks and biology of a generalist marine bacterial pathogen, and provides evidence that temperature regulates multiple physiological and virulence-related functions in Pdd.