Transcriptional Responses of Resistant and Susceptible Fish Clones to the Bacterial Pathogen <em>Flavobacterium psychrophilum</em>

<div><p><em>Flavobacterium psychrophilum</em> is a bacterial species that represents one of the most important pathogens for aquaculture worldwide, especially for salmonids. To gain insights into the genetic basis of the natural resistance to <em>F. psychrophilum</em>, we selected homozygous clones of rainbow trout with contrasted susceptibility to the infection. We compared the transcriptional response to the bacteria in the pronephros of a susceptible and a resistant line by micro-array analysis five days after infection. While the basal transcriptome of healthy fish was significantly different in the resistant and susceptible lines, the transcriptome modifications induced by the bacteria involved essentially the same genes and pathways. The response to <em>F. psychrophilum</em> involved antimicrobial peptides, complement, and a number of enzymes and chemokines. The matrix metalloproteases <em>mmp9</em> and <em>mmp13</em> were among the most highly induced genes in both genetic backgrounds. Key genes of both pro- and anti-inflammatory response such as IL1 and IL10, were up-regulated with a greater magnitude in susceptible animals where the bacterial load was also much higher. While higher resistance to <em>F. psychrophilum</em> does not seem to be based on extensive differences in the orientation of the immune response, several genes including complement C3 showed stronger induction in the resistant fish. They may be important for the variation of susceptibility to the infection.</p>