A multiplex PCR-based method was designed for the simultaneous detection of the main pathogens involved in warm-water streptococcosis in fish ( Streptococcus iniae , Streptococcus difficilis , Streptococcus parauberis , and Lactococcus garvieae ). Each of the four pairs of oligonucleotide primers exclusively amplified the targeted gene of the specific microorganism. The sensitivity of the multiplex PCR using purified DNA was 25 pg for S. iniae , 12.5 pg for S. difficilis , 50 pg for S. parauberis , and 30 pg for L. garvieae . The multiplex PCR assay was useful for the specific detection of the four species of bacteria not only in pure culture but also in inoculated fish tissue homogenates and naturally infected fish. Therefore, this method could be a useful alternative to the culture-based method for the routine diagnosis of warm-water streptococcal infections in fish.

Flavobacterium psychrophilum 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 F. psychrophilum, 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 F. psychrophilum involved antimicrobial peptides, complement, and a number of enzymes and chemokines. The matrix metalloproteases mmp9 and mmp13 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 F. psychrophilum 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.