In 1948, Flavobacterium psychrophilum, Gram-negative bacteria, was isolated after very high mortality rates in salmon fish farms, in the United States. The bacteria is to blame for cold water flavobacteria mainly affecting salmonids. Depending on the age of the contaminated fish, the disease has different clinical forms, viz., brachial necrosis, ulcers, skin and muscle necrosis. As no vaccine is available, the only efficient way to fight the disease is the oral administration of massive, repeated doses of antibiotics. However, the large-scale use of antibiotics causes environmental impact problems and the emergence of resistant strains.
Although the bacteria was long confined to the North American West, the first strains appeared in Europe during the mid-eighties. Today, it can be found in every salmon fish-farming region in the world. This is why the research of the scientists at the INRA Jouy-en-Josas Center, who have completed the sequence of the F. psychrophilum genome, is so important. The complete genome sequence of a pathogen fish bacteria provides the bases for understanding the host-pathogen relation in an original animal model. It also clears the way for the environmentally friendly development of disease control, specifically through vaccination.
The genome, which has 2.8 million base pairs, is fairly small compared to the genome of the other member of the genera isolated in the environment. The genome has a circular chromosome encoding for 2,432 proteins. Among the proteins, the scientists have identified mediators of the response to oxidative stress, which probably enables the bacteria to resist the host's defense mechanisms. Numerous secreted proteases were also discovered. The scientists assume that they are involved in the invasion and destruction of host tissues. F. psychrophilum probably uses them in association with toxins to destroy fish tissue.
