An understanding of the molecular mechanisms which confer an aptitude for colonization, invasiveness, and reorientation of the physiology of a host constitutes a challenge of major importance, from academic, medical and agronomic points of view: these studies serve to guide the conception of new strategies for fighting pathogenic organisms.

Our group has been studying pathogenic bacteria from the Flavobacterium genus since the first strains appeared in Europe in the 1960s. Our work has notably led to the complete revision of the genus (Bernardet, J-F, & Bowman), and the description of the Flavobacteriaceae family (Bernardet J-F, & Nakagawa,Y). We have developed methods for molecular typing, and standardized culture techniques and methods of experimental infection of the rainbow trout.

The genus Flavobacterium includes a large diversity of species, principally isolated from aquatic environments. These species include notably three which are pathogenic for fish: F. psychrophilum, F. columnare and F. branchiophilum. We have already sequenced and analyzed the genome of the JIP 02/86 strain of the bacterium which is pathogenic for the salmonides, F. psychrophilum (Duchaud E.)

Whereas F. columnare and F. psychrophilum are sometimes capable of causing branchial lesions in several species of fish, a third species, F. branchiophilum, is mainly responsible for gill disease (Kimura et al., and Wakabayashi H. et al.), a pathology whicy primarily affects salmonides. Since the first description of this bacterium in Japan, F. branchiophilum has been detected in numerous regions in which salmoniculture is practiced (USA, Hungary, Canada, Korea).

In contrast to F. psychrophilum and F. columnare, F. branchiophilum is considered to be a non-invasive bacterium. The branchial tropism is probably linked to proteolytic and hemagglutinating activities of extracellular products as well as to the adherence capacities of the bacterium, which affects the respiratory capacity of the colonized hosts (Wakabayashi, H., and Iwado, T.).

The complete sequencing of the FL15 strain of F. branchiophilum should lead to an understanding of the molecular mechanisms implicated in virulence, and in particular proteolytic, hemolytic and adherence properties. A comparative genomics program including the genomes of F. psychrophilum, F. columnare and the environmental bacterium, F. johnsoniae, should allow characterization of the central genome and define the accessory genome which carries the virulence determinants we are looking for.

Bibliography

• Bernardet, J.-F. & Bowman, J. P. The Genus Flavobacterium. in The Prokaryotes, a Handbook on the biology of bacteria, Edn. 3 , Vol. 7 (eds. Dworkin, M. et al.) Springer-Verlag, New York, 2006, pp. 481-531).
• Bernardet, J.-F. & Nakagawa, Y. An introduction to the family Flavobacteriaceae. in The Prokaryotes: a Handbook on the biology of bacteria, Edn. 3 , Vol. 7 (eds. Dworkin, M. et al.) (Springer-Verlag, New York, 2006, pp. 455-480).
• Duchaud E., Boussaha M., Loux V., Bernardet J-F., Michel C., Kerouault B., Mondot S., Nicolas P., Bossy R., Caron C., Bessières P., Gibrat J-F., Claverol S., Dumetz F., Le Hénaff M. & Benmansour A. Complete genome sequence of the fish pathogen Flavobacterium psychrophilum. Accepté pour publication.
• Kimura, N., Wakabayashi H. & Kudo S.. 1978. Studies on bacterial gill disease in salmonids I. Selection of bacterium transmitting gill disease. Fish pathol., vol. 12, pp. 233-242 (in Japanese with English abstract).
• Wakabayashi, H., and T. Iwado. 1985. Effects of a bacterial gill disease on the respiratory functions of juvenile rainbow trout. In: A. E. Ellis (ed.), Fish and Shellfish Pathology. Academic Press Inc. Ltd., London, UK, pp. 153-160.
• Wakabayashi H., G. J. Huh, and N. Kimura. 1989. Flavobacterium branchiophila sp.nov., a causative agent of bacterial gill disease of freshwater fishes. Int. J. Syst. Bacteriol., vol. 39, pp. 213-216.