Charlène Siekoula-Nguedia

Abstract :

Flavobacterium psychrophilum is the bacterial agent of “cold water disease” and “rainbow trout fry syndrome” affecting mainly the fish of the salmon family, especially the rainbow trout. This disease causes massive mortality in these species resulting in significant economic losses. With the emergence of antibiotics resistance and the lack of vaccines against this disease, the improvement of management strategies to limit the spread requires a better understanding of the genetic diversity and population structure of its agent. In this context, this thesis aims to study the genetic diversity of F. psychrophilum in France, where no such investigation has yet been conducted. For this purpose, genotyping of F. psychrophilum by MLST method was carried out on a population of sampled rainbow trout rainbow with clinical signs of the disease, in the main French regions of aquaculture production. This study showed that the fish were mainly infected by a complex of genetically closed isolates, without being able to observe a geographic structure. A second method of genotyping, PFGE, applied to a population of bacteria sampled on rainbow trout rainbow apparently healthy, at the finer scale of a sites located in a watershed in Britain, showed great genetic diversity: on the same fish, in a same farm and between sites, with no evidence of a major genetic variant. Finally, MLST and PFGE methods applied to a selection of isolates showed concordant results and similar discriminatory powers.

Key words :

Flavobacterium psychrophilum, genetic diversity, MLST, PFGE, rainbow trout, France