Mutations involved in the emergence of Yersinia ruckeri biotype 2 in France.

Yersina ruckeri is an enterobacteria responsible for Enteric redmouth disease (ERM), which causes significant economic losses in the aquaculture industry worldwide. Two biotypes have been described within Y. ruckeri: biotype 1 (BT1) and biotype 2 (BT2). Unlike BT1, BT2 is negative for motility and lipase secretion. The emergence of BT2 Y. ruckeri has been associated with disease outbreaks in vaccinated fish in several countries, notably France in the early 2000s. In this study, 15 BT2 strains (14 BT2 strains isolated in France and the BT2 reference strain EX5) were studied to compare the phenotypic characters of the BT1 and BT2 strains and to determine the genetic origin of the emergence of BT2 in France. BT1 bacteria are significantly longer in size than BT2 bacteria (a difference of 0.222 µm). The loss of motility of some French BT2 strains could be due to the loss of their ability to produce flagella caused by three mutations within the fliG, flhC and flgA genes. In the light of these results, the emergence of BT2 Yersinia ruckeri in France is discussed.