Influence of strains in development of francisellosis in the blue mussel Mytilus edulis during experimental challenges

The bacterium Francisella halioticida, known to induce francisellosis in abalone and Yesso scallop, is suspected of being involved in the blue mussel mortalities observed in France. Recently, several isolates of F. halioticida were obtained from moribund mussels and categorized into two type strains, FR21 and FR22. Two other strains, AG1 and AG3, determined to belong to the genus Francisella were isolated. To determine the virulence of these isolates, juvenile and adult blue mussels were injected with bacterial solutions at high dose and monitored for 11 days. FR22c and FR22d were found to induce 80% mortality in less than seven days. The isolates AG1 and AG3 led to over 50% mortality in adult mussels but only AG1 led to significant mortality in juveniles (41%). FR22c and FR22d, the most virulent isolates, were selected to determine their respective lethal dose at 50% (LD50) in juveniles and adults. This analysis was performed with bacterial solutions ranging from 102 to 106 CFU/mussel and monitored for 30 days. The isolate FR22c was found to be the most virulent. Observed LD50 for the isolate FR22c was 4.14 ×103 CFU/juvenile and 3.45 ×103 CFU/adult and for the isolate FR22d, 1.89 ×104 CFU/juvenile and 1.52 ×104 CFU/adult. To confirm Koch's postulate, a selection of moribund, surviving and control animals were plated on specific media. The isolates were reisolated from moribund animals but not from surviving or controls. To confirm the proliferation, a specific real-time PCR was performed. All moribund individuals were positive by PCR. The main Ct values were lower for moribund compared to surviving animals and a dose effect was observed in DNA bacterial load. This study shows that some F. halioticida isolates are able to induce francisellosis in mussels and lead to high mortality, highlighting differences in virulence among the strains.