DAO prepress abstract   -  DOI: https://doi.org/10.3354/dao03803

ABSTRACT: Piscine francisellosis is one of the most important bacterial diseases affecting various fish species worldwide. Francisella orientalis, F. noatunensis and F. salimarina (F. marina) have been reported as etiological agents of disease in fish. A Francisella sp. was isolated from several diseased red drum (Sciaenops ocellatus) experiencing mortality in Florida, United States of America in 2008. In this study, molecular and phenotypic characterization of the recovered isolate was conducted. Phenotypically, the isolate showed a biochemical reaction profile distinct from that of F. orientalis and F. salimarina. Though 16S rRNA sequence of this isolate shared 99.61% identity to the type strain of F. philomiragia O#319L^T, whole genome analysis (average nucleotide identity <95%; digital DNA-DNA hybridization <70%) and multilocus sequence analysis of eight concatenated housekeeping genes comparison with other Francisella spp. indicated that this isolate was a novel Francisella species, more closely related to F. orientalis. Immersion, intracoelomic injection, and co-habitation challenges using a Nile tilapia Oreochromis niloticus fingerling model of infection were done to investigate virulence in a piscine model. Variably pigmented granulomas and pigmented macrophage aggregates were observed in the kidneys and spleens of the challenged fish, but no mortality was recorded during the 15 days of the challenge period, suggesting that this novel Francisella sp. might be an opportunistic pathogen of fish. Based on the phenotypic and genotypic differences from other Francisella spp. observed in this study, we proposed the name Francisella sciaenopsi sp. nov. for this novel isolate.