Francisella sciaenopsi sp. nov. isolated from diseased red drum Sciaenops ocellatus in Florida, USA.

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 morbidity in Florida, USA, 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. Although the 16S rRNA sequence of this isolate shared 99.61% identity to the type strain of F. philomiragia O#319LT, whole genome analysis (average nucleotide identity <95%; digital DNA-DNA hybridization <70%) and a multilocus sequence analysis of 8 concatenated housekeeping genes in 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 d 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 propose the name Francisella sciaenopsi sp. nov. for this novel isolate.

Francisellosis of Yesso scallops Mizuhopecten yessoensis in Japan is caused by a novel type of Francisella halioticida.

Francisella halioticida, the causative agent of francisellosis of the giant abalone Haliotis gigantea, has also been isolated from Yesso scallops Mizuhopecten yessoensis, which presented with orange/pinkish lesions in the adductor muscle and experienced high mortality. However, it is not clear whether the F. halioticida isolated from the giant abalone and Yesso scallops are phenotypically and genetically identical to each other. The present study revealed that isolates from the giant abalone and Yesso scallops were phenotypically different, with slower growth in modified eugon broth and a lack of prolyl aminopeptidase and phenylalanine aminopeptidase in Yesso scallop isolates. Additionally, we found that 3 of 8 housekeeping genes were different between them. Based on these phenotypic and genetic differences, we propose that F. halioticida isolated from Yesso scallops in Japan be designated as the 'J-scallop type' to distinguish it from strains from abalone ('abalone type'). Whole-genome sequencing analysis of a strain belonging to the J-scallop type showed that the overall similarity between the J-scallop and abalone type strains was estimated to be 99.84%. In accordance with a lack of prolyl aminopeptidase activity, in general, all of the J-scallop type strains examined have a 1 bp deletion in the responsible gene encoding prolyl aminopeptidase. This deletion was confirmed in all F. halioticida in diseased Yesso scallops examined, suggesting that in Japan, francisellosis of Yesso scallops is caused by a novel type of F. halioticida and not by the abalone type.

Parallel studies confirm Francisella halioticida causes mortality in Yesso scallops Patinopecten yessoensis.

Francisella halioticida is a marine bacterium originally described as the causative agent of mass mortality among giant abalone Haliotis gigantea. Recent field studies in Canada and Japan have suggested that this bacterium is also the cause of adductor muscle lesions and high mortality of Yesso scallops Patinopecten yessoensis, although a causal relationship has not been established. In the present study, the pathogenicity of F. halioticida in Yesso scallops was assessed in both Canada and Japan using bacteria isolated from diseased Yesso scallops in each respective country. Independent laboratory experiments revealed that scallops challenged with F. halioticida via bath exposure resulted in high mortality and histological lesions characterized by massive haemocyte infiltration. The presence of F. halioticida was confirmed using PCR, and F. halioticida was re-isolated from a portion of dead and surviving specimens. These results fulfill Koch's classic criteria for establishing disease causation and provide conclusive evidence that F. halioticida causes adductor muscle lesions and high mortality in Yesso scallops.