First report of Rahnella aquatilis infection in crucian carp Carassius auratus in China.

Rahnella aquatilis infection is rare in aquaculture. Here, a Gram-negative rod-shaped bacterium was isolated from diseased crucian carp Carassius auratus in Xuzhou City, Jiangsu Province, eastern China. The isolate was tentatively named strain KCL-5, and subsequently identified as R. aquatilis by biochemical properties and molecular techniques. The results showed that the isolate KCL-5 was most closely related to the type strain ATCC33071 (= DSM4594) of R. aquatilis, which shared 99.67, 96.26 and 99.58% nucleotide sequence identities for 16S rDNA, gyrB and toxin yhaV genes, respectively. Experimental challenges were conducted which demonstrated pathogenicity of the isolate in crucian carp. Antimicrobial susceptibility testing showed that the isolated strain was susceptible to piperacillin, gentamicin, kanamycin, nalidixic acid, norfloxacin, ofloxacin, azithromycin and erythromycin. To our knowledge, this is the first report on R. aquatilis infection in crucian carp, and the first evidence of pathogenicity in fish.