Iodobacter fluviatilis, a New Potential Opportunistic Pathogen Associated with Skin Lesions, First Report in Hypophthalmichthys nobilis in China

In the spring of 2023, a disease outbreak occurred in Lake Taihu in China, which caused a large number of deaths of <i>H. nobilis</i>. In order to investigate the cause of morbidity and mortality of the <i>H. nobilis</i>, the diseased fish were collected for histopathological and etiological studies. Histopathological observation revealed that substantial inflammatory cell infiltration was observed around skin lesion in diseased fish, extensive degeneration and necrosis were observed in the hepatic parenchymal cells, the spleen exhibited congestion, and the kidney showed congestion. A bacterial strain, C1, isolated from diseased <i>H. nobilis</i> was identified as <i>Iodobacter fluviatilis</i> through <i>16S rRNA</i> gene sequencing and biochemical phenotypic characterization. Experimental infection of the fish via intramuscular injection induced a localized abscess in a subset of fish. Antimicrobial susceptibility testing revealed that the isolate was susceptible to aminoglycosides, tetracyclines, quinolones and amphenicols, but resistant to sulfonamides commonly used in aquaculture. Here, we describe an association between <i>I. fluviatilis</i> and skin lesions in <i>H. nobilis</i>. Furthermore, we report the biochemical characteristics and drug resistance profile of the isolated bacteria. These findings also facilitate further investigations into the role of <i>I. fluviatilis</i> associated with skin diseases of <i>H. nobilis</i> and other freshwater fish.