Phenotypic and Genetic Characterization of Flavobacterium psychrophilum Recovered from Diseased Salmonids in China

• Published in: Microbiology spectrum 2021-10, Vol.9 (2), p.e0033021-e0033021
• Main Authors: Li, Shaowu, Chai, Jingru, Knupp, Christopher, Nicolas, Pierre, Wang, Di, Cao, Yongsheng, Deng, Furong, Chen, Fuguang, Lu, Tongyan, Loch, Thomas P
• Format: Article
• Language: English
• Subjects: Animals; Anti-Bacterial Agents - pharmacology; antimicrobial susceptibility; Aquaculture - economics; bacterial coldwater disease; Bacteriology; Biodiversity; China; Epidemiology; Fish Diseases - drug therapy; Fish Diseases - microbiology; Fish Diseases - prevention & control; Flavobacterium - drug effects; Flavobacterium - genetics; Flavobacterium - isolation & purification; Flavobacterium - pathogenicity; Flavobacterium psychrophilum; genetic diversity; Life Sciences; Microbial Sensitivity Tests; Microbiology and Parasitology; MLST; Multilocus Sequence Typing; Oncorhynchus kisutch - microbiology; Oncorhynchus mykiss - microbiology; Osmeriformes - microbiology; Quantitative Methods; Research Article; serotype; Systematics, Phylogenetics and taxonomy; Virulence Factors - genetics
• ISSN: 2165-0497; 2165-0497
• DOI: 10.1128/Spectrum.00330-21

Flavobacterium psychrophilum, the etiological agent of bacterial coldwater disease (BCWD) and rainbow trout fry syndrome, causes great economic losses in salmonid aquaculture worldwide. Recent molecular studies have uncovered important epidemiological and ecological aspects of this pathogen; however, such data are lacking for F. psychrophilum populations affecting aquaculture in China. Herein, F. psychrophilum phenotype, genotype, and virulence were characterized for isolates recovered from epizootics in multiple salmonid aquaculture facilities across China. Thirty-one F. psychrophilum isolates, originating from four provinces and three host fish species, were predominantly homogeneous biochemically but represented 5 sequence types (STs) according to multilocus sequence typing (MLST) that belonged to clonal complex CC-ST10 or 3 newly recognized singleton STs. PCR-based serotyping classified 19 and 12 F. psychrophilum isolates into molecular serotypes 1 and 0, respectively, showing an obvious relationship with host species. Antimicrobial susceptibility analysis via broth microdilution revealed reduced susceptibility to enrofloxacin, flumequine, and oxolinic acid, moderate susceptibility to gentamicin, erythromycin, and florfenicol, and variable susceptibility to ampicillin and oxytetracycline. challenge experiments confirmed the ability of two representative Chinese F. psychrophilum isolates to induce typical signs of BCWD and mortality in 1-year-old rainbow trout (Oncorhynchus mykiss). Findings collectively demonstrate (i) that BCWD outbreaks in China studied thus far are caused by F. psychrophilum lineages that are common on other continents (e.g., CC-ST10) and others that have not been reported elsewhere (e.g., ST355, ST356, ST357), (ii) that F. psychrophilum molecular serotypes distinguish isolates from different host fish species, even within STs, and (iii) reduced F. psychrophilum antimicrobial susceptibility against compounds used for BCWD control in China. Flavobacterium psychrophilum causes substantial economic losses in salmonid aquaculture worldwide. Although this bacterium is also believed to be a disease source in China, published reports of its presence do not yet exist. Herein, F. psychrophilum was linked to multiple disease outbreaks in several salmonid aquaculture facilities within four Chinese provinces, and polyphasic characterization revealed that most isolates were genetically distinct from strains recovered on other continents. Analyse