Biological Characteristics, Pathogenicity Assessment, and Whole genome analysis of Aeromonas salmonicida Strain SGQ-1 Isolated from Crucian Carp.

Abstract

Aeromonas salmonicida is an important fish pathogen, but its pathogenicity in non-salmonid hosts remains poorly characterized. In this study, strain SGQ-1 was isolated from diseased crucian carp (Carassius auratus) and subjected to phenotypic, genomic, antimicrobial susceptibility, and pathogenicity analyses. SGQ-1 was a Gram-negative, β-hemolytic, short rod-shaped bacterium. The optimal growth was observed at 28 °C and pH 7.0 in separate experiments. Analyses of average nucleotide identity, digital DNA-DNA hybridization, and core-genome phylogeny identified SGQ-1 as A. salmonicida. However, the isolate could not be assigned unequivocally to a recognized subspecies. Experimental challenge resulted in gross lesions, histopathological changes in multiple tissues, and dose-dependent mortality. The preliminary LD50 estimated was 1.40 × 107 CFU/mL. Disk diffusion testing revealed variable inhibition-zone diameters among the tested antimicrobial agents, with no inhibition zone observed for tetracycline under the test conditions used. Separately, database-based genome annotation identified candidate sequences putatively associated with adhesion, motility, secretion systems, iron acquisition, and toxin production. These findings support the pathogenic potential of SGQ-1 in crucian carp under experimental conditions. The study evaluates a single bacterial isolate using an intracoelomic challenge model and database-based genomic predictions. Therefore, the findings support the experimental pathogenic potential of SGQ-1 in crucian carp but do not establish natural transmission dynamics, field-level disease risk, or the functional activity of the predicted virulence-associated genes. These provide baseline phenotypic, pathogenic, and genomic information on SGQ-1, an A. salmonicida isolate from a non-salmonid host.