Characteristics and Complete Genome Analysis of a Pathogenic Aeromonas Veronii SJ4 from Diseased Siniperca Chuatsi.

As an opportunistic pathogen, Aeromonas veronii can cause hemorrhagic septicemia of various aquatic animals. In our present study, a dominant strain SJ4, isolated from naturally infected mandarin fish (Siniperca chuatsi), was identified as A. veronii according to the morphological, physiological, and biochemical features, as well as molecular identification. Intraperitoneal injection of A. veronii SJ4 into S. chuatsi revealed clinical signs similar to the natural infection, and the median lethal dosage (LD50) of the SJ4 to S. chuatsi in a week was 3.8 × 105 CFU/mL. Histopathological analysis revealed that the isolate SJ4 could cause cell enlargement, obvious hemorrhage, and inflammatory responses in S. chuatsi. Detection of virulence genes showed the isolate SJ4 carried act, fim, flgM, ompA, lip, hly, aer, and eprCAL, and the isolate SJ4 also produce caseinase, dnase, gelatinase, and hemolysin. In addition, the complete genome of A. veronii SJ4 was sequenced, and the size of the genome of A. veronii SJ4 was 4,562,694 bp, within a G + C content of 58.95%, containing 4079 coding genes. Nine hundred ten genes encoding for several virulence factors, such as type III and VI secretion systems, flagella, motility, etc., were determined based on the VFDB database. Besides, 148 antibiotic resistance-related genes in 27 categories related to tetracyclines, fluoroquinolones, aminoglycosides, macrolides, chloramphenicol, and cephalosporins were also annotated. The present results suggested that A. veronii was etiological agent causing the bacterial septicemia of S. chuatsi in this time, as well as provided a valuable base for revealing pathogenesis and resistance mechanism of A. veronii.