Whole-genome characteristics, antimicrobial resistance and virulence of pathogenic Aeromonas veronii from diseased Macrobrachium rosenbergii.

Aeromonas veronii is an opportunistic pathogen capable of infecting various aquatic organisms. In this study, a pathogenic strain, designated MRG1, was isolated from diseased Macrobrachium rosenbergii and identified as A. veronii through morphological, biochemical, and molecular analyses, including whole-genome average nucleotide identity (ANI). Injection challenge trials indicated a 7-day median lethal dose (LD50) of A. veronii strain MRG1 for M. rosenbergii was 1.33 × 106 CFU/mL. Histological examination indicated that MRG1 infection caused pathological alterations, including deformation of the hepatopancreatic lumen, gill filament adhesion, and detachment of gill epithelial cells. Plate assays detected the production of extracellular caseinase, lecithinase, gelatinase, and hemolysin. Whole-genome sequencing determined the MRG1 genome to consist of a 4,673,298 bp chromosome (58.36% GC) and two plasmids (6,199 bp and 8,444 bp; 55.38% and 55.03% GC), encompassing 4,330 predicted genes. Virulence Factor Database (VFDB) screening identified 121 putative virulence-associated genes, primarily involved in bacterial motility and adherence. Phenotypic susceptibility testing indicated broad susceptibility to multiple cephalosporins and aminoglycosides, with resistance to several penicillins (ampicillin, piperacillin, oxacillin), tetracycline, and fluoroquinolones (norfloxacin, ciprofloxacin). Comprehensive Antibiotic Resistance Database (CARD) analysis predicted multiple antibiotic resistance determinants, including tet(E), KPC-27, cphA3, rsmA, OXA-912, qnrS2, and EF-Tu. Importantly, we report the first complete whole-genome sequence of an A. veronii strain isolated during a mortality event affecting M. rosenbergii. These findings confirm MRG1 as a pathogenic isolate associated with the mortality event. The complete genome sequence provides a valuable reference for future comparative genomic studies of virulence and resistance variation among A. veronii strains infecting M. rosenbergii.