Diversity, virulence factors and antibiotic resistance profiles of Aeromonas species isolated from farmed fish in Belarus

Aeromonas species are regarded as significant opportunistic pathogens of fish, causing notable economic losses to aquaculture. The present study aimed to investigate the diversity of aeromonads isolated from farmed fish in Belarus, along with their virulence and antibiotic resistance profiles. A total of 40 isolates of Aeromonas spp. were recovered from diseased fish from 9 fish farms and 12 recreational reservoirs. Based on 16S rRNA and cnp60 genes sequencing the majority (37.5%) of the isolates were identified as A. veronii , followed by A. sobria (15%), A. salmonicida (10%), A. rivipollensis (10%), A. piscicola (10%), A. media (7.5%), A. allosaccharophila (7.5%), and A. bestiarum (2.5%). Motility was observed in 90% of isolates, including A. salmonicida strains, and 70% of them formed capsules. All Aeromonas strains exhibited proteolytic and lipolytic activities, while hemolytic activity was detected in 75% of them. The prevalent toxin-coding gene was alt, encoding cytotonic heat-labile enterotoxin, detected in 92% of isolates. The genes that encode aerolysin, aerolysin-related cytotoxic enterotoxin, and hemolysin were identified in 75%, 75% and 25% of isolates, respectively. Conversely, ast gene, encoding heat-stable cytotonic toxin, was identified in 15% of the strains. The results of in vivo pathogenicity testing confirmed that isolated aeromonads could cause fish diseases. Furthermore, Aeromonas strains were resistant to ampicillin (100%), amoxicillin (100%), imipenem (80%), kanamycin (45%), tetracycline (42.5%), trimethoprim/sulfamethoxazole (22.5%), gentamicin (20%), norfloxacin (15%), and ceftriaxone (12.5%), with 42.5% of strains demonstrated multidrug resistance. Collectively, these findings contribute to the advancement of knowledge concerning Aeromonas infections in aquaculture. • Diverse Aeromonas species were isolated from diseased farmed fish in Belarus • All isolates carried toxin-coding genes and produced extracellular enzymes • Number of toxin-coding genes not always an indicator of virulence in aeromonads • All isolates were resistant to beta-lactams and 42.5% were multidrug-resistant • Antibiotic resistance profiling of aeromonads could help prevent misuse of antibiotics