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Aeromonads are ubiquitous in aquatic environments and have been implicated in fish and human infections. In this study, we isolated, studied antimicrobial susceptibility patterns and screened the existence of 15 virulence genes in aeromonads from two famously consumed fish species—seven marine Tiger Grouper (Epinephelus fuscoguttatus) and eight freshwater Marble Goby (Oxyeleotris marmoratus) from the aquaculture hatchery in Sabah, Malaysia. A total of 30 aeromonads (17 A. caviae, 9 A. rivuli, 4 A. dhakensis) were identified using PCR targeting GCAT gene, rpoD‐restriction fragment length polymorphism and multi‐locus phylogenetic analysis. All 30 strains were resistant to amoxicillin and cephalothin and five strains were multidrug‐resistant. Nine virulence genes (lip, ela, eno, fla, aerA, hylA, dam, alt and ser) present in A. dhakensis, suggesting the virulence potential of this species as a fish pathogen. This study offers as a baseline for future studies in monitoring and managing these two fish in aquaculture industry. [ABSTRACT FROM AUTHOR]

Aeromonas dhakensis possesses dual flagellar systems for motility under different environments. Flagella-mediated motility is necessary for biofilm formation through an initial attachment of bacteria to the surface, but this has not been elucidated in A. dhakensis. This study investigates the role of polar (flaH , maf1) and lateral (lafB , lafK and lafS) flagellar genes in the biofilm formation of a clinical A. dhakensis strain WT187 isolated from burn wound infection. Five deletion mutants and corresponding complemented strains were constructed using pDM4 and pBAD33 vectors, respectively, and analyzed for motility and biofilm formation using crystal violet staining and real-time impedance-based assays. All mutants were significantly reduced in swimming (p < 0.0001), swarming (p < 0.0001) and biofilm formation using crystal violet assay (p < 0.05). Real-time impedance-based analysis revealed WT187 biofilm was formed between 6 to 21 h, consisting of early (6–10 h), middle (11–18 h), and late (19–21 h) stages. The highest cell index of 0.0746 was recorded at 22–23 h and biofilms began to disperse starting from 24 h. Mutants Δ maf1 , Δ lafB , Δ lafK and Δ lafS exhibited reduced cell index values at 6–48 h when compared to WT187 which indicates less biofilm formation. Two complemented strains c maf1 and c lafB exhibited full restoration to wild-type level in swimming, swarming, and biofilm formation using crystal violet assay, hence suggesting that both maf1 and lafB genes are involved in biofilm formation through flagella-mediated motility and surface attachment. Our study shows the role of flagella in A. dhakensis biofilm formation warrants further investigations. • Flagella-mediated motility involved in Aeromonas dhakensis biofilm formation. • Polar Δ flaH and Δ maf1 were reduced 64–67% in swimming. • Lateral Δ lafB , Δ lafK and Δ lafS were abolished 85–86% in swarming. • Biofilm formation of all mutants was reduced 21–40% using crystal violet assay. • RTCA: maf1 and lafB genes involved in A. dhakensis biofilm formation. [ABSTRACT FROM AUTHOR]