Isolation and characterization of gill mucus-associated antagonistic bacteria in the Asian swamp eel (<i>Monopterus albus</i>)

This study investigated the gill mucus of Asian swamp eel (Monopterus albus) for the presence of antagonistic bacteria against known aquatic pathogens, Vibrio harveyi and Aeromonas hydrophila. Initial screening of 500 bacterial isolates via the spot-on-lawn and co-incubation assays, identified five with significant antagonistic activity, which were further subjected to morphological, enzymatic, biochemical, and molecular characterization. All five isolates were Gram-negative bacilli. Four isolates were identified as belonging to the Pseudomonas genus, a known probiont with documented biocontrol properties in various plant and animal species. The remaining isolate exhibited high 16S rRNA gene sequence similarity to Aeromonas dhakensis, a known human pathogen. All isolates demonstrated the ability to produce at least three of six tested extracellular enzymes: catalase, amylase, protease, lipase, gelatinase, and urease. Given the observed antagonistic activity of these isolates, further research is warranted to evaluate their potential application as probiotics. This study represents the first investigation of antagonistic bacteria in swamp eel gill mucus and contributes to the limited research on fish gill mucus as a source of such bacteria.