Characterization of Antagonistic Bacteria from Gill Mucus of Marine Cage-Cultured Milkfish (Chanos chanos)

With the rise of aquaculture, rearing intensification and irresponsible use of antibiotics have led to chemical pollution, antibiotic resistance, and proliferation of pathogens. The milkfish (Chanos chanos) is one of the economically important aquaculture species in the Philippines threatened by bacterial pathogens, including Vibrio spp., which cause mortalities leading to economic losses. To combat these pathogens, probiotics have been proposed as alternatives since they aid in the growth and health of fish. Hence, this study aimed to determine the bacterial and Vibrio count of the milkfish and to screen for potential probiotics from their gill mucus. Two hundred forty-five (245) isolates were collected from the gills of six milkfish samples and were tested against Vibrio harveyi for antagonistic activity. Out of the 245 isolates, five showed antagonistic activity against V. harveyi through the spot-on-lawn and the cross-streak assays. These isolates were characterized morpho-biochemically and identified through 16S rRNA sequencing. Three isolates were identified to be closely related to either Vibrio alginolyticus or Vibrio neocaledonicus, while the other two were putative Oceanimonas and Marinomonas. Findings of this study might have a potential contribution to further characterizing the probiotic potential of the Oceanimonas and the Marinomonas species as they appear understudied in the area of probiotics research in aquaculture.