Potential Antibacterial Use of Selected Seaweed Species Against Aquaculture Pathogens

Pathogens in the aquaculture industry have threatened commercially important fish species, leading to significant losses, and conventional drugs used to combat these pathogens are usually expensive. Plants are good medicinal alternatives as they contain bioactive compounds. Seaweeds Ulva spp. and Padina spp. are abundant at the intertidal zones of Nangaramoan Beach in Sta. Ana, Cagayan, Philippines and have shown potential inhibitory properties against pathogenic bacteria. Despite their abundance, these remain understudied. This study aimed 1) to evaluate the presence of bioactive compounds in the methanolic and ethanolic extracts from Ulva spp. and Padina spp., and 2) to assess their effectiveness in inhibiting the growth of selected pathogens at various concentrations: 1.0 mg/mL, 0.75 mg/mL, and 0.5 mg/mL. The phytochemical screening of Ulva spp. and Padina spp. utilized qualitative test tube screening methods. The methanolic and ethanolic extracts of both seaweeds were subjected to the paper disc agar diffusion method against two aquaculture pathogens, namely Escherichia coli and Staphylococcus aureus, to assess the antibacterial activities with Amoxicillin (Benedex) and distilled water as the positive and negative controls, respectively. The phytochemical analysis revealed that Ulva spp. methanolic extract contains flavonoids, while its ethanolic extract contains flavonoids and tannins similar to Padina spp. methanolic extract. The ethanolic extract of Padina spp. contains all three bioactive compounds: flavonoids, saponins, and tannins. In addition, the antimicrobial screening revealed that S. aureus is the most susceptible pathogen to both extracts due to factors, such as bacteria type and dosage dependency. While the results showed that inhibitory activity is significantly lower than the positive control (p>0.05), the study revealed that Ulva spp. and Padina spp. have promising antibacterial properties. These extracts are, therefore, recommended to be tested against other aquaculture pathogens and further explored for possible drug formulation.