Potential Antibacterial Use of Selected Seaweed Species Against Aquaculture Pathogens

Abstract

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, 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.