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Abstract

Seaweed-associated bacteria participate in complex interactions necessary for the health and defense of their host. It involves the production of antifouling metabolites and antimicrobials to prevent secondary colonization of pathogenic epibiota. Halymenia durvillei, commonly known as “red sea lettuce,” is a red alga ubiquitously distributed in the Indo-Pacific region. It is cultivated in the Philippines due to its abundance in high-value pigments such as phycoerythrin and r-phycocyanin. However, there are no studies regarding the microbial community associated with H. durvillei and its potential applications. This study aimed to isolate and identify the epiphytic bacteria of H. durvillei and determine their antimicrobial and quorum-sensing inhibitory (QSI) effects. The thalli of H. durvillei were collected at the shores of Santa Fe, Bantayan, Cebu, Philippines. The bacterial isolates were identified using 16S rRNA, and their ethyl acetate (EtOAc) extracts were subjected to antimicrobial susceptibility tests against representative species of yeast, gram-negative, and gram-positive bacteria. Their QSI activity against Chromobacterium violaceum was also determined. Fourteen distinct bacterial colonies belonging to four genera, namely, Alteromonas (3), Bacillus (5), Oceanobacillus (1), and Vibrio (5) were successfully isolated and identified. All fourteen bacterial isolates exhibited antibiotic effects against gram-negative and gram-positive bacteria. In particular, EPB9, identified as B. safensis, showed consistently strong potent inhibition against Escherichia coli, Staphylococcus aureus, and S. epidermidis having minimum inhibitory concentrations (MIC) ranging from 0.0625 – 1.0 mg mL-1. In contrast, all fourteen isolates showed weak antifungal effects. Both B. safensis (EPB 9) and B. australimaris (EPB15) exhibited QSI effects at 100 mg mL-1 showing opaque zones of 3.1 ± 0.9 mm and 3.8 ± 0.4 mm, respectively. This study is the first to isolate and identify the distinct microbial epiphytic bacterial community in H. durvillei and show that they can be used as an abundant resource for new antibiotics and QSI bioactive.