Antimicrobial and quorum sensing inhibitory activity of epiphytic bacteria isolated from the red alga Halymenia durvillei

Abstract

Halymenia durvillei is a red alga that is commonly utilized in the Philippines as food and as a source of high value natural products for industrial applications. 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. 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 antibacterial effects. EPB9, identified as B. safensis, consistently showed the strongest 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 of H. durvillei and its potential as an abundant resource for new antibacterial and QSI bioactives.