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.

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.

Plant-based protein is being sought after as a substitute for fish meals (powdered fish) in tilapia feeds. This is to promote sustainable aquaculture, as fish meals contribute to the dwindling marine fish catch. Amaranthus spinosus is an edible weed that shows potential to improve the growth and immunity of Nile tilapia. However, most studies only consider the survivability of fish to evaluate the benefit of using plant-based feeds and do not necessarily elucidate whether a pathogen is affected in vivo. A. spinosus leaf meals (ASLMs) were used to determine effectiveness against Aeromonas hydrophila (BIOTECH 10089) injected intraperitoneally into Nile tilapia. Formulated feeds with fish meals substituted with 50 % (ASLM50) and 75 % (ASLM75) A. spinosus leaves were fed to Nile tilapia challenged with A. hydrophila . Then spleen and kidney tissue were collected and analysed 10 days post-injection for total plate count. The fish fed with ASLM50 appeared healthier than those fed with ASLM75 and those fed with control feeds. Fish fed with ASLMs had lower A. hydrophila counts (P=0.03). Phytochemical screening and antimicrobial activity determination for crude methanolic A. spinosus leaf (ASL) and ASLMs were also conducted to enhance the in vivo results. The metabolites present in the extracts were carbohydrates, amino acids and proteins, cardiac glycosides, saponins and terpenoids. The ASL and ASLM extracts had antimicrobial activity (MIC=115 mg ml−1). Overall, the study showed that ASLMs can make tilapia more resilient against A. hydrophila infections. Fish meal substitution was best at 50 %. Higher substitution had unwanted effects (more bacterial counts), possibly due to antinutritional factors.