Antimicrobial Activity of Crude Extracts from Ascophyllum nodosum Obtained by Microwave-Assisted Extraction

Ascophyllum nodosum (Linnaeus) Le Jolis is a brown alga from the Fucaceae family and a unique species from the Ascophyllum genus. This brown alga is an edible macroalga from the North Atlantic Ocean, commonly found on the European north-western coast. High-value bioactive molecules such as pigments, polyphenols, and phlorotannin were found in the macroalgae composition, which makes this alga particularly interesting for exploring potential biological activities. Among sustainable extraction technologies, microwave-assisted extraction (MAE) has many advantages such as a short extraction time and fewer solvent requirements. On the other hand, ethanol and water are eco-friendly solvents that have already been proven to be effective for obtaining bioactive compounds with antimicrobial capacity. Therefore, in this work, analytical conditions of MAE: t = 5 min; pressure = 10.5 bar; ethanol concentration (37%) as solvent were applied to obtain a polyphenol-rich extract from A. nodosum. The antimicrobial effect of the resulting extract against five food-borne microorganisms (Bacillus cereus, Escherichia coli, Salmonella enteritidis, Pseudomonas aeruginosa, Staphylococcus aureus), and the opportunistic bacteria Staphylococcus epidermidis was assessed. The antimicrobial activity was performed through the Kirby–Bauer disk diffusion susceptibility test protocol and the microdilution method. The analytical results indicated that the A. nodosum extract was effective against all tested bacteria except for Escherichia coli. The highest antimicrobial activity was found against Staphylococcus aureus, presenting inhibitory capacity with a concentration of 400 µg/mL and an inhibition halo of 11.79 ± 1.92 mm.