The antimicrobial activity of extracts derived from macroalgal species against hospital- and community-acquired methicillin-resistant Staphylococcus aureus (MRSA) isolates

The rise of antimicrobial resistance (AMR) has not coincided with novel antibiotic development, highlighting the necessity for alternative antimicrobial agents. In recent years, there has been growing interest in deriving antimicrobial compounds from macroalgae with hopes to alleviate the burden of AMR. This study aims to evaluate the antimicrobial activity of macroalgal extracts against a range of methicillin-resistant Staphylococcus aureus (MRSA) clinical isolates. Thirteen macroalgae species underwent sequential solid–liquid extraction using organic solvents of increasing polarity. The minimum inhibitory concentration (MIC) of 65 crude extracts was determined via a broth microdilution assay. Extracts were analysed for brominated compounds and their respective fatty‑acid profiles using GC-MS, while statistical analyses explored correlations between extract polarity, extract composition and antimicrobial activity. Of the extracts tested, n = 22 showed antimicrobial activity (MICs ≤ 1000 µg ml‒1) across all tested isolates. Hexane and dichloromethane (DCM) extracts were the most effective (p < 0.001), with Asparagopsis armata and Alaria esculenta hexane extracts exhibiting the lowest MICs (15.63–125 µg ml‒1). GC-MS analysis identified the presence of palmitoleic acid (C16:1n9) in both species, while brominated compounds were identified exclusively in A. armata and Asparagopsis Taxiformis. Increased abundance of palmitoleic acid was significantly associated with reduced MIC values (Spearman’s r = −0.6, p < 0.001), supporting a potential role for this fatty acid in the antimicrobial activity of the extracts. In conclusion, this study has identified the influence of extract type and composition on the antimicrobial activity of macroalgae against several clinical MRSA isolates. The work outlined may serve as a bioactivity guide for future studies.