Padina pavonica (Pp) is a common brown alga worldwide that has been reported to harbour bioactive substances with diverse potential applications in biotechnology. Among other, extracts of Pp brown alga have been reported with antibacterial, antifungal, anti-inflammatory, cytotoxic, and algicidal activity. However, these findings were shown to vary by sampling locations, seasons of the year, solvents and extraction protocols used.
This study investigated antibacterial, antifungal, and embryotoxic activity of Pp from the Adriatic Sea as no reported study included this location. Algal samples were taken near Zadar, Croatia in summer 2016, were triple washed with filtered seawater, single washed with filtered freshwater, and dried in shade for 10 days. Dried samples were pulverised and suspended in 96% ethanol, sonicated for 15 minutes, and filtered through a sintered glass funnel. Subsequently, the same sonication and filter procedures were applied using acetone and chloroform. The residual material was then extracted with 0.1 M HCl (60 °C, 2 hours), centrifuged at 5000 rpm for 30 minutes, and the supernatant precipitated with ethanol (overnight, 2-8 °C). The obtained crude extract was dissolved in both MiliQ water and DMSO. The antibacterial and antifungal susceptibility testing were performed according to CLSI guidelines and acute fish embryotoxicity testing according to the OECD guidelines. Depending on the assay and solvent used, extracts were tested in concentrations between 2560 and 2.5 µg/mL and results obtained by visual and spectrophotometric readings.
By visual reading of results in the tested range between 2560 and 5 µg/mL, no complete inhibition of growth was observed using the aqueous solution of Pp extract with any microorganism tested except for Photobacterium damselae subsp. piscicida that returned MIC value of 1000 µg/mL. Dose-response modelling of the spectrophotometric data resulted in EC50 value of 598 µg/mL (95% CI 252-944) using the four-parameter log-logistic Hill model. Furthermore, using the spectrophotometric data partial bacterial inhibition was observed in testing of aqueous solution of Pp extract with Escherichia coli, Pseudomonas aeruginosa, and Photobacterium damselae subsp. damselae but no inhibition of growth of Salmonella typhimurium, Staphylococcus aureus, Vibrio anguillarum, and of Saccharomyces cerevisae yeast. Pp extract dissolved in DMSO did not inhibit growth of any microorganism in the tested range between 1280 and 2.5 µg/mL. No acute toxicity to embryos of Danio rerio zebrafish was observed in the tested range between 2500 and 78 µg/mL. Crude extracts of Pp from the Adriatic Sea showed antibacterial activity that will be further investigated upon fractionation of the extract and isolation of single compounds.