Macroalgal extracts as an alternative for the control of Paenibacillus larvae

The present study evaluated the potential use of methanolic extracts from two seaweeds from the Argentine Patagonian coasts, Macrocystis pyrifera and Undaria pinnatifida, as natural alternatives against Paenibacillus larvae. We carried out a detailed characterization of both seaweeds, comparing vegetative and reproductive thallus portions. We also assessed their phenolic content and antioxidant activity and evaluated the in vitro antibacterial action. Based on the minimal inhibitory concentration results, extracts from sporophyll of Undaria (US) and basal thallus portion of Macrocystis (MB) are strong inhibitors. US showed the best activity against P. larvae strain CMM(B)35 (12.2 μg/mL), and MB showed the same activity for strains CMM(B)35 and MAR1 (18.6 μg/mL). Antioxidant activity was higher in US, followed by MB, and a positive correlation was found with phenolic content. Taken together, our results indicate the role of specific chemical compounds present in algal extracts on antioxidant properties and antibacterial activity against P. larvae. We conclude that these seaweeds are promising alternatives against P. larvae.