Extracts of macroalgae from the Brazilian coast inhibit bacterial quorum sensing

The ability of polar (1:1 water/methanol) and non-polar (dichloromethane) extracts of macroalgae from Arraial do Cabo, RJ, Brazil, to inhibit quorum sensing (QS) was investigated. Before extraction, half of the algae were treated with 30% ethanol to kill and remove surface-associated bacteria. Twenty of 22 polar extracts inhibited the QS of the acyl homoserine lactone (AHL) producer and reporter Chromobacterium violaceum CV017 with minimal inhibitory concentrations (MIC) ranging from 0.28 μg ml^-1 (Ulva fasciata) to 189 μg ml^-1 (Codium sp.). The MIC of non-polar extracts varied from 69 μg ml^-1 (Sargassum furcatum) to 2730 μg ml^-1 (Peyssonnelia capensis). Usually, extracts from algae with associated bacteria had a higher bioactivity than ones without them. Among 11 algal species tested, 50% of extracts with associated bacteria were toxic to C. violaceum CV026. None of the extracts contained AHLs, which was shown using reporters C. violaceum CV026 and Agrobacterium tumefaciens NTL4 (pZLR4). Polar (11%) and non-polar extracts (2%) inhibited QS-dependent attachment of Pseudomonas aeruginosa PA01 at tissue-level concentrations. Our data suggest that secondary metabolites from algae and their associated microbiota are an important source of QS inhibitors, which potentially can be used in future biotechnological applications.