Locations from which samples were taken are listed in Table 1. 1 g of sample material (soil or water) was suspended in 10 ml of saline (0.9% NaCl) and subjected to three cycles of 30 s vigorous vortexing and 30 s sonication. After briefly spinning down the sample at 3000 rpm to remove larger particles and debris, 2 ml of the resulting supernatant was centrifuged for 4 min at 4500 rpm. Subsequently, pellets were resuspended in 5 ml saline and 250 μl of this inoculum suspension was added to the wells of a 24-well microtitre plate (MTP). Subsequently, 200 μl of a minimal medium was added, together with 50 μl of a mixture of AHLs (final concentrations: 2 mMN-(3-oxo-hexanoyl)-L-homoserine lactone (3OC6HSL) and 1 mM N-(3-oxo-dodecanoyl)-L-homoserine lactone (3OC12HSL); Sigma Aldrich, Bornem, Belgium).