To quantify the resulting biofilm biomass, the washed biofilms were stained with 200 μl of a 0.1% crystal violet solution (1:4 dilution in DDW of the Gram's crystal violet solution, Merck) for 20 min at room temperature [24 (link)]. Thereafter the biofilms were washed twice with DDW and the stain dissolved in 200 μl of a 33% acetic acid solution. The OD at 595 nm was measured spectrophotometrically using the M200 Tecan microplate reader. Different dilutions of the test compounds in the absence of bacteria were used to measure background signals.
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