Fig 3. The pH of the growth medium affects the synergistic anti-biofilm activity of PMZ in combination with tobramycin.

P. aeruginosa static biofilms grown in 96-well plates were treated with increasing concentrations of tobramycin (TOB) with or without the addition of 100 μM promethazine (PMZ) in buffered LB. The pH of the medium was buffered to 7.2 (a) 5.5 (b) or 8.5 (c), respectively. Colony forming units (CFU) were determined from the tobramycin treated (dark grey) and tobramycin plus PMZ treated (light grey) biofilms. Isopropanol treatment (IPA) was used as a killing control. Mean CFU counts were performed on pools of three wells in at least three independent experiments. Error bars represent the standard error of the mean, while the dashed line indicates the lower limit of detection of the assay.