Screening for biosurfactant activity. PAH-utilizing bacteria were first screened for the ability to produce surfactants by cultivation in liquid BH supplemented with 2% dextrose. Many Pseudomonas strains are known to produce biosurfactants
on dextrose, glycerol, or mannitol. The drop-collapsing test was used for rapid determination of surface tension lowering. The emulsifying activity of culture supernatants was estimated by adding 0.5 ml of sample fluid and 0.5 ml of hexadecane to 4.0 ml of distilled water. The tube was vortexed for 10 s, held stationary for 1 min, and then visually examined for turbidity of a stable emulsion.Blue agar plates containing cetyltrimethylammonium bromide (CTAB) (0.2mg/ml; Sigma Chemical Co., St. Louis, Mo.) and methylene blue (5 mg/ml) were used to detect extracellular glycolipid production with 2% dextrose or mannitol as a substrate. Biosurfactants were observed by the formation of dark blue halos around the colonies.