FIG. 2.

Rhamnolipids increase PQS solubility in aqueous solutions. Synthetic PQS (5 μg) and increasing amounts of rhamnolipids were dried in 13-ml polystyrene tubes. Distilled water (circles), phosphate-buffered saline (triangles), or PTSB (squares) (0.5 ml of each) was then added to produce the indicated concentrations of rhamnolipid, and the mixture was incubated at 37°C with vigorous shaking (250 rpm) for 2 h. Aliquots removed from each tube were then organically extracted and analyzed as described in Materials and Methods. The amount of PQS in solution in each mixture was calculated and is presented as the mean ± standard deviation from three separate experiments. The insert is a representative TLC plate photographed under UV light. Numbers within the inset indicate rhamnolipid concentration, and the arrowhead indicates PQS. The PQS solubility assay was performed as in Fig. 1, and the photograph is included to visually show the effects of lower rhamnolipid concentrations on PQS solubility in distilled water.