Figure 6.

The expression of genes for biosurfactant synthesis is growth-rate dependent and is triggered by nitrogen limitation. (A) Increasing nutrients (casamino acids, the sole carbon and nitrogen source in the medium) increases growth, showing that entry into the stationary phase is due to nutrient depletion. (B) rhlAB expression data measured at all nutrient levels collapse to a single curve when plotted against growth rate - but not OD600 (inset panel) - revealing that expression is a function of growth rate, not cell density per se. (C) Complementing media with a nitrogen source (ammonium sulfate) has the same effect as increasing casamino acids, showing that growth was originally limited by nitrogen in the standard medium. (D) 2-D matrices of OD, GFP expression and GFP/OD measured at 24 h for a range of carbon (glycerol) and nitrogen (ammonium sulfate) source levels. These data show that rhlAB expression is favored at lower nitrogen/carbon ratios, when compared to the conditions producing optimal cell growth (highest OD).