Figure 9.

Our data suggest that Pseudomonas aeruginosa regulates gene expression to secrete rhamnolipid biosurfactants only when carbon source is in excess – a mechanism we call ‘metabolic prudence’. (A) Bacteria can grow and divide if both carbon-and nitrogen-source are available. In casamino acids media, carbon is initially in excess relative to nitrogen, and therefore bacteria grow exponentially until the nitrogen becomes limiting. Subsequently, cells express the biosurfactant synthesis genes and use the excess carbon at a low cost to their fitness (unlike cell biomass, rhamnolipid bisurfactants contain no nitrogen in their composition). (B) The enzyme RhlA plays a central role in biosurfactant synthesis. This enzyme is under the genetic regulation of rhlAB, the expression of which requires both quorum sensing and adequate nutrient conditions.