Osmotic stress and phosphate limitation alter production of cell-to-cell signal molecules and rhamnolipid biosurfactant by Pseudomonas aeruginosa

In Pseudomonas aeruginosa, rhamnolipid production is controlled by the quorum-sensing system RhlRI, which itself depends on LasRI. These systems use cell-to-cell signal molecules: N-butyryl- l-homoserine lactone (C 4–HSL) and N-(3-oxododecanoyl)- l-homoserine lactone (3OC 12–HSL), respectively. Whereas both HSLs were produced in M63 medium, rhamnolipid synthesis was not achieved. Phosphate limitation reduced the HSL concentrations, while allowing rhamnolipid production. Hyperosmotic shock applied during the exponential growth phase stopped the accumulation of 3OC 12–HSL, and prevented C 4–HSL and rhamnolipid production. These defects result from lower expression of genes involved in C 4–HSL and rhamnolipid syntheses. The osmoprotectant glycine betaine partially restored C 4–HSL and rhamnolipid production.