RESUMO
While variable production of the biosurfactant, rhamnolipid, by Pseudomonas aeruginosa has been shown to be dependent on growth conditions, no research has evaluated potential relationships between rhamnolipid production and the presence of heavy metals. The current investigation evaluates the influence of Cd(2+) on rhamnolipid synthesis. Cultures grown in the presence of 0.45 and 0.89 mM Cd(2+) were monitored for rhlB/rhlC expression, rhamnolipid yield, and the ratio of monorhamnolipid (RL1) and dirhamnolipid (RL2) produced. Results show a Cd-induced enhancement of rhlB expression in mid-stationary phase (53 h). In addition, sustained production of rhamnolipid through late stationary growth phase (96 h) was observed for Cd-amended cultures, unlike Cd-free control cultures that ceased rhamnolipid production by mid-stationary growth phase. Most significant was an observed increase in the ratio of RL2 to RL1 congeners produced by cultures grown in the presence of Cd(2+). Previous results have shown that the complexation constant for RL2-Cd is several orders of magnitude larger than that of RL1-Cd thus the preferential production of RL2 in the presence of Cd(2+) impacts its bioavailability and toxicity both for the cell and in the surrounding environment.
