Effects of rhamnolipids from pseudomonas aeruginosa DS10-129 on luminescent bacteria: Toxicity and modulation of cadmium bioavailability

In this study, the mixture of mono- and di-rhamnolipids produced by Pseudomonas aeruginosa DS10-129 was characterized for its toxicity and modulatory effects on Cd availability to different bacteria. Gram-negative naturally bioluminescent Vibrio fischeri and recombinant bioluminescent Pseudomonas fluorescens, P. aeruginosa, Escherichia coli, and Gram-positive Bacillus subtilis were used as model organisms. Rhamnolipids reduced the bioluminescence of these bacteria in less than a second of exposure even in relatively low concentrations (30-min EC₅₀ 45-167 mg l^-1). Toxicity of Cd to Gram-negative bacteria (30-min EC₅₀ values 0. 16 mg l^-1 for E. coli, 0. 96 mg l^-1 for P. fluorescens, and 4. 4 mg l^-1 for V. fischeri) was remarkably (up to 10-fold) reduced in the presence of 50 mg l^-1 rhamnolipids. Interestingly, the toxicity of Cd to Gram-positive B. subtilis (30-min EC₅₀ value 0. 49 mg l^-1) was not affected by rhamnolipids. Rhamnolipids had an effect on desorption of Cd from soil: 40 mg l^-1 rhamnolipids increased the water-extracted fraction of Cd twice compared with untreated control. However, this additionally desorbed fraction of Cd remained bound with rhamnolipids and was not available to bacteria. Hence, in carefully chosen concentrations (still effectively complexing heavy metals but not yet toxic to soil bacteria), rhamnolipids could be applied in remediation of polluted areas.