Effect of a Pseudomonas rhamnolipid biosurfactant on cell hydrophobicity and biodegradation of octadecane

In this study, the effect of a purified rhamnolipid biosurfactant on the hydrophobicity of octadecane-degrading cells was investigated to determine whether differences in rates of octadecane biodegradation resulting from the addition of rhamnolipid to four strains of Pseudomonas aeruginosa could be related to measured differences in hydrophobicity. Cell hydrophobicity was determined by a modified bacterial adherence to hydrocarbon (BATH) assay. Bacterial adherence to hydrocarbon quantitates the preference of cell surfaces for the aqueous phase or the aqueous-hexadecane interface in a two- phase system of water and hexadecane. On the basis of octadecane biodegradation in the absence of rhamnolipid, the four bacterial strains were divided into two groups: the fast degraders (ATCC 15442 and ATCC 27853), which had high cell hydrophobicities (74 and 55% adherence to hexadecane, respectively), and the slow degraders (ATCC 9027 and NRRL 3198), which had low cell hydrophobicities (27 and 40%, respectively). Although in all cases rhamnolipid increased the aqueous dispersion of octadecane at least 10⁴- fold, at low rhamnolipid concentrations (0.6 mM), biodegradation by all four strains was initially inhibited for at least 100 h relative to controls. At high rhamnolipid concentrations (6 mM), biodegradation by the fast degraders was slightly inhibited relative to controls, but the biodegradation by the slow degraders was enhanced relative to controls. Measurement of cell hydrophobicity showed that rhamnolipids increased the cell hydrophobicity of the slow degraders but had no effect on the cell hydrophobicity of the fast degraders. The rate at which the cells became hydrophobic was found to depend on the rhamnolipid concentration and was directly related to the rate of octadecane biodegradation. These results suggest that the bioavailability of octadecane in the presence of rhamnolipid is controlled by both aqueous dispersion of octadecane and cell hydrophobicity.