Cell surface hydrophobicity of Pseudomonas aeruginosa: Effects of monorhamnolipid and substrate on fatty acid and lipopolysaccharide content

Abstract

Cell surface hydrophobicity is a biosurfactant inducible parameter associated with increased biodegradation rates of hydrocarbons. Little is known about the types of physiological changes that are induced by a biosurfactant to change cell surface hydrophobicity. The objective of this study was to investigate the rhamnolipid-induced chemical and structural changes that cause the increase in cell surface hydrophobicity of two P. aeruginosa strains; P. aeruginosa ATCC 27853 and P. aeruginosa ATCC 9027. Both fatty acid and lipopolysaccharide content of cells were measured during growth on a soluble substrate; glucose, and a slightly soluble substrate; hexadecane in the presence and absence of monorhamnolipid. Cell surface hydrophobicity is a dynamic surface property that changes depending on strain, growth stage, substrate, and rhamnolipid addition. Results showed a general decline in the readily extractable lipid content that was correlated with increase in cell surface hydrophobicity. This decline took place only when growing cultures was supplied with rhamnolipid. In addition, rhamnolipid treatment caused a partial release of lipopolysaccharides (LPS) from the cells. This was indicated by KDO analysis and by SDS-PAGE analysis of LPS from culture supernatant. Also, LPS release from both strains was rhamnolipid concentration-dependent. Rates of LPS release from suspensions prepared from cells of both strains were highest at low rhamnolipid concentrations. Although increase in cell surface hydrophobicity was associated with LPS release, the amount of LPS released did not correlate with cell surface hydrophobicity. Rather the amount of LPS release was strain dependent. Cell surface ultrastructure revealed by scanning electron microscopy showed that the cells studied have a naturally rough surface. Cells grown in the presence of rhamnolipid had a smooth surface indicating a loss of the LPS from the outer membrane. Cells grown on hexadecane in the presence of rhamnolipid had deep pits on the cell surface which may act as hydrophobic sites that allow increased hexadecane absorption. These data suggest that biosurfactant addition caused LPS loss resulting in development of cell surface hydrophobicity.