Rhamnolipid-Induced Removal of Lipopolysaccharide from Pseudomonas aeruginosa : Effect on Cell Surface Properties and Interaction with Hydrophobic Substrates

Abstract

Little is known about the interaction of biosurfactants with bacterial cells. Recent work in the area of biodegradation suggests that there are two mechanisms by which biosurfactants enhance the biodegradation of slightly soluble organic compounds. First, biosurfactants can solubilize hydrophobic compounds within micelle structures, effectively increasing the apparent aqueous solubility of the organic compound and its availability for uptake by a cell. Second, biosurfactants can cause the cell surface to become more hydrophobic, thereby increasing the association of the cell with the slightly soluble substrate. Since the second mechanism requires very low levels of added biosurfactant, it is the more intriguing of the two mechanisms from the perspective of enhancing the biodegradation process. This is because, in practical terms, addition of low levels of biosurfactants will be more cost-effective for bioremediation. To successfully optimize the use of biosurfactants in the bioremediation process, their effect on cell surfaces must be understood. We report here that rhamnolipid biosurfactant causes the cell surface of Pseudomonas spp. to become hydrophobic through release of lipopolysaccharide (LPS). In this study, two Pseudomonas aeruginosa strains were grown on glucose and hexadecane to investigate the chemical and structural changes that occur in the presence of a rhamnolipid biosurfactant. Results showed that rhamnolipids caused an overall loss in cellular fatty acid content. Loss of fatty acids was due to release of LPS from the outer membrane, as demonstrated by 2-keto-3-deoxyoctonic acid and sodium dodecyl sulfate-polyacrylamide gel electrophoresis analysis and further confirmed by scanning electron microscopy. The amount of LPS loss was found to be dependent on rhamnolipid concentration, but significant loss occurred even at concentrations less than the critical micelle concentration. We conclude that rhamnolipid-induced LPS release is the probable mechanism of enhanced cell surface hydrophobicity.Cell surface properties result from the unique chemical structure of the cell surface. In the case of a gram-negative bacterium such as Pseudomonas aeruginosa, this structure is the outer membrane, the outer leaflet of which is primarily composed of lipopolysaccharide (LPS). The outer leaflet contains LPS molecules which are composed of three components (25). The first is the lipid A tail which is anchored into the hydrophobic region of the outer membrane. The second is the core oligosaccharide, which contains a unique eight-carbon sugar called 2-keto-3-deoxyoctonic acid (KDO). The core oligosaccharide is connected to the lipid A tail through its reducing end and is positioned at the surface of the membrane in a manner analogous to the glycerol-phosphate head group of a phospholipid. The core oligosaccharide is negatively charged, and the association of adjacent LPS molecules is stabilized by Mg 2ϩ ions at the membrane surface. The third component of LPS is the O antigen which consists of 15 to 20 repeating mon...

Biodegradation of n-hexadecane by bacterial strains B1 and B2 isolated from petroleum-contaminated soil

“…Some investigators have reported that changes in the carbon source can affect the cell wall, thus affecting cell surface hydrophobicity [40,41]. The surface hydrophobicity of some Pseudomonas bacteria has been shown to decrease when the bacteria are grown on carbon sources such as glucose and acetate [42,43].…”

Biofilm exopolysaccharides: a strong and sticky framework

“…Al-Tahhan et al (2000) pointed out that even very low levels of a rhamnolipid biosurfactant could render the cell surface more hydrophobic, causing loss of LPS in the process. It has also been suggested that biosurfactants might be involved in the horizontal transfer of exopolymer from one bacterial species to another (Osterreicher-Ravid et al, 2000).…”

Quorum sensing: implications on Rhamnolipid biosurfactant production

“…The permeability barrier property is largely caused by the presence of the LPS layer. Increased cell permeability induced by rhamnolipid biosurfactant was most likely caused by the release of LPS from the outer membrane (Al-Tahhan et al, 2000 Rhamnolipids have been used as emulsifying agents for the transport of drugs to the site of action Rhamnolipids in combination with the antibiotic, azithromycin facilitated destruction of the bacterial cells by increasing the bacterial membrane permeability . P aeruginosa rhamnolipid mixture was found to inhibit a majority of pathogenic bacteria such as A. faecalis, E. coli, Micrococcus luteus, Mycobacterium phlei, Serratia marcescens and S. epidermidis.…”

“…The permeability barrier property is largely caused by the presence of the LPS layer. Increased cell permeability induced by rhamnolipid biosurfactant was most likely caused by the release of LPS from the outer membrane (Al-Tahhan et al, 2000. Rhamnolipids have been used as emulsifying agents for the transport of drugs to the site of action.…”

“…to facilitate hydrocarbon utilization. Interestingly, one study showed that the P aeruginosa outer membrane was removed in the presence of rhamnolipids, such that the hydrophobic membrane interior could bind directly to lipids (Al-Tahhan et al, 2000).…”