Adsorption Of Rhamnolipid On Microorganisms And The Effect On Cell Surface Lypohydrophilic Property

In soil remediation, one of the effects of biosurfactants to enhance degradation of hydrophobic organic contaminants is to increase the microbial cell surface hydrophobicity and facilitate the direct contact of microorganisms with the contaminants trapped in soil pores. However, the manner of interaction between biosurfactants and microorganisms as well as the rules of cell surface hydrophobicity change as the function of biosurfactants were not fully understood. In this paper, the adsorption of the rhamnolipid biosurfactant on microorganisms and change of cell surface hydrophobicity due to the adsorption was studied.First the rhamnolipid biosurfactant was produced by aerobic fermentation using a Pseudomonas aeruginosa strain. It was separated from the culture by acid precipitation and purified by column chromatography until monorhamnolipid and dirhamnolipid were obtained. HPLC-MS examination showed that both of the rhamnolipids contained three main components and they were used in the following studies.Then the adsorption of dirhamnolipid and other four synthetic surfactants on Pseudomonas aeruginosa cells was studied, and the adsorption-induced change of cell surface hydrophobicity was investigated. The objective was to find the adsorption rules and reveal the adsorption mechanism considering both adsorbate and adsorbent, as well as to disclose the relation between the adsorption and the change of cell surface hydrophobicity. Result of the experiment showed that the adsorption kinetics of all the surfactants on the Pseudomonas aeruginosa cells followed the second-order law, and the adsorption isotherms of all the surfactants on the bacterium fitted Freundlich equation well. The strength of the adsorption was highly related to the surfactant molecular structure and size. The adsorption mode for the surfactants was probably hydrophilic interaction such as electrostatic attraction, because except for SDS the adsorption totally turned the cell surface to be more hydrophobic.At last, the adsorption of monorhamnolipid and dirhamnolipid on cells of two Pseudomonas aeruginosa strains was studied, and the change of cell surface hydrophobicity by adsorption was also investigated. Result showed that the adsorption capacity of all the cells to monorhamnolipid was much stronger than to dirhamnolipid, indicating that the structure of polar moiety of rhamnolipid has strong effect on the adsorption. Although both bacteria were Pseudomonas aeruginosa, the difference between strains also caused different adsorption manner in that the rhamnolipid-sourced P. aeruginosa cells had weaker adsorption capacity to dirhamnolipid or even released extra dirhamnolipid when aqueous concentration of the surfactant is high enough. The difference in the carbon source to grow the bacteria had little influence on the adsorption. Adsorption of both monorhamnolipid and dirhamnlipid tended to turn the cell surface more hydrophobic and the effect of dirhamnolipid was stronger.The paper disclosed the rules of adsorption of rhamnolipid biosurfactants to microorganisms and demonstrated the effect of adsorption to change cell surface hydrophobicity. Because this effect occurs at low biosurfactant concentrations, it implies the possibiltiy of economical and effective application of biosurfactant in soil remediation, especially in situ remediation with exogenous bacterial amendments.