Physiological studies on a locally isolated rhamnolipid producer, Pseudomonas strain

Abstract

Aim Bacterial strains were isolated from different soil samples and evaluated for their ability to produce biosurfactants. Preliminary identification was made of the selected strain Kh4, as well as of the type of produced biosurfactant. Finally, the most optimum conditions for improving growth and the percentage of reduction in the culture surface tension (ST) were investigated. Materials and methods The genus of the selected strain was preliminarily identified on the basis of some biological tests and characterized pigmentation on a specific medium. Rhamnolipid production was confirmed using both Siegmund Wagner and Dubois methods as well as thin-layer chromatography. Quantitative measurements of the ST of cell-free supernatants were determined using the Du Nouy ring method in order to estimate the amount of biosurfactant produced. Cultivation conditions were studied on the basis of both flask and 7 l stirred tank bioreactor levels. Results A total of 66 bacterial strains were isolated from Egyptian soil and assessed for biosurfactant productivity. The selected isolate, preliminarily identified as a Pseudomonas spp., has the ability to produce rhamnolipid and hence to decrease the ST of the fermentation medium from 60 to about 35 mN/m. The optimum fermentation medium, used for the growth and maximum decrease in ST, was determined. Some physiological parameters such as the initial pH, incubation temperature, and time were also studied. Furthermore, the effect of pH control during the fermentation production of rhamnolipid in a 7 l stirred bioreactor was also studied. Accordingly, a maximum decrease in the ST of the cell-free culture medium of 32 mN/m was reached, confirming the production of a satisfactory level of rhamnolipid biosurfactant. Conclusion These results qualified the locally isolated Pseudomonas spp. Kh4 for use in the bioremediation of hydrocarbon-contaminated sites as well as in petroleum industries.