ABSTRACT
Optimal conditions for obtaining elevated biosurfactant production by a local strain of Bacillus licheniformis strain No. 5 were investigated. Modified minimal salt medium, pH 7.0, containing crude oil [1%], urea [2 g/l], KH[2]PO[4] + K[2]HPO[4] [2+2 g/l], MgSO[4] [0.2 g/l], yeast extract [1 g/l], and trace elements solution [0.1%] was found to be the most suitable for growth and emulsifying activity by this bacterium. High biosurfactant production was obtained after incubation for 7 days at 30° C. By providing the previous conditions, the emulsion index [E[24]%] was increased 3-fold as compared to that obtained via growth in the original minimal salt medium. In biorcactor batch culture, an agitation speed of 300 rpm attained the highest microbial growth [1.9 g cell dry weight/l] and an E[24]= 50.90% after 7 clays of incubation. In fed-batch culture, the pulsed addition of crude oil during the first 2 - 3 days of incubation enhanced the emulsification activity by 1.3-fold. The greatest E[24] was obtained using black grain oil [89.09%], followed by that obtained against diesel oil [87.27%]. The highest stability of emulsion index was recorded on diesel oil, which remained stable for 10 days [E[240]= 81.81%]. The biosurfactant showed an almost stable surface activity profile over a wide range of pH values [from 6 to 12]. The maximum emulsification activity was obtained at pH 8. The reduction in E[24] after exposure of the biosurfactant to 121°C for 15 min against diesel oil and toluene was 43.7% and 28.6%, respectively. Chemical analyses of the purified biosurfactant showed that it is a lipoprotein. Significant emulsification activity was detected towards different aliphatic and aromatic hydrocarbons and vegetable oils. The purified biosurfactant contained 41.7% C, 7.4% H and 5.8% N and was comprised of 36.2% proteins, 12.3% lipids and 5.6% carbohydrates