Production optimization, stability and oil emulsifying potential of biosurfactants from selected bacteria isolated from oil-contaminated sites

Abstract

Oil pollution is of increasing concern for environmental safety and the use of microbial surfactants in oil remediation has become inevitable for their efficacy and ecofriendly nature. In this work, biosurfactants of bacteria isolated from oil-contaminated soil have been characterized. Four potent biosurfactant-producing strains (SD4, SD11, SD12 and SD13) were selected from 27 isolates based on drop collapse assay and emulsification index, and identified as species belonging to Bacillus, Burkholderia, Providencia and Klebsiella, revealed from their 16S rRNA gene-based analysis. Detailed morphological and biochemical characteristics of each selected isolate were determined. Their growth conditions for maximum biosurfactant production were optimized and found quite similar among the four isolates with a pH of 3.0 and temperature 37°C after 6 or 7 days of growth on kerosene. The biosurfactants of SD4, SD11 and SD12 appeared to be glycolipids and that of SD13 a lipopeptide. Emulsification activity of most of the biosurfactants was stable at low and high temperatures (4-100°C), a wide range of pH (2-10) and salt concentrations (2-7% NaCl). Each biosurfactant showed antimicrobial activity against two or more pathogenic bacteria. The biosurfactants were well-capable of emulsifying kerosene, diesel and soya bean, and could efficiently degrade diesel.

Keywords: biosurfactant-producing bacteria; crude oil biodegradation; emulsification index; microbial surfactants; petroleum hydrocarbon bioremediation; production optimization.

Figure 1.

Phylotypes of the selected isolates. (a) Taxonomic affiliations of the four isolates based on sequence identity of their 16S rRNA genes. Accession numbers of the strains are provided in parentheses. (b) Phylogenetic tree of the isolates and their closest type strains (T).

Figure 2.

Effect of growth conditions on emulsification activity of culture filtrates. The isolates were grown at different sets of culture conditions such as incubation period (a), temperature (b), pH (c) and carbon source (d), and the emulsification index of culture supernatant was recorded. Error bars represent one standard deviation of the mean of three experiments.

Figure 3.

Stability of the biosurfactants at various (a) temperature, (b) pH and (c) salinity. The error bars represent one standard deviation of the mean, n = 3.

Figure 4.

Emulsification activity of the biosurfactants on different types of oil (a), and degradation of diesel by the isolates (b). The error bars represent one standard deviation of the mean, n = 3.