Novel lipopeptide biosurfactant produced by hydrocarbon degrading and heavy metal tolerant bacterium Escherichia fergusonii KLU01 as a potential tool for bioremediation

Abstract

Escherichia fergusonii KLU01, a propitious bacterial strain isolated from oil contaminated soil was identified to be hydrocarbon degrading, heavy metal tolerant and a potent producer of biosurfactant using diesel oil as the sole carbon and energy source. The biosurfactant produced by the strain was characterized to be a lipopeptide. The minimum active dose and critical micelle concentration of the biosurfactant were found as 0.165 ± 0.08 μg and 36 mg/L, respectively. In spite of being an excellent emulsifier, the biosurfactant showed an incredible stability at extremes of temperature, pH and at various concentrations of NaCl, CaCl₂ and MgCl₂. Also the bacterium manifested tolerance towards Manganese, Iron, Lead, Nickel, Copper and Zinc. The strain emerges as a new class of biosurfactant producer with potential environmental and industrial applications, especially in hydrocarbon degradation and heavy metal bioremediation.

Introduction

Biosurfactants are the extracellular or membrane bound surface active compounds that are mainly produced by microorganisms. These biosurfactants are amphiphilic and exist in a wide variety of chemical structures such as lipopeptides, glycolipids, phospholipids, fatty acids, neutral lipids and polymeric biosurfactants and are produced by microorganisms of diverse origin (Ron and Rosenberg, 2001). Despite the broad applications of chemical surfactants, they are environmentally hazardous and can lead to ecological imbalance when accumulated in excess. In such a scenario, the performance of biosurfactants is most promising and they can act as an effective alternative to the chemical surfactants.

Petroleum hydrocarbons are the major pollutants leading to environmental degradation as a consequence of terrestrial freshwater runoff, refuse from oil refineries, shipping industries and accidental spillage during petroleum refining and transport activities (Thavasi et al., 2011). According to Itoh and Suzuki (1972), the ability of a rhamnolipid-negative mutant strain of Pseudomonas aeruginosa to utilize hydrocarbons as its carbon source was restored, only after the addition of rhamnolipid to the medium. This confirmed the indomitable role of surface active agents in the ability of the microorganisms to grow on oils, n-alkanes or other hydrocarbons. Since then, a large number of hydrocarbon degrading microorganisms were reported to produce biosurfactants (Bordoloi and Konwar, 2009, Zhao et al., 2011).

With the rapid advancements in technology, the natural environment often gets exacerbated by harmful effects of industrial pollution. The heavy metals which are utilized in a vast array of industrial activities, when released into the environment via waste waters, were known to severely affect the micro and macro biota inhabiting the water bodies, thereby perturbing the entire ecosystem (Joseph et al., 2009). Further, the exposure to heavy metals through the uptake of contaminated drinking water and foods can result in bioaccumulation of heavy metals in animals and humans eventually leading to biomagnification. In such a situation, the microbial means of heavy metal bioremediation can act as a potential eco-friendly solution for environmental clean up.

Considering the biosurfactant production by Escherichia species, so far there is no scientific report and the strain Escherichia fergusonii had been primarily isolated from clinical specimens (Mahapatra et al., 2005). The potential biotechnological applications of this strain remain unexplored and to our surprise, we identified the presence of this strain in oil contaminated soil which substantiates its diverging natural habitats. Hence in the present study, the isolated strain Escherichia fergusonii KLU01 was evaluated for its ability to produce biosurfactants, utilizing diesel oil as its sole carbon source and to bioremediate heavy metals. To our knowledge this is the first report, unveiling the ability of a hydrocarbon degrading and heavy metal tolerant E. fergusonii strain to produce potent lipopeptide biosurfactants which exhibited extreme stability, striking surfactant activity and excellent emulsifying capability.