Biosurfactants and Bioemulsifiers in Accessing Hydrophobic Substrates

Biosurfactants are one of the numerous adaptations of microorganisms metabolizing hydrocarbons and broadly represent a physiological response to specific requirements encountered by the cells depending on their environment. Some bacteria have developed the ‘‘pseudosolubilization’’ strategy to gain access to poorly soluble substrates and therefore produce highly dynamic low-molecular mass biosurfactants characterized by the capability to self-assembly in micelles, hemi-micelles or aggregates. Other bacteria interact with hydrocarbons directly by means of wall-bound biosurfactants that confer on the cell surface the appropriate hydrophobicity. High molecular mass bioemulsifiers in general adsorb tightly and cover the hydrocarbons thus dramatically increasing their apparent solubility. Although a variety of mechanisms and specializations distinguish biosurfactants, they all share a few essential traits. They are part of the process of interaction of microbial cells with surfaces as well as substrates. In addition to working on hydrocarbon solubilization they also act at the level of the cellular outer membrane allowing temporary and reversible modifications that control substrate access by the cells. As a consequence, a ‘‘substrate effect’’ can often be observed. Although the biosynthetic mechanisms for most remain unclear, substrates seem able to influence structural variations that make the biosurfactants particularly active towards the same substrate. Finally, many biosurfactants and their producing strains stimulate the growth of differently specialized bacteria which suggests that these molecules play a vital role in the interaction between the oil-degrading microbial communities and their environment.