**2.3.1 Natural roles**

Although biosurfactants are produced by a large number of microorganisms and are clearly significant in many aspects of growth, it is difficult to generalize on their roles in microbial physiology. Due to their very diverse chemical structures and surface properties, different groups of biosurfactants may have different natural roles in the growth of the producing microorganisms and probably provide advantages in a particular ecological niche. Ron & Rosenberg, (2001) and Van Hamme et al., (2006) recently reviewed the physiological roles of microbial surface-active compounds. Some biosurfactants are essential for the motility of the microorganisms (gliding and swarming). For example, serrawettin plays a fundamental role for surface locomotion and access to water repelling surfaces for *Serratia marcescens* whereas surfactin, together with flagellar biosynthesis, are crucial for swarming motility in *B. subtilis* (Arutchelvi et al., 2008; Van Hamme et al., 2006). Bioemulsifiers also play an important role in regulating the attachment-detachment of microorganisms to and from surfaces (Van Hamme et al., 2006).

In addition, bioemulsifiers are involved in cell-to-cell interactions such as bacterial pathogenesis, quorum sensing and biofilm formation, maintenance and maturation. Rhamnolipids, for example, are essential to maintain the architecture of the biofilms and are considered as one of the virulence factors in *Pseudomonas* sp. (Arutchelvi et al., 2008; Ron & Rosenberg, 2001; Van Hamme et al., 2006). Rhamnolipids, mannosylerythritol lipid and surfactin show antimicrobial and antibiotic properties thus conferring a competitive advantage to the organism during colonization and cell–cell competition. In addition cellular differentiation, substrate accession and resistance to toxic compounds are all roles attributed to microbial surface-active compounds. Their most widespread role however is believed to be the interaction between microbes and insoluble substrates such as

hydrocarbon ratios of 1:100 to 1:1000, while exhibiting considerable substrate specificity (Ron & Rosemberg, 2001). Polymeric surfactants offer additional advantages because they coat droplets of oil, thereby forming stable emulsions. This property is especially useful for making oil/water emulsions for cosmetics and food industries (Muthusamy et al., 2008).

Fig. 4. Example of emulsion produced by the bioemulsifier extracted from the bacterial strain 7bCT5, isolated from a Cambodian soil. This emulsion is stable since 2009.

Although biosurfactants are produced by a large number of microorganisms and are clearly significant in many aspects of growth, it is difficult to generalize on their roles in microbial physiology. Due to their very diverse chemical structures and surface properties, different groups of biosurfactants may have different natural roles in the growth of the producing microorganisms and probably provide advantages in a particular ecological niche. Ron & Rosenberg, (2001) and Van Hamme et al., (2006) recently reviewed the physiological roles of microbial surface-active compounds. Some biosurfactants are essential for the motility of the microorganisms (gliding and swarming). For example, serrawettin plays a fundamental role for surface locomotion and access to water repelling surfaces for *Serratia marcescens* whereas surfactin, together with flagellar biosynthesis, are crucial for swarming motility in *B. subtilis* (Arutchelvi et al., 2008; Van Hamme et al., 2006). Bioemulsifiers also play an important role in regulating the attachment-detachment of microorganisms to and from surfaces (Van

In addition, bioemulsifiers are involved in cell-to-cell interactions such as bacterial pathogenesis, quorum sensing and biofilm formation, maintenance and maturation. Rhamnolipids, for example, are essential to maintain the architecture of the biofilms and are considered as one of the virulence factors in *Pseudomonas* sp. (Arutchelvi et al., 2008; Ron & Rosenberg, 2001; Van Hamme et al., 2006). Rhamnolipids, mannosylerythritol lipid and surfactin show antimicrobial and antibiotic properties thus conferring a competitive advantage to the organism during colonization and cell–cell competition. In addition cellular differentiation, substrate accession and resistance to toxic compounds are all roles attributed to microbial surface-active compounds. Their most widespread role however is believed to be the interaction between microbes and insoluble substrates such as

**2.3.1 Natural roles** 

Hamme et al., 2006).

hydrocarbons. Some biosurfactants/bioemulsifiers enhance the growth of bacteria on hydrophobic water-insoluble substrates by increasing their bioavailability, presumably by increasing their surface area, desorbing them from surfaces and increasing their apparent solubility (Neu, 1996; Ron & Rosenberg, 2001; Van Hamme et al., 2006).
