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The use of surfactants in almost every sector of modern industry reveals
their importance in the industrial chemical production. Surfactants are
surface-active compounds capable of reducing surface and interfacial tension between liquids,
solids and gases (Desai and Banat, 1997). Though chemical surfactants are widely used
in many industries, they are a cause for concern because of their low biodegradability
and high toxicity to the environment. Increase in environmental awareness about
chemical surfactants paved the way for the production of biological surfactants as a
possible alternative (Christofi and Ivshina, 2002). The most important advantage of
biosurfactants is probably their being ecologically acceptable. Biosurfactants are biodegradable
and can be produced from renewable substrates (Fiecther, 1992). Microbial surfactants
are surface-active metabolites produced by microorganisms when grown on water
miscible or oily substrates: they either remain adherent to microbial cell surfaces or are
secreted in the culture broth. They possess the characteristic property of reducing the
surface and interfacial tensions, using the same mechanisms as chemical surfactants.
Nowadays, the use of biosurfactants has been limited due to the high production
cost. Nevertheless, biosurfactants can be produced with high yield by some
microorganisms, especially Pseudomonas species. Pseudomonads are the best-known bacteria capable
of utilizing hydrocarbons as carbon and energy sources and producing biosurfactants
that enhance the uptake of such immiscible hydrophobic compounds (Al-Tahhan et al., 2000; Beal and Betts, 2000; Noordman and Janssen, 2002; and Rahman et al., 2002). Rhamnolipid compounds are frequently the main biosurfactants produced by Pseudomonas aeruginosa as a mixture of mono—and dirhamnolipids, which have quite
different physico-chemical properties (Benincasa et al., 2004). Rhamnolipid can reduce
surface tension of water from 72 mN/m to 30 mN/m (Arino et al., 1996; Abalos et al., 2001; and Ron and Rosenberg, 2001) with a critical micelle concentration of 50-65 mg/L
(Mata-Sandoval et al., 1999). Although rhamnolipid is an effective biosurfactant and
is well suited for applications in bioremediation of oil pollutants (Mulligan, 2005), the
major hurdle for commercial application of the biosurfactant has been the low yield and
high production cost (Yu-Hong et al., 2005). Therefore, there is an urgent demand to
develop an efficient biosurfactant producer and a cost-effective bioprocess for the production
of rhamnolipid.
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