Amphiphilic Molecules of Microbial Origin Classification, Characteristics, Genetic Regulations, and Pathways for Biosynthesis - Biosurfactants: Research Trends and Applications - page 39

P. aeruginosa was stable over the pH range of 5-12, temperature up to 120°C, and
up to 20 g/L NaCl concentrations (Xia et al., 2011). Few other bioemulsifiers and
biosurfactants have also been reported to tolerate wide range of pH, temperature,
and salinity (Amaral et al., 2006; Chen et al., 2012; Kim et al., 1997).
l ow t oxiCity
It is extremely essential to investigate the pharmacological properties and toxicity
of the antibiotic biosurfactants to exploit them as safe drugs. Hwang et al. exam-
ined the subacute toxicity of surfactin from B. subtilis in adult Sprague-Dawley
rats for 28 days. Rats survived even with a high dose of surfactin (2 g/kg), and the
no-observed-adverse-effect-level of surfactin was found to be 500 mg/kg (Hwang
et al., 2009). The cytotoxicity of the MEL from C. antarctica was examined using
a neutral red assay in mouse fibroblast L929 cells. It was much less toxic than the
synthetic surfactants and was reported to be not harmful to human skin and eyes,
indicating its potential applicability to industries such as cosmetics or personal care
(Kim et al., 2002). A glycolipid biosurfactant from Rhodococcus ruber was exam-
ined for its acute toxicity against outbred male albino mice. No effect on central
nervous system or weight loss was found during the 14-day observation (Kuyukina
et al., 2007). Few other reports have also compared the toxicity of biosurfactants
and chemical counterparts, indicating the low toxicity of biosurfactants (Dehghan-
Noudeh et al., 2005; Edwards et al., 2003; Hirata et al., 2009; Poremba et al., 1991).
B ioDeGraDaBility
As the concern about the environment is increasing, easily biodegradable microbial
surfactants are preferred over the synthetic surfactants for environmental applica-
tions (Nitschke and Costa, 2007). Pei et al. examined the biodegradability of rham-
nolipids by incubating it in black loamy soil and red sandy soil for 1 week. The
degradation rate was slow initially, but 92% of rhamnolipid was found to be min-
eralized in both kinds of soil at the end of the week (Pei et al., 2009). Kim et al.
compared the biodegradability of MEL and two chemical surfactants by modified
biochemical oxygen demand method. MEL was readily degraded by activated sludge
microorganisms in just 5 days, whereas the chemical surfactants showed a lower
degradation rate even after 7 days (Kim et al., 2002). Recently, the biodegradabil-
ity of biosurfactants produced by five different bacterial strains was compared with
sodium dodecyl sulfate (SDS). The biosurfactants were highly biodegradable when
compared to SDS (Lima et al., 2011).
BIOSYNTHESIS AND GENETICS OF MAJOR
MICROBIAL AMPHIPHILES
Hydrophilic and hydrophobic domains of the biosurfactants are synthesized by
separate pathways and are then amalgamated to form an amphipathic structure
(Hommel and Ratledge, 1993). These two domains may be synthesized de novo