Biomedical Engineering Reference
In-Depth Information
INTRODUCTION
Amphiphilic molecules with hydrophobic moiety at one end and hydrophilic moiety
at the other end possess the ability to reduce the interfacial/surface tension (ST) at
a liquid-liquid interface or a gas-liquid interface. In general, synthetic surfactants
produced from petroleum feedstock are used in food, cosmetic, and pharmaceutical
industries as emulsifiers, lubricants, stabilizers, wetting agents, etc. (Banat et al.,
2000; Mukherjee et al., 2006). Since they are very harmful to the environment, use
of more efficient and ecofriendly surfactants is necessary. Various microorganisms
produce such risk-free surface active metabolites known as biosurfactants. When
compared to synthetic surfactants, biosurfactants are less toxic, highly biodegrad-
able, and stable at extremes of pH, temperature, and salinity (Mukherjee et al., 2006).
Furthermore, microbial surfactants can be produced by using low-cost agro-based
raw materials as potential carbon sources. Hence, biosurfactants are considered to
have strategic advantages over their chemically synthesized counterparts.
Microorganisms synthesize amphiphilic molecules or biosurfactants extracellularly
or as part of the cell membrane. These molecules facilitate the growth of their produc-
ers by increasing the substrate availability, transporting nutrients, and by acting as
biocide agents (Rodrigues et al., 2006). Biosurfactants are classified into many groups,
since they are produced from wide variety of microorganisms and have very different
chemical structures (Ron and Rosenberg, 2001). These structurally diverse molecules
have different properties and inherent functions. In addition to their surface active and
emulsification properties, biosurfactants possess antiadhesive, antimicrobial, and anti-
carcinogenic properties, which make them a versatile class of biomolecules. Owing to
their diverse physicochemical characteristics, biosurfactants find myriad applications
in pharmaceutical, food, and cosmetic industries as well as in enhanced oil recovery
and environmental bioremediation (Sen, 2008; Sen et al., 2011). This chapter deals
with the microbial synthesis, classification, and properties of the biosurfactants.
MAJOR CLASSIFICATION OF MICROBIAL AMPHIPHILES
Biosurfactants are classified according to their chemical composition, molecular
weight, and microbial source of origin. Consequently, the major classes of biosurfac-
tants include glycolipids, lipopeptides and lipoproteins, fatty acids, phospholipids and
neutral lipids, polymeric surfactants, and particulate surfactants (Desai and Banat,
1997) as reported in Table 2.1. The lipophilic moiety of the biosurfactant is usually
the hydrocarbon chain of a fatty acid or fatty acid derivatives, but it can also be a pro-
tein or a peptide with a high proportion of hydrophobic side chains. The hydrophilic
moiety can be a peptide, a carbohydrate, or an ester group (Nitschke and Costa, 2007).
Bacteria account for the greatest production of biosurfactants followed by yeasts.
G
lyColiPiDs
The most commonly described microbial surfactants are the glycolipids. Glycolipids
are carbohydrates (hydrophilic moiety) linked with long-chain aliphatic acids or
hydroxy aliphatic acids (hydrophobic moiety). The linkage is by means of either
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