Trehalose Biosurfactants - Biosurfactants: Research Trends and Applications - page 198

Potential applications of naturally occurring surface-active compounds are quite
diverse and at present main applications are related to environmental concerns, such
as bioremediation of hydrocarbon and heavy metal contaminated sites, enhanced oil
recovery, and treatment of oil spills (Hester 2001; O'Connor 2002). Alternatively to
synthetic compounds, biosurfactants can be utilized in agriculture, mining, and food-
processing, with functional properties such as wetting and foaming agents and as
emulsifiers in pharmaceutical and cosmetic products (Banat et al. 2000). Recently,
some biosurfactants were proved to be suitable alternatives to synthetic medicines and
antimicrobial agents and may be used as effective therapeutic agents (Rodrigues et al.
2006). Currently biosurfactants have not yet been commercialized extensively due
to their relatively high production and recovery costs. Improvement of biosynthesis
efficiency through the use of low-cost medium components coupled with efficient
downstream processing techniques and development of hyperproducing strains can
make biosurfactant production commercially profitable (Mukherjee et al. 2006).
Glycolipids are the most common class of biosurfactants. They are carbohy-
drates in combination with long-chain aliphatic or hydroxyaliphatic acids. Among
the glycolipids, the best-known subclasses are the rhamnolipids from Pseudomonas
aeruginosa , sophorolipids from yeasts, and trehalose lipids from Mycobacteria and
related bacteria (Desai and Banat 1997). Trehalose lipids possess unique surface
properties and wide biological activities, as well as other favorable characteristics
like low CMC, biodegradability, and low toxicity. The present chapter focuses on
their diverse chemical structures, some aspects of their production, physicochemical
and biological properties, and possible applications.
TREHALOSE LIPIDS
It is of great importance to know the chemical structure of a surfactant molecule,
because slight differences in structure can lead to pronounced differences in its sur-
face properties and bioactivity. Based on the knowledge to date, the members of the
trehalose glycolipid family can be divided into three general subclasses: I-subclass-6,6
substituted trehalose esters [such as fatty acid trehalose diesters (TDEs), trehalose dico-
rynomycolates (TDCMs), and trehalose dimycolates (TDMs)]; II-subclass-2,3-diesters
of trehalose sulfates; and III-sub class-succinoyl diesters and tetraesters (Figure 8.1).
C hemiCal s truCtures of G lyColiPiDs P roDuCeD from m ycoBacteria
Trehalose lipids were first discovered as an important factor in the virulence of
Mycobacteria . The so-called cord factor is the best known and was isolated as a
pure compound in 1956 from the lipids of Mycobacterium tuberculosis (Asselineau
and Lederer 1956). The chemical structure of cord factor of Mycobacterium tuber-
culosis comprises a branched-chain mycolic acid esterified to the 6-hydroxyl
group of each glucose to give 6,6′-dimycoloyl-α,α′-trehalose. Mycolic acids pos-
sess a great variety of structures, the number of their carbon atoms varies from
approximately 60 to 90; they contain not only a normal long-chain saturated
alkane, C 22 H 45 or C 24 H 49 , but a different number of double bonds, methyl groups, and
cyclopropane rings (Lederer 1967) (Figure 8.1, I-subclass). Later, several groups of