Green Chemistry and Biosurfactant Research - Biosurfactants: Research Trends and Applications - page 17

metabolite leakage. This occurs due to changes in physical membrane structure or
through disrupting protein conformations that alter important membrane functions
such as transport and energy generation (Van Hamme et al., 2006).
Antimicrobial activity of biosurfactants : The high demand for new antimicrobial
agents following increased resistance shown by pathogenic microorganisms against
existing antimicrobial drugs has drawn attention to BSs as antibacterial agents (Běhal,
2006). Some BSs have been reported to be suitable alternatives to synthetic medicines
and antimicrobial agents and may therefore be used as effective and safe therapeutic
agents (Cameotra and Makkar, 2004; Singh and Cameotra, 2004).
Lipopeptides form the most widely reported class of BSs with antimicrobial activity.
Surfactin, produced by B. subtilis , is the best-known lipopeptide. Other antimicrobial
lipopeptides include fengycin, iturin, bacillomycins, and mycosubtilins produced by
B. subtilis (Vater et al., 2002). Lichenysin, pumilacidin, and polymyxin B (Naruse et al.,
1990) are other antimicrobial lipopeptides produced by B. licheniformis , B. pumilus ,
and B. polymyxa, respectively. The production of antimicrobial lipopeptides by
Bacillus probiotic products is one of the main mechanisms by which they inhibit the
growth of pathogenic microorganisms in the gastrointestinal tract (Hong et al., 2005).
Other reported BSs having antimicrobial activity are daptomycin, a cyclic lipopeptide
from Streptomyces roseosporus (Baltz et al., 2005), viscosin, a cyclic lipopeptide
from Pseudomonas (Neu et al., 1990; Saini et al., 2008), rhamnolipids produced by
P. aeruginosa (Abalos et al., 2001; Benincasa et al., 2004), and sophorolipids produced
by C. bombicola (Kim et al., 2002; Van Bogaert et al., 2007). MEL-A and MEL-B
produced by C. antarctica strains have also been reported to exhibit antimicrobial
action against gram-positive bacteria (Kitamoto et al., 1993).
Recently, a lipopeptide BS produced by a marine organism, B. circulans , was
found to be active against Proteus vulgaris , Alcaligenes faecalis , methicillin-
resistant Staphylococcus aureus (MRSA), and other multidrug-resistant pathogenic
strains (Das et al., 2008a), while not having any hemolytic activity. A rhamnolipid
surfactant produced from soybean oil waste had antimicrobial activity against sev-
eral bacteria and fungi, namely, B. cereus , S. aureus, Micrococcus luteus, Mucor
miehei , and Neurospora crassa (Nitschke et al., 2009b). Flocculosin, a cellobiose
lipid produced by the yeast-like fungus Pseudozyma flocculosa, , was tested against
clinical bacterial isolates and the pathogenic yeast C. albicans (Mimee et al., 2009).
The glycolipid was particularly effective against Staphylococcus species, including
MRSA, and its antibacterial activity was not influenced by the presence of common
resistance mechanisms (e.g., against methicillin and vancomycin) in tested strains.
In addition, flocculosin was able to kill C. albicans cells in a very short period of
time. Huang et al. (2007) observed that a lipopeptide antimicrobial substance pro-
duced by the strain B. subtilis fmbj, which is mainly composed of surfactin and
fengycin, was able to inactivate endospores of B. cereus . Observation by TEM indi-
cated that the lipopeptide could damage the surface structure of the spores. The anti-
fungal activities of BSs have long been known, although their action against human
pathogenic fungi has been rarely described (Tanaka et al., 1997; Chung et al., 2000;
Abalos et al., 2001). Recently, a glycolipid isolated from the yeast-like fungus P. floc-
culosa , named flocculosin, was shown to display in vitro antifungal activity against