Environmental Engineering Reference
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and antimicrobial functions on other living systems
(Bergstrom et al.
1946
; Abalos et al.
2001
).
Excellent emulsifying properties relative to syn-
thetic surfactants make rhamnolipids viable alter-
natives due to their effi cacy at low concentrations
and their inherent biodegradability (Ron and
Rosenberg
2002
; Calvo et al.
2009
; Rahman et al.
2003
), for example, the bioremediation of
hydrocarbon-polluted sites (Thavasi et al.
2011a
;
Jorfi et al.
2013
) and in enhanced oil recovery
(Amani et al.
2013
). However, the industrial-
scale application of
P. aeruginosa
strains for
rhamnolipid production is an unrealistic task: this
organism is an opportunistic human pathogen
(Rahman et al.
2010
) and is responsible for infec-
tious diseases in immune-compromised individuals
(Goethals et al.
2001
). Addressing these safety
issues renders
P. aeruginosa
-derived biosurfac-
tant uneconomical (Ochsner et al.
1995
; Tuleva
et al.
2002
) and unsafe for industrial processes.
The cultivation of
P. aeruginosa
, a Gram-
negative bacterium belonging to the taxonomical
class of Gammaproteobacteria and the family
Pseudomonadaceae, has been well studied, and
the genome of strain PAO1 has been fully
sequenced and annotated. The organism is found
in various habitats including water, soil, plants
and air, is resistant to a variety of antibiotics
(Tummler et al.
1991
) and is the main causative
organism for cystic fi brosis and nosocomial
infections. The pathogenicity of
P. aeruginosa
in
cystic fi brosis was confi rmed by Kownatzi et al.
(
1987
) who discovered rhamnolipids of up to
8
Rhamnolipids from
P. aeruginosa
stimulate
the release of allergy and infl ammatory media-
tors from the mast cells such as histamine, sero-
tonin and 12-hydroxyeicosatetraenoic acid
(Bergmann et al.
1989
; McClure and Schiller
1992
; König et al.
1992
; Cosson et al.
2002
;
Andrä et al.
2006
). Furthermore, rhamnolipids
have been associated with several dysfunctions in
the lungs and the entire respiratory tract. These
include inhibition of ciliary function, damage to
bronchial epithelium, alteration of respiratory
epithelial ion movement and induction of the
release of mucus conjugates from human bron-
chial mucosa, and very recently it has been
reported that rhamnolipid synthesis is an impor-
tant prerequisite for the invasion of
P. aeruginosa
into human respiratory epithelial cells (Stutts
et al.
1986
; Graham et al.
1993
; Fung et al.
1995
;
Zulianello et al.
2006
).
P. aeruginosa
has been
classifi ed as a biosafety level 2 organism due to
the health and safety issues associated with the
organism as well as the cytotoxic properties of
their rhamnolipid species.
These safety concerns inhibit the economic
viability of large-scale rhamnolipid production
from
P. aeruginosa
. Such viability is further
compromised by the involvement of complex
quorum sensing and transcriptional mechanisms
(Soberón-Chávez et al.
2005
). Furthermore, the
purifi cation and treatment of the rhamnolipid
yield from
P. aeruginosa
are also costly at the
industrial scale, and it is important to note that
rhamnolipids produced from
P. aeruginosa
fer-
mentation cannot be considered as safe raw mate-
rials in the production of food, pharmaceutical
and cosmetics products. Thus there is need to
explore for new nonpathogenic natural rhamno-
lipid producers. A survey of biosurfactants pro-
duced by pathogenic strains is presented in
Table
1
together with their pathogenesis in
humans, plants and animals.
Rhamnolipids are also produced by
Burkholderia
species; some of these organisms
such as
B. cepacia
have been identifi ed as human
pathogens. This organism capable of producing a
biosurfactant exhibiting a surface tension value of
45.7 mN m
−1
(Yalçin and Ergene
2010
) has been
l/ml in the sputum of
P. aeruginosa
-colonised
cystic fi brosis patients. Read et al. (
1992
) also
showed the presence of 65
ʼ
l/ml rhamnolipid in
the secretions of a lung removed from a cystic
fi brosis patient. The cytotoxic and haemolytic
effects of crude rhamnolipids from other rham-
nolipid producers have been demonstrated by
researchers (Häussler et al.
1998
,
2003
; Rahman
et al.
2010
). Rhamnolipids have been shown to
be heat-stable haemolysins having haemolytic
activity on various erythrocyte species (Fujita
et al.
1988
; Johnson and Boese-Marrazzo
1980
)
and cytotoxic at high concentration to immune
cells (Bjarnsholt et al.
2005
; Jensen et al.
2007
).
ʼ
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