Environmental Engineering Reference
In-Depth Information
Table 3
Recombinant biosurfactant producers
Biosurfactants
utilise hydrophilic substances, reaching a maximum
rhamnolipid concentration in medium after
1-2 days. This is compared with the wild-type
strain of
P. aeruginosa
PA01 that although it
exhibits higher rhamnolipid production, it
requires 1-3 days to reach their maximal levels.
This reduction in process time exhibited by
recombinant strains enhances the space-time
yield for the process (Wittgens et al.
2011
).
According to this investigation, the engineered
P. putida
KT2440 is the second recombinant
strain featuring the highest recorded space-time,
and yield, therefore making this strain a potential
industrial tool for biotechnological rhamnolipid
synthesis.
Burkholderia kururiensis
KP23(T), a
trichloroethylene- degrading, nitrogen-fi xing and
plant growth-promoting bacterium, has been
reported to produce rhamnolipids, and when
genetically engineered with two biosynthetic
enzymes from
P. aeruginosa
RhlA and RhlB,
rhamnolipid production increased by sixfold
compared to wild-type strain. Rhamnolipids pro-
duced by the engineered strains were mainly
mono-RL as opposed to wild-type strains
B.
kururiensis
and
P. aeruginosa
that predominantly
produce di-RLs. This organism is a promising
biosurfactant producer with potential environ-
mental and biotechnological application espe-
cially due to its nonpathogenicity and
compatibility with metabolic engineering proce-
dures (Tavares et al.
2013
).
Microbial strains
References
Rhamnolipids
Pseudomonas putida
1067(PNE2)
Cha et al.
(
2008
)
Pseudomonas
fl uorescens
Ochsner
et al. (
1995
)
E. coli
W3110,
E. coli
HB101
Cabrera-
Valladares
et al. (
2006
)
P. putida
KT2440
Wittgens
et al. (
2011
)
Burkholderia
kururiensis
KP23(T)
Tavares
et al. (
2013
)
carbon substrate (Cabrera-Valladares et al.
2006
).
Similarly, recombinant
E. coli
HB101 was also
detected to produce 52 mg L
−1
rhamnolipid with
oleic acid as substrate (Cabrera-Valladares et al.
2006
). A successful development of a non-
pathogenic host capable of producing mono-
rhamnolipids using glucose as substrate has been
reported by Wittgens et al. (
2011
). In this study,
the synthesis of mono-rhamnolipid independent
from biomass formation was done using
P. putida
KT2440 expressing the
rhlAB
genes from
P.
aeruginosa
PA01. A sevenfold increase in the
fi nal rhamnolipid concentration from 0.22 to
1.5 g L
−1
was recorded after genetic optimisation
of the strain. The engineered strain exhibited
some advantages compared to rhamnolipid pro-
duction in
P. aeruginosa
. Firstly, these strains are
nonpathogenic and can appropriately substitute the
opportunistic pathogenic
P. aeruginosa
. Secondly,
rhamnolipid production in the recombinant
strains was void of the complex quorum sensing
regulation. However, the production rate in the
recombinant strain was about two thirds of that
usually obtained in optimised fermentation with
P. aeruginosa
although there remains the possibility
of increasing the production rate by increasing the
availability of activated rhamnose in the medium.
More importantly, compared to
P. aeruginosa
, it
is the utilisation by the recombinant strains of
glucose instead of hydrophobic substances.
Glucose as a carbon substrate is relatively cheap
and is applied in many biotechnological processes
(Blank et al.
2008
). Furthermore, these recombinant
strains capable of substituting
P. aeruginosa
can
5
Conclusion
The development of biosurfactant production in
nonpathogenic organisms is a current challenge
that is receiving increased attention in order to
avoid pathogenicity and complex metabolic regu-
lations. Most biosurfactant-producing organisms
are pathogenic and diffi cult to handle in large-
scale industrial processes. Pathogenicity of bio-
surfactant producers is key factor to prevent
large-scale production; therefore, the nonpathogenic
eco-friendly organisms need to be explored further,
and a library of collections is important for the
future development in this area of study.
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