Rhamnolipids Characteristics, Production, Applications, and Analysis - Biosurfactants: Research Trends and Applications - page 72

inhibited in the presence of 90 g/L dirhamnolipid. At this concentration, the
growth rate of the bacteria was slowed down by 50%.
Ochsner et al. (1995) studied and compared the rhamnolipid production of
P. fluorescens, P. putida, P. oleovorans , and E. coli , which were genetically altered
through introducing the genes responsible for rhamnolipid production into their chro-
mosomes. These researchers observed that P. putida had the highest yield of rham-
nolipids and P. fluorescens had the second highest production rate. Furthermore,
the studies of other researchers such as Cha et al. (2008) involved transferring the
genes responsible for synthesizing rhamnolipids from P. aeruginosa to P. putida .
They reported that this recombinant strain was able to produce rhamnolipids. When
soybean oil was used as the substrate, and the rate of production of rhamnolipids was
up to 7.3 g/L.
Ochsner et al. (1995) noted that even though the enzyme rhamnosyltransferase
was synthesized in the recombinant strains E. coli and P. oleovorans, these two
strains didn't produce rhamnolipids. This latter observation contradicts the results
from some other researchers who worked with genetically engineered E. coli .
Cabrera-Valladares et al. (2006) observed that by altering the DNA of E. coli , it
could become a monorhamnolipid producer. Their experiments on the recombinant
E. coli showed that although this metabolic engineered strain is able to produce
rhamnolipids, the rhamnolipid production rate for these E. coli was much lower than
the rate of the production in P. aeruginosa . However, they stated that the rate of the
production in recombinant E. coli was more than the previous reports on E. coli .
They noted that by optimization of the substrate and its availability, the rate of the
production was increased substantially.
Cabrera-Valladares et al. (2006) reported that recombinant E. coli HB101 grown
on a hydrophobic substrate (e.g., oleic acid) demonstrated a high production rate of
rhamnolipids. They also stated that the recombinant strain of E. coli W3110, which
expressed the P. aeruginosa operons rhlAB and rmlBDAC, growing on glucose as
the carbon source, was able to produce more than twice the amount of rhamnolipids
produced by the recombinant E. coli HB101 .
Wang et al. (2007) conducted some experiments by transferring the RhlAB gene,
the gene responsible for rhamnolipid production, into the chromosomes of two strains,
the P. aeruginosa PAO1-rhlA and Escherichia coli BL21 (DE3), which were not able to
produce rhamnolipids before. It was observed that following the integration of the gene
into the chromosomes, both strains were able to produce rhamnolipids (Wang et al.,
2007). In Table 3.2, some of the known producers of rhamnolipids are introduced.
u se of l ow -C ost s uBstrates
As it has been noted before, one of the important approaches to decrease the cost
of the rhamnolipid production involves using low-cost substrates such as waste
frying oil (Banat et al., 2010; Lotfabad et al., 2010; Mulligan and Gibbs, 2004;
Wang et al., 2007; Zhu et al., 2007) and industrial wastes (Nitschke et al., 2005).
Pornsunthorntawee et al. (2010) indicated that not only genetic regulatory gov-
erns the rhamnolipid production, but it was also affected by the main meta-
bolic pathways such as fatty acid synthesis and activated sugars and enzymes.