Biomedical Engineering Reference
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need for more purification, as they didn't use any oil as the substrate and its carbon
metabolism didn't produce any by-products. The produced rhamnolipids were mainly
monorhamnolipids (C
10
:C
10
). These researchers also performed some metabolic opti-
mization, including separating the biomass synthesis pathway from rhamnolipid
synthesis pathway, eliminating polyhydroxyalkanoate (PHA) synthesis pathway,
which is competing, and optimizing the pathway of synthesizing rhamnolipids from
glucose. These metabolic modifications result in the use of the majority of substrates
toward rhamnolipid synthesis.
P
seudomonas
fluorescens
P. fluorescens
are a group of gram-negative, obligate aerobic, rod-shaped bacteria
that live in the soil and water and can be recognized by their green-yellow, water-
soluble fluorescent pigments. They have an extremely versatile metabolism, and
some of the strains in this group could survive in anaerobic environments by using
nitrate as the electron acceptors. Most of the strains of fluorescent pseudomonads
grow well on a media composed of nitrate, glucose, and inorganic salts. Their opti-
mal temperature is 25°C-30°C (Rhodes, 1959).
Experiments conducted by Gunther et al. (2005) on
P. fluorescens
strains ATCC
17397, ATCC 17824, ATCC 17577, and ATCC 17816 showed no sign of rhamnolipid
production. Bondarenko et al. (2010) conducted some experiments to study the resil-
ience of the indigenous bacteria when they were exposed to rhamnolipids supple-
mented to a polluted area. Experiments showed that exposure to rhamnolipids, even
in low concentration, was able to reduce the bioluminescence of the
P. fluorescens
strains. They reported that the growth rate of
P. fluorescens
was affected by concen-
trations as low as 0.96 mg/L for 30 min. Researchers such as Ochsner (2005) showed
that the recombinant strain
P. fluorescens
ATCC 15453 growing on glycerol as the
substrate was able to produce rhamnolipids, but the concentration of the produced
rhamnolipids, which was mainly rhamnolipid type 2, was 0.15 g/L, 10% of what
P. aeruginosa
produces.
The experiments of Healy et al. (1996) included growth of the bacterium
P. fluo-
rescens
NCIMB 11712 on virgin olive oil and demonstrated that production of a
glycolipid in the form of a rhamnolipid took place. Furthermore, Abouseoud
et al. (2008a,b) conducted some research on the production of rhamnolipids by
P. fluorescens
Migula 1895-DSMZ. They studied the effect of a variety of carbon
and nitrogen sources and also different C:N ratios on bacterial cultures growing at
constant temperature and pH. They reported that optimal productivity was obtained
when using olive oil and ammonium nitrate as carbon and nitrogen sources and C:N
ratio was 10. The produced rhamnolipids could decrease the surface tension of the
supernatant to less than 32 mN/m, and it demonstrated emulsification of 65% in
36-48 h. Recovery of the rhamnolipids was done through the precipitation method
by using acetone. Afterward, there were more essays to determine the quality of the
recovered product. According to Abouseoud et al. (2008a,b), the recovered rham-
nolipids not only showed good foaming and emulsifying and antimicrobial activi-
ties, but also were stable throughout its exposure to high temperatures, when it was
exposed to 120°C for 15 min. Produced rhamnolipids also showed resilience to
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