Biomedical Engineering Reference
In-Depth Information
to other nonpathogenic microorganisms (Abdel-Mawgoud et al., 2011; Mulligan and
Gibbs, 2004; Nguyen and Sabatini, 2011). Besides, the focus of other research has
been to determine the potential of rhamnolipids for a variety of applications.
Some microorganisms evolved to survive in partially nutrition-depleted envi-
ronments by the secretion of rhamnolipids, and some microorganisms using the
coexistence with these bacteria to survive. There are two known mechanisms that
describe how rhamnolipids improve the substrate uptake. It is being done by solu-
bilizing the otherwise insoluble substrate, making them more usable for cells. The
second mechanism is by introducing changes into the membrane of cells. Numerous
investigations demonstrated that rhamnolipids make changes in microbial cell sur-
face properties by increasing the contact area between the cells and carbon source
thus increasing the carbon uptake (Chrzanowski et al., 2011). The mechanism of
changes that rhamnolipids initiate on the membrane needs to be investigated further.
Rhamnolipids also facilitate the bacterial movement from depleted environments
toward nitrogen- and phosphorus-rich ones (Chrzanowski et al., 2011). These find-
ings coincide with the findings of Mulligan and Gibbs (1989a) that the production of
rhamnolipids increases in nitrogen- and phosphorus-depleted environments.
Rhamnolipid is a valuable glycolipid biosurfactant that is primarily produced
extracellular by
P. aeruginosa
(Mulligan and Gibbs, 1993). There has been some
research to find new sources of bacteria for rhamnolipid production, but it should be
noted that the producing strains affect the quality of rhamnolipids and their efficiency
in specific applications, as well as the effect they have on other microorganisms.
For example, rhamnolipids produced by
P. aeruginosa
are able to increase nutrient
uptake for specific groups of bacteria but have no effect on other unrelated spe-
cies (Banat et al., 2010). At the same time, biosurfactants produced by
Rhodococcus
erythropolis
are able to stimulate the uptake even in a few unrelated species (Banat
et al., 2010).
By using the appropriate reactor, not only the rate of the productions can be
increased but also the cost of the production can be decreased. The experiments of
Rahman et al. (2010) consisted of cultivating
P. aeruginosa
DS10 in a microbiore-
actor that was made of polytetra fluoroethylene and compared the result with those
from bacteria cultivated in the regular bioreactors. Their experiments, cultivating the
bacteria in the microbioreactor, resulted in producing 106 μg/mL of rhamnolipids
during the stationary phase of bacterial growth. The produced rhamnolipids were
able to reduce the surface tension of distilled water from 72 to 27.9 mN/m and emul-
sify kerosene by 71.3%.
For cultivating the microorganisms that are aerobic, aeration is an important fac-
tor that affects the production rate. Speed of agitation is another factor that affects the
production rate. Mulligan and Gibbs (2004) conducted some experiments on com-
paring the production of rhamnolipid in a continually stirred tank reactor (CSTR)
with a sequencing batch reactor (SBR) and by using diesel-contaminated soil. They
stated that the production of rhamnolipid occurred only in the SBR reactor, and no
production of rhamnolipid was observed in the CSTR reactor. They also reported
that the produced rhamnolipid had a CMC value of 70 mg/L and was able to emulsify
the diesel. Although this emulsifying property is highly concentration dependent, it
was observed that by increasing the concentration of biosurfactant, emulsification of
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