Biomedical Engineering Reference
In-Depth Information
Fig. 4 Double-layer agar
plate for screening cellulase-
producing filamentous fungi
Another plating technique was used by Chinese scientists in selecting fungal
strains secreting bG with high activity. Esculin and ammonium ferric citrate were
added to the top layer of a double-layer agar plate. Esculin can be digested by bG,
generating esculetin, which reacts with ferric ions and turns black. Black circles
were observed around colonies secreting bGs, and the size of the circle can be used
for determining the level of bG secretion. Through this approach, P. decumbens
strain JU5-5 was obtained by Liu et al. [
66
], which has 100% greater bG secretion
than the traditional industrial strain JU-A10.
The genera Trichoderma and Aspergillus are the most extensively studied
cellulase-producing microorganisms. Enzymes produced by these fungi are com-
mercially available for industrial production. Microorganisms of the genus
Trichoderma produce relatively large quantities of EGs and CBHs, but only low
levels of bG, whereas Aspergillus species produce relatively large quantities of
EGs and bGs and low levels of CBHs.
To obtain higher titers and more effective cellulolytic systems, many novel
cellulase-producing strains have been screened by Chinese scientists. Although
T. reesei is the most extensively studied cellulase-producing industrial microor-
ganism for its high cellulolytic capabilities, many other novel cellulase-producing
strains have also been screened.
T. reesei produces only low levels of bG despite its high level of extracellular
protein expression. Chinese scientists therefore attempted to look for Trichoderma
strains that have higher bG activity. Two new Trichoderma viride strains, obtained
using random mutagenesis, were isolated by Jiang et al. [
67
] and Xu et al. [
68
].
These strains were observed to produce higher bG activity than T. reesei. Wang
et al. [
69
] suggested that the optimum temperature for the psychrophilic cellulases
produced by oceanic bacteria is 15-25 C, lower than that of regular cellulases
(45-55 C). Recently, psychrophilic cellulases produced by Trichoderma and
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