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characteristics. If one looks for a particular strain, the World Data Center on microorganisms is available
to locate the strain in the particular affiliated culture collection. The majority of these available strains,
however, have neither been isolated nor explored with an aim to process development. It is therefore
necessary to search for new, more suitable cultures, which possess the properties for producing the
desired product in high yield, or reinvestigate the existing strains from culture collections with the same
aim, and at the same time economically utilize the available substrate. New cultures may be found by
chance observations (e.g. Fleming's culture of Penicillium notatum) or more likely by a systematic search.
A systematic search for new cultures may depend on two major approaches:
1. the pure scientific approach;
2. the process development oriented search.
Whichever direction is chosen, it is absolutely necessary to be well acquainted with the
microorganisms, that is, one must be able to place them correctly into the system of living entities. Every
isolation is connected with an evaluation of various features of microorganisms. The initial features in
microbial process development would undoubtedly be related to resource utilization and/ or product
formation. In sharp contrast to the usual requirements of academic research, organism isolation and initial
selection for an industrial process is dependent on a range of criteria that are relevant to the optimization
of the particular process. Their features may be morphological, physiological, genetically, immunological
etc. and the sum of all these features of a microorganism is referred to as its phenotype. A phenotype
therefore represents any measurable characteristic or distinctive trait possessed by an organism. In
contrast, genotype can be explored via the phenotypic expression.
The isolation, identification and initial selection of organisms for microbial process development
depends therefore on the phenotypic expression of the organism. Despite the selective aim, one should not
forget that every microbial culture must possess certain general attributes:
a. the strain should be a pure culture and be free of phages;
b. the strain must be genetically stable;
c. the strain must produce readily many vegetative cells, spores or other reproductive units;
d. the strain should grow vigorously and rapidly after inoculation;
e. the strain should produce the required product within a short period of time;
f. if possible, the strain should be able to protect itself against contamination;
g. the strain should produce the desired product, which should be easily separable from all others;
and
h. the strain should be amenable to change by certain mutagenic agents.
In most cases it is useful to isolate a culture from a natural resource of decomposing or organic
materials. Rapid screening techniques for testing the phenotypic expression normally combines isolation
and selection simultaneously. The techniques used for these tests are numerous and depend, of course, on
the expected phenotypic expression. Any isolated culture should immediately be deposited with a culture
collection for maintenance and preservation.
The isolation and identification of a new culture on phenotypic expression also gives some
indication on the metabolism of the organism. It is of utmost importance, however, to investigate in
details the basic metabolic processes of the organism as part of the selection program. Traditionally,
screening procedures are based on agar plate techniques or enrichment cultures. It should be realized that
both could be very restrictive if one aims at certain microbial process developments. The agar plate
techniques are very important for enzyme - and antibiotic - producing strains. They give excellent results
for polymer degradation (e.g. starch, cellulose) by exoenzymes or antibiotic production, that is,
phenotypic expression related to products excreted out of the cell. They also could be indicators for acidic
or alkaline product formation. However, these procedures are very labor-intensive and time-consuming.
Enrichment cultures, on the other hand, are carried out under substrate excess conditions and thus select
organisms on the basis of maximum specific growth rate. This characteristic may not be the key criterion
for the process being developed. It also must be realized that in batch enrichment the time of sampling is
important for the selection of the most desirable organism, since the growth conditions change as a
function of time. It could therefore be possible to miss the particular stage when the particular organism is
present in sufficient numbers to guarantee its isolation.
An attractive alternative has been developed more recently, which involves a continuous flow
enrichment technique. This technique allows the selection and isolation of organisms on the basis of their
substrate affinity (using a chemostat), maximum specific growth rate (using a turbidostat), resistance to
toxic materials, etc.
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