The most appropriate micro-organism for a potential process is usually found by isolation from a variety of sources, most commonly soil. The classical method of screening to obtain a suitable organism tended to be very time consuming, expensive and often without a very clear objective. Eli Lilly and Company Ltd. discovered three new antibiotics in 10 years while screening 400,000 micro-organisms (Nelson, 1961). More recently Berdy (1989) has speculated that the screening of 100,000 soil micro-organisms may lead to the isolation of 5 to 50 new compounds, but there is no guarantee after evaluation that a useful new drug or other product will be found.
If a desired characteristic, which gives the organism a selective advantage, has already been recognized, a screen might be designed incorporating this characteristic as a selective factor (Chapter 3). The isolation may begin with pretreatment of samples which favour the survival of the preferred organism. This is followed by growth on selective or non-selective media and often associated with batch or continuous enrichment. Important factors which will be of economic significance which might be selected in a well planned screen could include:
1. Growth on a simple cheap medium.
2. Growth at a higher temperature (to reduce cooling costs).
However, the synthesis of other microbial products (e.g. antibiotics) does not give the producing organism any selective advantage which might be used in an isolation procedure (Chapter 3). Therefore a collection of these organisms must be made before testing for the desired characteristic. Because many isolation procedures will lead to the rediscovery of known organisms with known activities it is important to use well planned, efficient isolation procedures which can prove very productive.
A number of approaches are currently being used to improve isolation procedures (see also Chapter 3). Numerical taxonomic data bases are being exploited to design selective media for certain microbial taxa. Using these data bases it has been possible to design media to encourage the growth of uncommon streptomycetes or discourage the growth of common species (Vickers et al, 1984; Williams and Vickers, 1988). Knowledge of antibiotic sensitivity gained from taxonomic data bases has led to the design of other selective media which will select for resistant groups of organisms (Goodfel-low and O'Donnell, 1989; Bull, 1992; Bull et al, 1992). The selection of antibiotic producing soil isolates has also been achieved using media designed by a stepwise discrimination analysis technique (Huck et al, 1991).
These 'designed' isolation media are now being used extensively for the isolation of novel and rare microorganisms. For example, within the actinomycetes, Streptomyces spp. have been extensively screened since the 1940s for antibiotic production with subsequent notable commercial success. More interest has now been shown in isolation of strains of the less common genera such as Actinomadura, Actinoplanes, Ki-tosatosporia, Streptoalloteichus, etc. which are producing other novel bioactive compounds (Goodfellow and O'Donnell, 1989; Bull, 1992).
It has become a common practice to obtain isolates from unusual habitats, which may include extreme environments, to ensure that the greatest microbial diversity is being examined (Bull, 1992; Bull et al., 1992; Chapter 3).
The screening tests which have been developed to detect new useful compounds of potential industrial interest have become much more selective and sensitive. Better knowledge of cell biochemistry has enabled the design of screens which are much more precisely targeted to detect the desired activity using specific detector strains (Chapter 3). Many large companies which undertake screening programmes have introduced some automation to enable high throughput rates which are cost effective and less labour intensive. In 1989, Nisbet and Porter considered that 105 tests per year with up to 20 assay tests should be undertaken in a worthwhile screening programme.
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