Athough an industrial micro-organism may produce large quantities of a desirable metabolite it may also produce a large amount of a metabolite which is not required, is toxic or may interfere with the extraction process. Thus, in these circumstances it would be an advantage to alter the strain such that the undesirable product is no longer produced. An example in the penicillin-producing strains is the elimination of the production of the yellow pigment, chrysogenein, by the selection of non-pigmented mutants which made the extraction of the antibiotic much simpler (Backus and Stauffer, 1955).
Dolezilova et al. (1965) considered the production of fungicidin (nystatin) and cycloheximide by mutants of S. noursei. These workers demonstrated that mutants could be isolated which produced increased levels of fungicidin but produced no cycloheximide. The use of protoplast fusion techniques for the elimination of p~0\i penicillin V has been considered earlier in the chapter.
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