Agar plates, visual detection

Pigment formation can be used to visually detect mutants constitutive for enzymes of the purine pathway (Dorfman, 1969). The technique uses an auxotroph blocked early in the pathway which forms red colonies because of the polymerization into a red pigment of accumulated S-amino-4-imidazole ribonucleotide. Normally, high levels of adenine in the medium inhibit pigment formation because AMP represses the pathway. After mutation of the auxotroph, the population is plated on agar containing adenine derepressed mutant colonies are red. 

A variation of the plate test is the clear zone technique [53], sometimes used to screen polymers for biodegradability. A fine snspension of polymer is placed in an agar gel as the sole carbon source and the test inocnlnm is placed in wells bored into the agar. After incubation, a clear zone aronnd the well, detected visually or instrumentally, is indicative of utilisation of the polymer. The method has, for example, been used in the case of starch plastics [54], varions polyesters [55-57] and PU [58]. 

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