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Testing Purified Cultures, Scoring
Once a strain has been purified, its phenotype can be tested. Suppose, for example, that a desired bacterial strain is claimed to be unable to use lactose as a sole source of carbon and to be unable to synthesize arginine. What is the simplest way to test or score for Lac- Arg-? One satisfactory growth medium for the mutant would be a medium con-taining K+, Na+, PO4- -, SO4- -, Mg++, Ca++, Fe++, NH4+, and other trace elements which are present as chemical contaminants of these ingredi-ents, plus glucose at a concentration of 2 gm/liter and L-arginine at 0.1 gm/liter. This is called a minimal salts medium with glucose and arginine. Each candidate colony could be spotted with sterile wooden sticks or toothpicks onto minimal salts medium containing the follow - ing additions: glucose, glucose plus arginine, and lactose plus arginine. The desired strain should grow only on glucose plus arginine medium (Table 8.1).
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