1 A plate of bacterial colonies is pressed onto the surface of sterile velvet. 2 Cells adhering to the velvet are transferred to the sterile media, resulting in exact replicas of the original plate. Master plate with bacterial colonies (nutrient agar) Sterile plate; nutrient agar Sterile plate; glucose-salts agar Pressed onto sterile velvet Pressed to velvet Sterile velvet - Colonies imprinted on velvet - Plates incubated - Position of missing auxotroph Auxotroph - Nutrient agar; all colonies grow. Glucose-salts agar; auxotrophs do not grow. 3 Auxotrophic mutants form colonies on the nutrient agar but not on the glucose-salts agar. FIGURE 8.14 Indirect Selection of Mutants by Replica Plating The procedure shown was used by the Lederbergs and continues to be used today in many laboratories. Mutants are identified by comparing the growth of colonies on the two plates.

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