营养缺陷到富集法 Penicillin enrichment Resuspend in medium minus Auxotrophs one nutrient Transfer to Centrifuge plus penicillin rich medium Replica plate on Cell pellet medium minus nutrient Mutagenized Prototrophs grow culture in rich and 99% are killed by medium contains penicillin. Auxotrophs auxotrophic and cannot grow and are prototrophic cells not killed. Culture is only prototrophs grow. enriched for Locate auxotrophic auxotrophs. colonies on origInal plate.营养缺陷型富集法