The formula of Lb medium is 1% tryptone, 1% sodium chloride, 0.5% yeast extract and 1% agar.

Materials: 50mL bacterial culture dish with LB medium (1% peptone, 1% sodium chloride, 0.5% yeast extract, 1.2% agar), 3 engineering strains of Escherichia coli DH5a (or other antibiotics such as kana, but the drug resistance gene changed correspondingly), and transformed into Amp (ampicillin). Hereinafter referred to as AMP-R (ampicillin resistance), take 100μL (bacterial density: OD about 0.6-0.8); Another plasmid with similar length and no ampicillin resistance gene is Escherichia coli (hereinafter referred to as AMP-S), which takes 200μL l.. (Bacterial density: OD about 0.6-0.8) Process: The following operations are all carried out in a clean room. 1, spread the LB culture medium under the pressure of damp heat and sterilize it, and add 10mL to each of the three Petri dishes, numbered as A, B and C; 2. Add antibiotics. Before curing, ampicillin with the concentration of 100μg/mL was added to Petri dishes A and B, but not to Petri dish C. Inoculation experimental group: Inoculation of Amp-r strain 100μ l negative control: Inoculation of AMP-R strain 100μ l positive control in Petri dish B: Inoculation of AMP-R strain 100μL positive control in Petri dish C. The expected experimental results of 12- 10 cultured in a 37-degree incubator show that there are a large number of colonies in experimental group A and positive control group C, while there are no colonies or only one or two colonies in negative control group B (because the possibility of spontaneous mutation producing ampicillin resistance still exists). It is proved that antibiotics can effectively inhibit the growth of sensitive strains, while drug-resistant strains are ineffective. The answers are all handwritten and original according to my experimental experience.