Why is ampicillin added to the agar plate in plasmid experiments?

Why is ampicillin added to the agar plate in plasmid experiments?

• A. To promote growth of all bacteria on the plate
• B. To select for cells that have integrated the plasmid into the chromosome
• C. To select for bacterial cells that contain the plasmid due to antibiotic resistance
• D. To measure transformation efficiency by colony color

Answer: (C)

Ampicillin selection relies on antibiotic resistance encoded on the plasmid. After you introduce the plasmid into bacterial cells, you plate them on medium containing ampicillin. Only the cells that have taken up the plasmid and express the resistance gene can survive the antibiotic, while non-transformed cells are inhibited or killed. This makes the growing colonies a direct readout of which cells acquired the plasmid. This approach doesn’t promote growth of all bacteria, since the antibiotic blocks growth of those without resistance. It isn’t about the plasmid integrating into the chromosome—selection is tied to the presence of the resistance gene, which is carried by the plasmid in standard experiments. And it isn’t a method to measure transformation efficiency by colony color; color screening uses a reporter gene (like lacZ or GFP) for visual signals, whereas ampicillin selection simply indicates which cells carry the plasmid.

Ampicillin selection relies on antibiotic resistance encoded on the plasmid. After you introduce the plasmid into bacterial cells, you plate them on medium containing ampicillin. Only the cells that have taken up the plasmid and express the resistance gene can survive the antibiotic, while non-transformed cells are inhibited or killed. This makes the growing colonies a direct readout of which cells acquired the plasmid.

This approach doesn’t promote growth of all bacteria, since the antibiotic blocks growth of those without resistance. It isn’t about the plasmid integrating into the chromosome—selection is tied to the presence of the resistance gene, which is carried by the plasmid in standard experiments. And it isn’t a method to measure transformation efficiency by colony color; color screening uses a reporter gene (like lacZ or GFP) for visual signals, whereas ampicillin selection simply indicates which cells carry the plasmid.