Why did E. coli only grow on the P+ side of the LB/amp plate?

Why did E. coli only grow on the P+ side of the LB/amp plate?

• A. The plasmid conferred ampicillin resistance, enabling survival.
• B. Ampicillin was absent on the P+ side.
• C. The plate had more nutrients on the P+ side.
• D. Arabinose was present on the P+ side to induce expression.

Answer: (A)

Antibiotic selection is being tested. On a plate that contains ampicillin, only bacteria that carry a plasmid with the ampicillin resistance gene can survive and form colonies. The P+ side has the plasmid, so those E. coli express resistance and grow despite the antibiotic. The P- side lacks the plasmid, making the cells susceptible to ampicillin, so they don’t grow there. The presence of arabinose isn’t needed for ampicillin resistance; it would affect expression of other genes on the plasmid, not survival in this antibiotic-containing medium.

Antibiotic selection is being tested. On a plate that contains ampicillin, only bacteria that carry a plasmid with the ampicillin resistance gene can survive and form colonies. The P+ side has the plasmid, so those E. coli express resistance and grow despite the antibiotic. The P- side lacks the plasmid, making the cells susceptible to ampicillin, so they don’t grow there. The presence of arabinose isn’t needed for ampicillin resistance; it would affect expression of other genes on the plasmid, not survival in this antibiotic-containing medium.