In a transformation experiment, a sample of E. coli bacteria was mixed with a plasmid containing the gene for resistance to the antibiotic ampicillin (ampr). Plasmid was not added to a second sample. Samples were plated on nutrient agar plates, some of which were supplemented with the antibiotic ampicillin. The results of E. coli growth are summarized below. The shaded area represents extensive growth of bacteria; dots represent individual colonies of bacteria.

Plates that have only ampicillin-resistant bacteria growing include which of the following?

Question

In a transformation experiment, a sample of E. coli bacteria was mixed with a plasmid containing the gene for resistance to the antibiotic ampicillin (ampr). Plasmid was not added to a second sample. Samples were plated on nutrient agar plates, some of which were supplemented with the antibiotic ampicillin. The results of E. coli growth are summarized below. The shaded area represents extensive growth of bacteria; dots represent individual colonies of bacteria.

Plates that have only ampicillin-resistant bacteria growing include which of the following?

BlackTom AI · Accepted Answer

To tackle this question, we must compare growth patterns across plates with and without ampicillin, and relate them to the presence or absence of the ampicillin resistance gene (ampr) on the plasmid.
Option I only: This choice would assert that only plate I shows growth of ampicillin-resistant bacteria. However, plate I is the wild-type E. coli with no plasmid, which should not acquire ampicillin resistance and thus should not grow on ampicillin-containing plates. Since plate II (ampicillin without plasmid) shows no growth, plate I alone does not indicate ampicillin-resistant growth on ampicillin-containing conditions. Therefore, this option misreads the expected pattern of resistance versus sensitivity.
Option II: This option claims that both plate I and plate II show ampicillin-resistant growth. Plate II is the wild-type strain without the resistance plasmid and, on ampicillin-containing medium, should be inhibited, not growing. The presence of resistance requires the ampr gene from the plasmid; without plasmid, there is no resistance, so plate II cannot show resistant growth. Hence this option is incorrect because it treats the wild-type as resistant.
Option III: This option states that only plates III would show ampicillin-resistant growth. Plate III is the same wild-type background as plate I but with the plasmid not present. Since plasmid is necessary to confer ampicillin resistance, and plate III corresponds to no plasmid, there would be no growth on ampicillin for this plate. The pattern on plate III thus does not reflect ampicillin resistance and makes this option invalid.
Option IV: This option says that IV only shows ampicillin-resistant growth. Plate IV is the plate where the E. coli carrying the ampr plasmid is plated on ampicillin-containing medium, and indeed colonies are visible, indicating that cells harboring the resistance gene survive in the presence of ampicillin. The shaded growth in other panels confirms that without the plasmid (plates I and II) or in the absence of antibiotic (plate III), there is no selective growth of resistant cells. Therefore, plate IV uniquely demonstrates ampicillin-resistant growth due to the plasmid-borne ampr gene, and this correctly identifies the plate with only ampicillin-resistant bacteria growing.
In summary, the core concept is the necessity of the ampr plasmid for survival on ampicillin-containing media; without the plasmid, cells cannot resist the antibiotic, so they do not form colonies on ampicillin plates, while only those with the plasmid (as in plate IV) will grow there. The incorrect options fail to align with this logic by either attributing resistance to the wild-type background or by misidentifying which plate contains resistant colonies.

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