Which technique is used to identify bacteria containing the plasmid by checking the marker gene expression after replica plating?

• A. Microscopy
• B. Growth rate
• C. Sequencing
• D. Replica plating

Answer: (D)

Replica plating relies on a phenotype produced by the plasmid’s marker gene. In this approach, colonies are transferred in the same pattern from a master plate onto plates with selective conditions (for example, an antibiotic-containing medium). Only bacteria that carry the plasmid and express the marker gene will survive or show the desired expression on the selective plate. This makes it a direct, phenotype-based screen to identify plasmid-bearing cells without needing to observe cells under a microscope, measure general growth rate, or sequence DNA. Microscopy, growth rate alone, and sequencing don’t align with this pattern-based, marker expression check, whereas replica plating specifically uses the marker’s expression to pinpoint the colonies that contain the plasmid.