Multiple Choice
Plasmids function in bacterial cells to:
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7. Prokaryotic Cell Structures & Functions
Introduction to Bacterial Cell Walls
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Which of the following best explains the presence of nontransformed bacteria growing on an LB/ampicillin (LB/amp) plate during a transformation experiment?
A
The LB/amp plate was prepared without ampicillin.
B
All bacteria were successfully transformed with the plasmid containing the ampicillin resistance gene.
C
Ampicillin is not effective against any bacterial cell walls.
D
The bacteria possess natural resistance to ampicillin due to a mutation in their genome.
Verified step by step guidance1
Understand the purpose of the LB/ampicillin (LB/amp) plate: it is designed to select for bacteria that have been transformed with a plasmid containing an ampicillin resistance gene. Only transformed bacteria should grow on this plate because ampicillin kills bacteria without resistance.
Consider what it means if nontransformed bacteria (those without the plasmid) are growing on the LB/amp plate. Normally, these bacteria should be killed by ampicillin, so their growth suggests an unusual situation.
Evaluate the possible explanations: if the plate was prepared without ampicillin, all bacteria would grow, but the problem specifies the plate is LB/amp. If all bacteria were transformed, there would be no nontransformed bacteria growing. Ampicillin is generally effective against bacteria by inhibiting cell wall synthesis, so that option is unlikely.
Focus on the explanation that nontransformed bacteria possess natural resistance due to a mutation. This means some bacteria have genetic changes that allow them to survive ampicillin without the plasmid, leading to their growth on the selective plate.
Conclude that the presence of nontransformed bacteria on the LB/amp plate is best explained by natural or spontaneous mutations conferring ampicillin resistance, allowing these bacteria to survive despite lacking the plasmid.
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