Question 212

Question:

Escherichia coli strains X and Y are both resistant to ampicillin. Ampicillin resistance is stable in strain X when it is grown for multiple generations in the absence of the antibiotic. However, strain Y loses ampicillin resistance when it is grown in media without the antibiotic. Which of the following best explains the acquisition of ampicillin susceptibility in strain Y?

Correct Answer: C.  Loss of a plasmid carrying the resistance gene

Antimicrobial resistance in Escherichia coli is primarily conferred through transfer of a plasmid (choice C). This is true in the case of strains of Excherichia coli that carry a resistance to ampicillin. Without the need to produce the beta-lactamases needed to be resistant to ampicillin, it will not preferentially transfer the plasmid and it may be lost in subsequent generations.
The other four answer choices discuss changes to the actual gene which are not ways in which E. coli develops resistance.