A food animal survey identifies an Escherichia coli isolate with colistin MIC of 16 mg/L despite no exposure to colistin. Sequencing reveals a plasmid-borne gene encoding a phosphoethanolamine transferase. Which gene has been detected and what is its effect?
- A pmrAB mutation, which removes the negative charge of lipopolysaccharide
- B mcr-1, which adds phosphoethanolamine to lipid A and reduces polymyxin binding ✓
- C arnBCADTEF operon, which adds 4-aminoarabinose to lipid A
- D phoPQ mutation, which downregulates the MgrB regulator
Correct answer: B. mcr-1, which adds phosphoethanolamine to lipid A and reduces polymyxin binding
Explanation
mcr-1 was the first described plasmid-mediated, horizontally transmissible colistin resistance gene. It encodes a phosphoethanolamine transferase that modifies the phosphate group of lipid B, decreasing the net negative charge of lipopolysaccharide so that polymyxins bind less avidly. pmrAB, phoPQ and arn operon changes are chromosomal, mutational routes seen mainly in Acinetobacter and Klebsiella, and cannot spread between strains by conjugation, which is what makes mcr-1 epidemiologically important.
Reference: Harrison's Principles of Internal Medicine, 21st ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
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