A urinary isolate of *Escherichia coli* carries the plasmid‑mediated *mcr‑1* gene. (a) Explain how the *mcr‑1* gene confers resistance to colistin. (b) Suggest an alternative oral antibiotic that would likely be effective against this strain and justify your choice based on its mechanism of action and the resistance profile.
Answer
(a) *mcr‑1* encodes a phosphoethanolamine transferase that adds phosphoethanolamine to lipid A, reducing colistin binding. (b) Oral ciprofloxacin is likely effective because it inhibits DNA gyrase and is not affected by *mcr‑1* resistance.
Step-by-step solution
- Describe *mcr‑1* as a gene encoding a phosphoethanolamine transferase.
- Explain the enzyme adds phosphoethanolamine to the lipid A of lipopolysaccharide.
- State that this modification reduces the negative charge, diminishing colistin binding.
- Choose ciprofloxacin as an oral option.
- Justify: ciprofloxacin targets bacterial DNA gyrase; *mcr‑1* does not confer resistance to fluoroquinolones.
Common mistake: Believing *mcr‑1* also confers β‑lactam resistance
Concept tested: Plasmid‑mediated colistin resistance mechanism
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