Describe how a point mutation in the gyrA gene can lead to antibiotic resistance in Escherichia coli, giving a specific example of the mutation and the antibiotic it confers resistance to.
Answer
A single‑nucleotide change (T86I) in gyrA alters DNA gyrase, conferring resistance to fluoroquinolones such as ciprofloxacin.
Step-by-step solution
- 1. Identify the target: DNA gyrase (GyrA subunit) is essential for bacterial DNA replication and the target of fluoroquinolones.
- 2. Explain the mutation type: A point mutation substitutes threonine (T) with isoleucine (I) at position 86 (T86I).
- 3. Describe the effect: The amino‑acid change reduces binding affinity of fluoroquinolones to GyrA, allowing the enzyme to function even in the drug’s presence.
- 4. State the antibiotic affected: Fluoroquinolones (e.g., ciprofloxacin, levofloxacin).
- 5. Conclude that this single mutation is sufficient to produce high‑level resistance in E. coli.
Common mistake: confusing mutation in DNA gyrase with mutation in efflux pumps
Concept tested: mutation conferring antibiotic resistance
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