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Difficulty: MediumInfectious Disease Mechanisms

A 34-year-old female teacher is evaluated in the urgent care center for a 2-day history of dysuria and increased urinary frequency. She reports no fever, flank pain, nausea, or vaginal discharge. Her medical history is significant for three uncomplicated urinary tract infections over the past year. Her most recent infection occurred three weeks ago and was treated with a 5-day course of ciprofloxacin. Vital signs are within normal limits. Physical examination reveals mild suprapubic tenderness on palpation but no costovertebral angle tenderness. Urinalysis is positive for leukocyte esterase and nitrites. A urine culture subsequently yields >100,000 colony-forming units/mL of Escherichia coli. Susceptibility testing indicates that the isolate is resistant to ciprofloxacin and levofloxacin, but remains susceptible to nitrofurantoin, fosfomycin, and trimethoprim-sulfamethoxazole. Which of the following is the most likely mechanism underlying this pathogen's resistance to the previously prescribed antibiotic?

  1. Amino acid substitutions in bacterial DNA gyrase and topoisomerase IVAnswer
  2. B
    Hydrolysis of the antimicrobial agent by plasmid-encoded beta-lactamases
  3. C
    Post-transcriptional methylation of the 23S ribosomal RNA subunit
  4. D
    Covalent modification of the drug by bacterial acetyltransferases
  5. E
    Overproduction of para-aminobenzoic acid by the target bacteria

Answer

Amino acid substitutions in bacterial DNA gyrase and topoisomerase IV
The patient has a urinary tract infection caused by a fluoroquinolone-resistant strain of Escherichia coli, likely selected for by her recent course of ciprofloxacin. Fluoroquinolones function by inhibiting bacterial DNA gyrase (topoisomerase II) and topoisomerase IV, enzymes critical for unwinding DNA during replication. In Gram-negative organisms such as E. coli, the most common and clinically significant mechanism of fluoroquinolone resistance involves chromosomal mutations in the genes coding for these enzymes (specifically gyrA and parC). These mutations result in single amino acid substitutions that alter the structure of the target binding site, significantly decreasing the affinity of the fluoroquinolone for the enzyme and allowing bacterial DNA replication to proceed unimpeded.

Step-by-Step Solution

1
Identify the antibiotic class that the pathogen is resistant to based on the clinical vignette.
The patient previously received ciprofloxacin, and the isolate is resistant to both ciprofloxacin and levofloxacin, which are fluoroquinolones.
Recognizing the drug class is the prerequisite for determining the specific molecular mechanism of action and resistance.
2
Recall the mechanism of action of fluoroquinolones.
Fluoroquinolones exert bactericidal effects by inhibiting bacterial DNA gyrase (topoisomerase II) and topoisomerase IV, thereby preventing DNA replication.
Resistance mechanisms directly relate to how the drug interacts with the bacterial cell.
3
Identify the primary mechanism by which Gram-negative bacteria like E. coli develop resistance to this drug class.
Resistance primarily emerges via chromosomal point mutations in the gyrA and parC genes, which lead to target site structural alterations (amino acid substitutions) in DNA gyrase and topoisomerase IV.
This structural change prevents the fluoroquinolones from effectively binding to the target enzymes.

Key Concept

Fluoroquinolone Resistance Mechanism
Estimated Time:1m 15s
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Topics

Infectious Disease Mechanisms

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