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Zorluk: Çok zorInfectious Disease Mechanisms

During routine inpatient monitoring, blood cultures drawn from a 5858-year-old male return positive for Gram-positive cocci in clusters. The patient was admitted 1818 days ago for right-sided infective endocarditis and was initially treated with intravenous vancomycin. On hospital day 66, his regimen was changed to daptomycin due to a rising vancomycin minimum inhibitory concentration (MIC) and persistent bacteremia. He initially improved, but over the last 4848 hours, he has developed recurrent high-grade fevers. Repeat susceptibility testing of the new isolate demonstrates a daptomycin MIC of 8 \mug/mL8 \text{ \mu g/mL} (susceptible is ≤1 \mug/mL\leq 1 \text{ \mu g/mL}). Which of the following best describes the underlying molecular mechanism responsible for this pathogen's newly acquired drug resistance?

  1. A
    Substitution of D-lactate for D-alanine at the peptidoglycan precursor terminus
  2. B
    Production of a hydrolytic enzyme that cleaves the drug's cyclic lipopeptide core
  3. Enhanced incorporation of cationic amino acids into the phospholipid bilayerCevap
  4. D
    Downregulation of outer membrane porin channel expression
  5. E
    Target site methylation of the 50S ribosomal subunit

Cevap

Enhanced incorporation of cationic amino acids into the phospholipid bilayer
The correct answer accurately describes the mprF-mediated mechanism of daptomycin resistance in Staphylococcus aureus. The mprF gene encodes a synthase/flippase enzyme that catalyzes the transfer of the cationic (positively charged) amino acid lysine to phosphatidylglycerol. Gain-of-function mutations in mprF, which frequently emerge during prolonged daptomycin therapy, lead to an accumulation of lysyl-phosphatidylglycerol in the outer leaflet of the cell membrane. Because the active daptomycin-calcium complex is also positively charged, this membrane alteration creates electrostatic repulsion, preventing the drug from inserting into the membrane and depolarizing the cell.

Adım Adım Çözüm

1
Analyze the clinical and microbiological data provided in the stem.
The patient has endocarditis caused by Gram-positive cocci in clusters, which is highly diagnostic for Staphylococcus aureus. The organism developed high-level resistance to daptomycin (MIC rising to 8 \mu g/mL) during active therapy.
Identifying the specific pathogen and the drug failing therapy is necessary to pinpoint the exact cellular mechanism of resistance.
2
Determine the mechanism of action of the drug in question (daptomycin).
Daptomycin is a cyclic lipopeptide antibiotic that requires calcium ions to insert into the Gram-positive bacterial cell membrane, leading to rapid depolarization, potassium efflux, and cell death.
Understanding how the drug attacks the cell provides the foundation for understanding how the pathogen evades it.
3
Identify the primary mechanism of daptomycin resistance in Staphylococcus aureus.
Resistance emerges primarily through mutations in the multiple peptide resistance factor (mprF) gene.
Step 3 level exams frequently test the exact genetic or molecular adaptations bacteria make to survive specific targeted therapies.
4
Translate the genetic mutation (mprF) into its structural and functional consequence.
The mprF mutation increases the addition of the positively charged amino acid lysine to membrane phosphatidylglycerol (forming lysyl-phosphatidylglycerol). This increases the net positive charge of the outer leaflet of the cell membrane, electrostatically repelling the positively charged daptomycin-calcium complex.
Connecting the genetic mutation to the phenotypic change (electrostatic repulsion via cationic amino acids) leads directly to the correct answer.

Anahtar Kavram

The primary mechanism of daptomycin resistance in Staphylococcus aureus involves mutations in the mprF gene, leading to increased incorporation of positively charged lysine into the cell membrane, which electrostatically repels the drug.
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Infectious Disease Mechanisms

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