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Zorluk: Çok zorInfectious Disease Mechanisms

A 68-year-old man is hospitalized in the intensive care unit for septic shock secondary to a complicated intra-abdominal infection following a ruptured appendix. He has a prolonged hospital course complicated by acute respiratory distress syndrome requiring mechanical ventilation. During his third week of hospitalization, he develops a new-onset fever, increased vasopressor requirements, and purulent endotracheal secretions. Blood and sputum cultures grow Klebsiella pneumoniae. Initial antimicrobial susceptibility testing reveals resistance to all tested penicillins, cephalosporins, and carbapenems. The patient is treated empirically with ceftazidime-avibactam based on local antibiogram data. However, 72 hours later, he remains febrile and hemodynamically unstable. Repeat susceptibility testing confirms that the isolate is resistant to ceftazidime-avibactam. Further specialized susceptibility testing reveals the isolate is susceptible to the combination of aztreonam and avibactam, as well as to cefiderocol. Which of the following best describes the underlying molecular mechanism responsible for this organism's specific resistance profile?

  1. Cleavage of the beta-lactam ring mediated by a zinc-dependent enzymeCevap
  2. B
    Cleavage of the beta-lactam ring mediated by an active-site serine enzyme
  3. C
    Structural alteration of penicillin-binding proteins preventing drug target engagement
  4. D
    Downregulation of outer membrane porin channels coupled with overexpression of efflux pumps
  5. E
    Enzymatic modification of the antimicrobial agent via covalent addition of an acetyl group

Cevap

Cleavage of the beta-lactam ring mediated by a zinc-dependent enzyme
The correct answer describes the mechanism of metallo-beta-lactamases (MBLs), such as the New Delhi metallo-beta-lactamase (NDM). The patient's Klebsiella pneumoniae isolate is a carbapenem-resistant Enterobacteriaceae (CRE). Novel beta-lactamase inhibitors like avibactam effectively inhibit serine-based carbapenemases (Ambler Class A, e.g., KPC), which is why ceftazidime-avibactam is often used for CRE. However, avibactam cannot inhibit zinc-dependent MBLs (Ambler Class B), explaining the isolate's resistance to ceftazidime-avibactam. Aztreonam, a monobactam, is structurally unique and is not hydrolyzed by MBLs. While MBL-producing organisms typically co-produce other beta-lactamases (like ESBLs or AmpC) that can destroy aztreonam, combining aztreonam with avibactam allows avibactam to inhibit the serine beta-lactamases while aztreonam evades the MBL, restoring activity. Therefore, the highly specific susceptibility pattern (ceftazidime-avibactam resistant, aztreonam-avibactam susceptible) is pathognomonic for an MBL, which utilizes an active-site zinc ion to hydrolyze beta-lactam antibiotics.

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1
Analyze the organism and the initial resistance profile.
Klebsiella pneumoniae resistant to penicillins, cephalosporins, and carbapenems.
This broad-spectrum resistance indicates the presence of a carbapenemase, which can be either a serine carbapenemase (e.g., KPC) or a metallo-beta-lactamase (e.g., NDM).
2
Evaluate the failure of ceftazidime-avibactam therapy.
The organism is resistant to ceftazidime-avibactam.
Avibactam is a novel beta-lactamase inhibitor that effectively inhibits active-site serine beta-lactamases (Ambler classes A, C, and some D) but has no activity against zinc-dependent metallo-beta-lactamases (Ambler class B). Resistance to ceftazidime-avibactam strongly points to a metallo-beta-lactamase.
3
Interpret the susceptibility to aztreonam-avibactam.
The organism remains susceptible to this specific drug combination.
Aztreonam is structurally unique and not hydrolyzed by metallo-beta-lactamases. However, metallo-beta-lactamase-producing organisms often co-produce ESBLs or AmpC enzymes that destroy aztreonam. Combining aztreonam with avibactam (which inhibits the ESBLs/AmpC) allows aztreonam to remain effective, confirming the presence of a zinc-dependent metallo-beta-lactamase.

Anahtar Kavram

Differentiating carbapenemase resistance mechanisms (metallo-beta-lactamases vs. serine beta-lactamases) based on susceptibility to novel beta-lactamase inhibitors and monobactams.
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Konular

Infectious Disease Mechanisms

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