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Difficulty: HardInfectious Diseases

A 62-year-old man is evaluated in the intensive care unit for antimicrobial management. He was admitted 8 days ago after a severe motor vehicle collision resulting in multiple rib fractures and a pulmonary contusion. He has required mechanical ventilation since admission. On hospital day 6, he developed a new fever, increased purulent endotracheal secretions, and a right lower lobe consolidation on chest radiography. He was empirically started on intravenous vancomycin and piperacillin-tazobactam for suspected ventilator-associated pneumonia.

Today (hospital day 8), his temperature is 37.6°C (99.7°F), blood pressure is 122/78 mmHg, and heart rate is 84/min. Endotracheal aspirate cultures obtained on day 6 now show >105>10^5 CFU/mL of *Enterobacter cloacae*. Blood cultures remain sterile. Antimicrobial susceptibility testing for the *E. cloacae* isolate is reported below:

AntibioticResult
AmpicillinResistant
CefazolinResistant
CeftriaxoneSusceptible
CefepimeSusceptible
Piperacillin-tazobactamSusceptible
MeropenemSusceptible
TobramycinSusceptible

Which of the following is the most appropriate modification to this patient's pharmacotherapy?

  1. Discontinue piperacillin-tazobactam and vancomycin, and initiate intravenous cefepimeAnswer
  2. B
    Discontinue piperacillin-tazobactam and vancomycin, and initiate intravenous ceftriaxone
  3. C
    Discontinue vancomycin and continue intravenous piperacillin-tazobactam
  4. D
    Continue intravenous vancomycin and piperacillin-tazobactam for a full 7-day course
  5. E
    Discontinue piperacillin-tazobactam and vancomycin, and initiate intravenous tobramycin monotherapy

Answer

The most appropriate step is to discontinue piperacillin-tazobactam and vancomycin, and initiate intravenous cefepime.
The patient has ventilator-associated pneumonia (VAP) caused by *Enterobacter cloacae*. This organism is notorious for harboring an inducible, chromosomal *AmpC* beta-lactamase gene. A critical tenet of infectious disease management is recognizing that organisms with moderate-to-high risk of AmpC production (such as *Enterobacter*, *Citrobacter freundii*, and *Klebsiella aerogenes*) should not be treated with third-generation cephalosporins (e.g., ceftriaxone) or beta-lactam/beta-lactamase inhibitor combinations (e.g., piperacillin-tazobactam) for invasive infections. Even when these drugs are reported as 'susceptible' by the microbiology laboratory, exposing the organism to them selects for derepressed mutants that hyperproduce the AmpC enzyme, rapidly destroying the antibiotic and causing clinical failure.

The drugs of choice for invasive AmpC-producing infections are cefepime and carbapenems (e.g., meropenem), as they are relatively stable against AmpC hydrolysis. To practice appropriate antimicrobial stewardship and preserve carbapenems for highly resistant organisms, cefepime is the preferred step-down agent. Additionally, vancomycin must be discontinued because the culture did not grow methicillin-resistant *Staphylococcus aureus* (MRSA).

Step-by-Step Solution

1
Analyze the culture data and clinical scenario.
The patient has ventilator-associated pneumonia caused by Enterobacter cloacae. Blood cultures are negative, and there is no MRSA isolated.
Establishing the definitive pathogen determines the required spectrum of antimicrobial coverage.
2
Evaluate the need for continued empiric therapy.
Vancomycin can be safely discontinued.
Since respiratory cultures did not grow MRSA, continuing vancomycin violates antibiotic stewardship principles.
3
Select the appropriate definitive beta-lactam agent based on the organism's intrinsic resistance mechanisms.
Cefepime is selected over ceftriaxone or piperacillin-tazobactam.
Enterobacter cloacae possesses an inducible AmpC beta-lactamase gene. Treating invasive infections with ceftriaxone or piperacillin-tazobactam can induce enzyme production leading to clinical failure, despite apparent in vitro susceptibility.

Key Concept

Invasive infections caused by organisms with a moderate-to-high risk of inducible AmpC production (e.g., Enterobacter cloacae, Klebsiella aerogenes, Citrobacter freundii) should be treated with cefepime or a carbapenem, avoiding third-generation cephalosporins and piperacillin-tazobactam even if they appear susceptible in vitro.
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Topics

Infectious Diseases

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