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Difficulty: Very hardInfectious Diseases

A 68-year-old male is currently receiving care in the intensive care unit for acute respiratory distress syndrome secondary to severe acute pancreatitis. He has been mechanically ventilated for the past 11 days. Over the last 48 hours, his oxygen requirements have increased, and he has developed copious purulent endotracheal secretions. His temperature is 38.8°C (101.8°F), blood pressure is 110/65 mm Hg, and pulse is 112/min. Physical examination reveals coarse crackles in the right lung base. A portable chest radiograph demonstrates a new right lower lobe consolidation. Endotracheal aspirate and blood cultures are obtained, and the patient is empirically started on intravenous vancomycin, meropenem, and tobramycin.

Two days later, the patient's clinical status has improved, and he is afebrile. Blood cultures show no growth. The endotracheal aspirate culture yields heavy growth of *Enterobacter cloacae*. The antimicrobial susceptibility report is as follows:

AntimicrobialInterpretation
AmpicillinResistant
CefazolinResistant
CefepimeSusceptible
CeftriaxoneSusceptible
MeropenemSusceptible
Piperacillin-tazobactamSusceptible
TobramycinSusceptible

Which of the following is the most appropriate next step in the pharmacologic management of this patient's infection?

  1. Discontinue vancomycin and tobramycin; switch meropenem to cefepimeAnswer
  2. B
    Discontinue vancomycin and tobramycin; switch meropenem to ceftriaxone
  3. C
    Discontinue vancomycin and tobramycin; switch meropenem to piperacillin-tazobactam
  4. D
    Discontinue vancomycin and tobramycin; continue meropenem
  5. E
    Continue vancomycin, meropenem, and tobramycin

Answer

Discontinue vancomycin and tobramycin; switch meropenem to cefepime
The correct approach is to discontinue vancomycin and tobramycin and switch meropenem to cefepime. The patient has ventilator-associated pneumonia caused by Enterobacter cloacae. Once culture and susceptibility data are available, empiric broad-spectrum therapy should be tailored to the narrowest effective agent. Enterobacter cloacae, Klebsiella aerogenes, and Citrobacter freundii are organisms with a moderate-to-high risk of inducible AmpC beta-lactamase production. When these organisms are exposed to third-generation cephalosporins (like ceftriaxone), derepressed mutants that hyperproduce AmpC are selected, leading to clinical failure despite reported in vitro susceptibility. Therefore, ceftriaxone must be avoided. Cefepime, a fourth-generation cephalosporin, forms a stable zwitterion that quickly penetrates the outer membrane and is a weak inducer of AmpC, making it stable against hydrolysis. It is the preferred agent when the minimum inhibitory concentration suggests susceptibility. Discontinuing vancomycin and tobramycin eliminates unnecessary MRSA and double-pseudomonal coverage, minimizing nephrotoxicity.

Step-by-Step Solution

1
Diagnose the clinical condition based on the vignette.
The patient has developed ventilator-associated pneumonia (VAP).
New fever, purulent secretions, increased oxygen requirements, and a new consolidation on chest radiography after 11 days of mechanical ventilation confirm VAP.
2
Analyze the culture data and pathogen characteristics.
The infection is monomicrobial, caused by Enterobacter cloacae. This organism possesses an inducible chromosomal ampC gene.
Enterobacter cloacae is one of the classic 'SPACE' or 'HECK-Yes' organisms that are at high risk for producing clinically significant AmpC beta-lactamases.
3
Select the optimal definitive antimicrobial therapy based on stewardship guidelines.
Cefepime is chosen to de-escalate therapy.
Cefepime is stable against AmpC hydrolysis and does not induce derepressed mutants. Ceftriaxone is contraindicated despite in vitro susceptibility. Meropenem is overly broad and should be spared for resistant pathogens.

Key Concept

Definitive antimicrobial selection for moderate-to-high risk AmpC-producing Enterobacterales requires agents stable against AmpC hydrolysis, such as cefepime, while avoiding third-generation cephalosporins despite apparent in vitro susceptibility.
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Topics

Infectious Diseases

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