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Difficulty: Very hardInfectious Diseases

A 64-year-old man is evaluated in the hospital for a new-onset fever. He was admitted 12 days ago for severe acute pancreatitis complicated by a prolonged ileus, which necessitated the placement of a right internal jugular central venous catheter for total parenteral nutrition. Yesterday, he developed a fever of 39.2°C (102.6°F) and chills. Blood cultures were drawn, and empiric intravenous vancomycin and piperacillin-tazobactam were initiated. Today, his temperature is 37.6°C (99.7°F), blood pressure is 118/76 mm Hg, and pulse is 88/min. The central venous catheter has been removed. Blood cultures from both the catheter tip and a peripheral vein are growing *Enterobacter cloacae*. Antimicrobial susceptibility testing results are as follows:

AntimicrobialInterpretation
AmpicillinResistant
CefazolinResistant
CeftriaxoneSusceptible
CefepimeSusceptible
Piperacillin-tazobactamSusceptible
MeropenemSusceptible
CiprofloxacinSusceptible

Which of the following is the most appropriate next step in this patient's pharmacotherapy?

  1. A
    Continue intravenous piperacillin-tazobactam
  2. B
    Change to intravenous ceftriaxone
  3. Change to intravenous cefepimeAnswer
  4. D
    Change to intravenous meropenem
  5. E
    Change to oral ciprofloxacin

Answer

The most appropriate next step is to change to intravenous cefepime.
The correct decision is to change therapy to intravenous cefepime. *Enterobacter cloacae* is a classic member of the organisms (along with *Citrobacter freundii* and *Klebsiella aerogenes*) that harbor a chromosomal AmpC beta-lactamase gene. Exposure to third-generation cephalosporins (such as ceftriaxone) can induce the hyperproduction, or derepression, of this enzyme. This leads to profound clinical failure even if the isolate initially tests susceptible in vitro, meaning third-generation cephalosporins are contraindicated for invasive AmpC infections. Cefepime, a fourth-generation cephalosporin, is a weak inducer of AmpC and structurally withstands hydrolysis by the enzyme, making it the preferred definitive therapy. While meropenem is also effective, cefepime is favored to maintain good antibiotic stewardship and spare carbapenems.

Step-by-Step Solution

1
Identify the organism and its characteristic resistance mechanisms.
The organism is Enterobacter cloacae, a known producer of chromosomal AmpC beta-lactamase.
Recognizing organisms with inducible AmpC (e.g., Enterobacter, Klebsiella aerogenes, Citrobacter freundii) is critical because in vitro susceptibility does not always equal in vivo efficacy.
2
Evaluate the susceptibility report in the context of AmpC production.
Although ceftriaxone and piperacillin-tazobactam test susceptible in vitro, they are associated with high failure rates in vivo due to AmpC derepression and weak inhibition, respectively.
Standard automated susceptibility testing does not account for the high likelihood of resistance emerging during therapy with specific beta-lactams.
3
Select the most appropriate definitive therapy balancing efficacy and antibiotic stewardship.
Cefepime is chosen because it is stable against AmpC hydrolysis and is narrower in spectrum than meropenem.
Guidelines recommend cefepime for invasive AmpC infections, reserving carbapenems to prevent the emergence of carbapenem-resistant Enterobacterales.

Key Concept

Definitive management of severe infections caused by organisms with inducible AmpC beta-lactamase (e.g., Enterobacter cloacae) requires cefepime or a carbapenem; third-generation cephalosporins and piperacillin-tazobactam should be avoided despite in vitro susceptibility.
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Infectious Diseases

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