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Difficulty: Very hardInfectious Diseases

A 65-year-old woman is hospitalized due to a 2-day history of high fever, right-sided flank pain, and altered mental status. She has a history of poorly controlled type 2 diabetes mellitus and recurrent urinary tract infections. On admission, her temperature is 39.2°C (102.6°F), blood pressure is 88/56 mm Hg, pulse is 118/min, and respirations are 22/min. Physical examination is notable for severe right costovertebral angle tenderness. Laboratory studies show a leukocyte count of 18,500/mm³. Urinalysis is positive for leukocyte esterase, nitrites, and numerous leukocytes. Blood and urine cultures are obtained, and the patient is started on empiric intravenous piperacillin-tazobactam. Over the next 48 hours, her fever resolves, and her hemodynamics normalize.

Preliminary results from both blood and urine cultures grow *Enterobacter cloacae*. Antimicrobial susceptibility testing reveals the isolate is susceptible to ceftriaxone, cefepime, piperacillin-tazobactam, meropenem, and gentamicin. It is resistant to ciprofloxacin and trimethoprim-sulfamethoxazole. Which of the following is the most appropriate next step in antimicrobial therapy?

  1. A
    Switch to intravenous ceftriaxone
  2. B
    Continue intravenous piperacillin-tazobactam
  3. Switch to intravenous cefepimeAnswer
  4. D
    Switch to intravenous meropenem
  5. E
    Add intravenous gentamicin to the current regimen

Answer

The most appropriate step is to switch the patient's therapy to intravenous cefepime.
The correct approach for treating invasive infections (e.g., bacteremia) caused by organisms with a high risk of clinically significant AmpC beta-lactamase production (such as *Enterobacter cloacae*, *Klebsiella aerogenes*, and *Citrobacter freundii*) is to use a fourth-generation cephalosporin, such as cefepime. Cefepime is highly stable against AmpC hydrolysis and is a weak inducer of the enzyme. Although carbapenems (like meropenem) are also highly effective, antimicrobial stewardship guidelines strongly recommend cefepime as a carbapenem-sparing targeted therapy for susceptible AmpC isolates.

Step-by-Step Solution

1
Identify the pathogen and its specific intrinsic resistance mechanisms.
The pathogen is *Enterobacter cloacae*, a classic organism known to harbor an inducible chromosomal AmpC beta-lactamase gene.
Recognizing the intrinsic resistance mechanisms of specific organisms is essential for safely interpreting laboratory susceptibility reports.
2
Interpret the susceptibility results in the context of the pathogen's AmpC production risk.
Although ceftriaxone and piperacillin-tazobactam show in vitro susceptibility, third-generation cephalosporins are strong inducers of AmpC and can lead to rapid hyperproduction and clinical failure in vivo.
In vitro susceptibility does not always guarantee in vivo clinical success, particularly with inducible beta-lactamase producers.
3
Select the optimal targeted therapy by balancing clinical efficacy and antimicrobial stewardship.
Cefepime, a fourth-generation cephalosporin, is highly stable against AmpC hydrolysis and is a weak inducer, making it the drug of choice. Meropenem is effective but should be reserved for cases where cefepime cannot be used.
Cefepime provides definitive, targeted therapy for invasive AmpC infections while adhering to stewardship principles by sparing overly broad-spectrum agents like carbapenems.

Key Concept

Management of invasive infections caused by moderate-to-high risk AmpC beta-lactamase-producing organisms (e.g., *Enterobacter cloacae*) requires avoiding third-generation cephalosporins despite in vitro susceptibility; cefepime is the preferred definitive therapy.
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Infectious Diseases

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