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Zorluk: ZorInfectious Diseases

During morning rounds in the surgical intensive care unit, a physician reviews the microbiological data for a patient recovering from an emergency laparotomy. The patient, a 67-year-old female, underwent surgery six days prior for perforated diverticulitis. At that time, she presented in septic shock and was resuscitated with intravenous fluids, vasopressors, and empiric broad-spectrum coverage utilizing vancomycin and piperacillin-tazobactam. Her clinical status has steadily improved; she was extubated on postoperative day 3 and is currently hemodynamically stable without vasoactive medications. Current vital signs show a temperature of 37.8°C (100.0°F). The electronic medical record flags a new critical result: blood cultures drawn 48 hours ago demonstrate growth of *Enterobacter cloacae*. The laboratory reports the isolate is susceptible to ceftriaxone, ceftazidime, cefepime, piperacillin-tazobactam, and meropenem. It shows resistance to ampicillin and first-generation cephalosporins. Based on these findings, which of the following is the most appropriate modification to her pharmacotherapy?

  1. Switch the regimen to intravenous cefepime onlyCevap
  2. B
    Switch the regimen to intravenous ceftriaxone only
  3. C
    Switch the regimen to intravenous meropenem only
  4. D
    Withdraw vancomycin but maintain piperacillin-tazobactam
  5. E
    Continue both vancomycin and piperacillin-tazobactam

Cevap

Switch the regimen to intravenous cefepime only.
The most appropriate action is to switch the regimen to intravenous cefepime only. The patient has a bloodstream infection caused by *Enterobacter cloacae*. This organism, along with *Klebsiella aerogenes* and *Citrobacter freundii*, typically harbors a chromosomal gene for AmpC beta-lactamase. While these organisms often test susceptible to third-generation cephalosporins (like ceftriaxone) *in vitro*, these antibiotics act as strong inducers of the AmpC enzyme and are susceptible to hydrolysis by it. Using them in vivo for a high-inoculum or serious infection (like bacteremia) frequently results in the rapid selection of derepressed mutants and subsequent treatment failure. Cefepime, a fourth-generation cephalosporin, is a weak inducer of AmpC and is highly stable against hydrolysis by the enzyme, making it the drug of choice. Vancomycin can be safely discontinued because the causative pathogen has been fully identified as a Gram-negative bacillus.

Adım Adım Çözüm

1
Evaluate the microbiology results.
Blood cultures are positive for Enterobacter cloacae, a Gram-negative bacillus known to harbor an inducible chromosomal AmpC beta-lactamase.
Identifying the specific pathogen allows for targeted antimicrobial therapy.
2
Analyze the resistance profile and implications for therapy.
Although ceftriaxone and piperacillin-tazobactam appear susceptible in vitro, they are highly associated with clinical failure in serious AmpC infections due to enzyme induction or weak inhibition.
In vitro susceptibility does not always equal in vivo clinical success, especially for AmpC-producing organisms.
3
Select the most appropriate targeted agent.
Cefepime is selected because it is a weak inducer of AmpC and is stable against hydrolysis. Vancomycin is discontinued as there is no Gram-positive pathogen.
Cefepime provides definitive efficacy while practicing good antimicrobial stewardship by sparing carbapenems.

Anahtar Kavram

Treatment of serious infections caused by organisms with inducible AmpC beta-lactamases (e.g., Enterobacter, Klebsiella aerogenes, Citrobacter freundii) requires agents that are weak inducers and stable against hydrolysis, such as cefepime or carbapenems. Third-generation cephalosporins should be avoided despite apparent in vitro susceptibility.
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