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Difficulty: Very hardInfectious Diseases

An infectious diseases consultation is requested for a 74-year-old woman admitted with subacute bacterial endocarditis. She has a history of osteoarthritis and underwent a screening colonoscopy with polypectomy one month ago. She presented to the hospital with three weeks of low-grade fever, severe fatigue, and a new holosystolic murmur heard loudest at the cardiac apex. Transesophageal echocardiography revealed a 14 mm14\text{ mm}, highly mobile vegetation on the anterior mitral valve leaflet.

Four sets of blood cultures drawn upon admission are positive for *Enterococcus faecalis*. The microbiology laboratory provides the following antimicrobial susceptibility report:

AntimicrobialSusceptibility Result
AmpicillinSusceptible
CeftriaxoneResistant
Gentamicin (high-level screen)Resistant
LinezolidSusceptible
VancomycinSusceptible

Based on the susceptibility profile provided, which of the following is the most appropriate targeted pharmacotherapy to achieve bactericidal activity in this patient?

  1. Intravenous ampicillin and intravenous ceftriaxoneAnswer
  2. B
    Intravenous ampicillin and intravenous gentamicin
  3. C
    Intravenous ampicillin monotherapy
  4. D
    Intravenous vancomycin and intravenous piperacillin-tazobactam
  5. E
    Intravenous daptomycin monotherapy

Answer

Intravenous ampicillin and intravenous ceftriaxone
The correct targeted therapy is intravenous ampicillin and intravenous ceftriaxone. Enterococcal infective endocarditis requires a bactericidal regimen, which historically consisted of ampicillin plus an aminoglycoside (e.g., gentamicin) to achieve synergy. However, this patient's isolate demonstrates high-level aminoglycoside resistance (HLAR), meaning gentamicin cannot penetrate the cell wall and will not provide synergy. The standard of care for HLAR Enterococcus faecalis endocarditis is dual beta-lactam therapy with ampicillin and ceftriaxone. Even though enterococci are intrinsically resistant to cephalosporins in vitro, ceftriaxone saturates penicillin-binding proteins (PBP) 2 and 3, allowing ampicillin to more effectively bind its targets (PBP 4 and 5). This mechanism results in profound synergistic bactericidal activity, effectively curing the endocarditis while avoiding aminoglycoside-induced nephrotoxicity.

Step-by-Step Solution

1
Analyze the clinical presentation and primary diagnosis.
The patient has native valve infective endocarditis caused by Enterococcus faecalis.
Establishing the specific pathogen and site of infection dictates the necessity for a prolonged, bactericidal antimicrobial regimen.
2
Evaluate the microbiology susceptibility report.
The isolate is susceptible to ampicillin but shows high-level resistance to gentamicin (HLAR).
Enterococci are intrinsically tolerant to many antibiotics; bactericidal activity usually requires synergy between a cell-wall active agent (like ampicillin) and an aminoglycoside. HLAR eliminates the option of using gentamicin for synergy.
3
Determine the alternative synergistic regimen for HLAR Enterococcus faecalis.
Ampicillin combined with ceftriaxone is selected as the optimal regimen.
Despite enterococci being intrinsically resistant to cephalosporins (as shown on the report), ceftriaxone synergizes with ampicillin by saturating complementary penicillin-binding proteins (PBPs), achieving the necessary bactericidal effect without nephrotoxicity.

Key Concept

Management of Enterococcus faecalis infective endocarditis with High-Level Aminoglycoside Resistance (HLAR) using dual beta-lactam synergy (Ampicillin + Ceftriaxone).
Estimated Time:1m 30s
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Topics

Infectious Diseases

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