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Difficulty: HardInfectious Diseases

Following a transurethral resection of the prostate four weeks ago, a 65-year-old man presents to the emergency department reporting two weeks of worsening fatigue, night sweats, and shortness of breath with mild exertion. He has no other significant past medical history and takes no daily medications. Vital signs show a temperature of 38.4°C (101.1°F), heart rate of 98/min, and blood pressure of 118/72 mm Hg. Cardiac auscultation reveals a new grade 3/6 holosystolic murmur at the apex radiating to the left axilla. Dermatologic examination shows dark red, linear lesions in the nail beds of both hands. Laboratory evaluation reveals a leukocyte count of 14,500/mm³. A transesophageal echocardiogram demonstrates a 1.4-cm mobile vegetation on the mitral valve with moderate regurgitation. Gram stain of blood cultures drawn at admission shows gram-positive cocci in pairs and short chains. Subsequent speciation confirms *Enterococcus faecalis*. The susceptibility report indicates the organism is susceptible to ampicillin, vancomycin, and daptomycin, but testing demonstrates high-level resistance to gentamicin. Which of the following is the most appropriate definitive intravenous pharmacotherapy for this patient?

  1. Ampicillin and ceftriaxoneAnswer
  2. B
    Ampicillin and gentamicin
  3. C
    Vancomycin and gentamicin
  4. D
    Daptomycin monotherapy
  5. E
    Linezolid monotherapy

Answer

Ampicillin and ceftriaxone
The correct answer provides the standard-of-care bactericidal regimen for *Enterococcus faecalis* infective endocarditis, particularly when the isolate exhibits high-level aminoglycoside resistance (HLAR). *E. faecalis* is inherently tolerant to cell-wall active agents alone, meaning monotherapy is often bacteriostatic rather than bactericidal. Eradication of endovascular vegetations requires bactericidal activity. Traditionally, this was achieved by combining ampicillin with an aminoglycoside (like gentamicin). However, if an isolate has HLAR (due to modifying enzymes), the aminoglycoside cannot enter the cell, and synergy is lost. In these cases, the combination of ampicillin and ceftriaxone is utilized. Although enterococci are intrinsically resistant to cephalosporins, ceftriaxone saturates penicillin-binding proteins 2 and 3 (PBP2 and PBP3), while ampicillin binds PBP4 and PBP5. This dual blockade yields potent synergistic bactericidal activity against *E. faecalis* and avoids the nephrotoxicity associated with aminoglycosides.

Step-by-Step Solution

1
Analyze the clinical presentation and diagnostic findings.
The patient has subacute infective endocarditis (fever, new murmur, splinter hemorrhages, echocardiographic vegetation) following a genitourinary procedure, with blood cultures growing Enterococcus faecalis.
Establishing the diagnosis and identifying the specific pathogen is the prerequisite for selecting targeted antimicrobial therapy.
2
Evaluate the antimicrobial susceptibility profile provided in the stem.
The isolate is susceptible to ampicillin but demonstrates high-level resistance to gentamicin (HLAR).
Enterococcal endocarditis requires bactericidal therapy, which typically necessitates combination therapy for synergy. The susceptibility data dictates which combination is viable.
3
Determine the appropriate synergistic regimen given the resistance profile.
Because HLAR is present, an ampicillin-aminoglycoside combination will not yield synergy. Therefore, dual beta-lactam therapy with ampicillin and ceftriaxone must be utilized.
Ampicillin and ceftriaxone work synergistically against E. faecalis by binding complementary penicillin-binding proteins (PBPs), providing effective bactericidal activity without relying on aminoglycosides.

Key Concept

Treatment of Enterococcus faecalis infective endocarditis with high-level aminoglycoside resistance (HLAR).
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Topics

Infectious Diseases

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