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Difficulty: Very hardInfectious Diseases

A 72-year-old man is hospitalized due to 3 weeks of progressive fatigue, low-grade fevers, and shortness of breath with exertion. Six weeks ago, he underwent a transurethral resection of the prostate for benign prostatic hyperplasia. On physical examination, his temperature is 38.2°C (100.8°F), pulse is 94/min, blood pressure is 132/84 mm Hg, and respirations are 18/min. Cardiac auscultation reveals a new grade 3/6 holosystolic murmur loudest at the apex with radiation to the axilla. Examination of the extremities shows splinter hemorrhages under the fingernails of both hands.

Laboratory studies show:
Leukocyte count: 14,500/mm³
Hemoglobin: 10.8 g/dL
Platelets: 130,000/mm³
Blood urea nitrogen: 42 mg/dL
Serum creatinine: 2.1 mg/dL (baseline 1.1 mg/dL)

Transesophageal echocardiography demonstrates a 1.4-cm oscillating vegetation on the anterior leaflet of the mitral valve with moderate-to-severe mitral regurgitation. Empiric therapy with intravenous vancomycin and gentamicin is initiated. Two days later, three sets of blood cultures drawn prior to antibiotic administration are positive for *Enterococcus faecalis*. Antimicrobial susceptibility testing indicates the isolate is susceptible to ampicillin, penicillin, and vancomycin, and exhibits no high-level resistance to gentamicin. The patient remains hemodynamically stable.

Which of the following is the most appropriate next step in the pharmacologic management of this patient?

  1. A
    Continue intravenous vancomycin and gentamicin
  2. Switch to intravenous ampicillin and ceftriaxoneAnswer
  3. C
    Switch to intravenous ampicillin and gentamicin
  4. D
    Switch to intravenous ampicillin monotherapy
  5. E
    Switch to intravenous daptomycin and linezolid

Answer

The most appropriate next step is to switch to intravenous ampicillin and ceftriaxone, which provides required synergistic bactericidal activity without worsening the patient's acute kidney injury.
The correct answer is to switch to intravenous ampicillin and ceftriaxone. This patient has *Enterococcus faecalis* infective endocarditis, likely arising from his recent transurethral resection of the prostate (a classic risk factor due to genitourinary instrumentation). Enterococci are intrinsically tolerant to beta-lactam antibiotics because their primary penicillin-binding proteins (PBPs) have a low affinity for them. Therefore, cell wall synthesis inhibitors (like ampicillin) are merely bacteriostatic against Enterococci when used alone. To achieve the bactericidal activity necessary to cure endocarditis and sterilize the cardiac vegetations, synergistic combination therapy must be used.

Historically, the standard of care was ampicillin combined with an aminoglycoside (gentamicin). However, gentamicin is highly nephrotoxic. This patient has developed an acute kidney injury (serum creatinine increased from 1.1 mg/dL to 2.1 mg/dL). In patients with impaired renal function, the combination of ampicillin and ceftriaxone is the definitively preferred regimen. Ceftriaxone saturates specific PBPs (PBP2 and PBP3) that ampicillin does not effectively target, forcing the bacteria to rely on PBP5, which is then overwhelmed by ampicillin. This dual-beta-lactam therapy provides excellent bactericidal synergy against *E. faecalis* without the severe risk of aminoglycoside-induced nephrotoxicity.

Step-by-Step Solution

1
Identify the patient's primary diagnosis and the causative organism.
The patient has native valve infective endocarditis caused by Enterococcus faecalis, likely secondary to his recent genitourinary procedure.
Establishing the diagnosis and microbiologic etiology dictates the required duration and type of antimicrobial therapy.
2
Determine the required properties of antimicrobial therapy for this condition.
Infective endocarditis requires prolonged, bactericidal therapy to sterilize avascular heart valve vegetations. For Enterococcus, beta-lactam monotherapy is only bacteriostatic.
Enterococci are inherently tolerant to beta-lactams due to low-affinity penicillin-binding proteins (PBPs), so combination therapy is required to achieve a bactericidal effect.
3
Evaluate the patient's clinical status to select the safest synergistic regimen.
The patient has an acute kidney injury (creatinine 2.1 mg/dL, elevated from baseline). Therefore, the traditional synergistic agent, gentamicin (an aminoglycoside), is contraindicated due to its nephrotoxicity.
Ceftriaxone, when combined with ampicillin, provides excellent synergistic bactericidal activity against E. faecalis and avoids nephrotoxicity, making it the regimen of choice.

Key Concept

For Enterococcus faecalis infective endocarditis, the combination of ampicillin and ceftriaxone provides synergistic bactericidal activity and is the preferred regimen for patients with, or at high risk for, nephrotoxicity.
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Infectious Diseases

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