A 62-year-old man is hospitalized due to a 4-week history of low-grade fever, progressive … — USMLE Step 2 CK

A $62$ -year-old man is hospitalized due to a $4$ -week history of low-grade fever, progressive fatigue, and generalized myalgias. He has a history of osteoarthritis and underwent a colonoscopy $2$ months ago for routine screening. His temperature is $38.3^\circ\text{C}$ ( $100.9^\circ\text{F}$ ), blood pressure is $122/74\text{ mm Hg}$ , and pulse is $92/\text{min}$ . Physical examination reveals a grade $3/6$ holosystolic murmur heard best at the cardiac apex that radiates to the axilla. Splinter hemorrhages are noted on the fingernails of both hands. Laboratory studies show a leukocyte count of $14,000/\text{mm}^3$ and an erythrocyte sedimentation rate of $65\text{ mm/hr}$ . A transesophageal echocardiogram demonstrates a $1.2\text{-cm}$ mobile vegetation on the mitral valve.

Three sets of blood cultures are drawn, and the patient is empirically started on intravenous vancomycin and ceftriaxone. Forty-eight hours later, all three blood culture sets grow *Enterococcus faecalis*. Antimicrobial susceptibility testing is reported as follows:

Antimicrobial	Susceptibility
Ampicillin	Susceptible
Penicillin	Susceptible
Vancomycin	Susceptible
Linezolid	Susceptible
Gentamicin	High-level resistance
Streptomycin	High-level resistance

The patient's serum creatinine is $0.9\text{ mg/dL}$ . He has no known drug allergies. Which of the following is the most appropriate next step in antimicrobial therapy?

Intravenous ampicillin and ceftriaxoneAnswer
B
Intravenous ampicillin and gentamicin
C
Intravenous vancomycin and ceftriaxone
D
Intravenous ampicillin monotherapy
E
Intravenous daptomycin and linezolid

Answer

Intravenous ampicillin and ceftriaxone

The correct answer provides the optimal definitive therapy for Enterococcus faecalis infective endocarditis with high-level aminoglycoside resistance (HLAR). Enterococci are intrinsically tolerant to many antibiotics; cell wall-active agents like ampicillin are only bacteriostatic when used alone. Eradication of vegetations in endocarditis requires bactericidal activity, which is traditionally achieved by combining ampicillin with an aminoglycoside (e.g., gentamicin). However, this patient's isolate demonstrates HLAR, meaning an aminoglycoside will not provide synergistic killing. Clinical trials have established that dual beta-lactam therapy with ampicillin and ceftriaxone is highly effective for E. faecalis endocarditis. Ceftriaxone saturates penicillin-binding proteins (PBP2 and PBP3), while ampicillin targets PBP4. Together, they achieve potent bactericidal synergy, offering a first-line alternative to aminoglycosides that also spares the patient from nephrotoxicity.

Step-by-Step Solution

Diagnose the clinical syndrome based on the presentation.

The patient has infective endocarditis (IE), evidenced by fever, new murmur, splinter hemorrhages, and a vegetation on echocardiography.

Establishing the severity and location of the infection dictates whether bacteriostatic or bactericidal therapy is required.

Interpret the microbiological data and susceptibility profile.

The pathogen is Enterococcus faecalis, which is susceptible to ampicillin but demonstrates high-level aminoglycoside resistance (HLAR) to both gentamicin and streptomycin.

Pathogen-specific susceptibility guides targeted therapy de-escalation.

Apply pharmacologic principles of enterococcal endocarditis treatment.

Recognize that cell-wall active agents (like ampicillin) are only bacteriostatic against enterococci. IE requires bactericidal therapy, traditionally achieved via ampicillin plus an aminoglycoside.

Bacteriostatic monotherapy is insufficient to sterilize avascular heart valve vegetations.

Select the appropriate alternative synergistic regimen given the HLAR.

Since HLAR precludes aminoglycoside synergy, the standard of care is dual beta-lactam therapy with ampicillin and ceftriaxone.

This combination provides synergistic bactericidal activity against E. faecalis and avoids the nephrotoxicity associated with aminoglycosides.

Key Concept

Infective endocarditis caused by Enterococcus faecalis with high-level aminoglycoside resistance (HLAR) requires targeted, synergistic bactericidal therapy with ampicillin and ceftriaxone.

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