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Difficulty: MediumInfectious Diseases

A 48-year-old woman is admitted to the hospital with a 2-day history of fever, chills, and progressive right lower extremity swelling and redness. Her past medical history is unremarkable. On admission, her temperature is 38.6°C (101.5°F), blood pressure is 110/70 mmHg, and pulse is 108/min. Physical examination reveals a 10×810 \times 8 cm area of well-demarcated erythema, warmth, and exquisite tenderness over the right calf. Two sets of peripheral blood cultures are obtained, and the patient is started on empiric intravenous vancomycin and piperacillin-tazobactam.

By hospital day 3, her temperature has normalized, and the erythema has significantly receded. The admission blood cultures grow *Staphylococcus aureus* in all four bottles. Antimicrobial susceptibility testing reveals the following:

AntimicrobialInterpretation
CefazolinSusceptible
DaptomycinSusceptible
OxacillinSusceptible
PenicillinResistant
Trimethoprim-sulfamethoxazoleSusceptible
VancomycinSusceptible

A subsequent transthoracic echocardiogram shows no valvular vegetations. Which of the following is the most appropriate next step in the pharmacologic management of this patient?

  1. Discontinue the current antibiotics and begin intravenous cefazolinAnswer
  2. B
    Discontinue piperacillin-tazobactam and continue intravenous vancomycin
  3. C
    Continue both intravenous vancomycin and piperacillin-tazobactam
  4. D
    Discontinue the current antibiotics and begin intravenous daptomycin
  5. E
    Discontinue the current antibiotics and begin oral trimethoprim-sulfamethoxazole

Answer

Discontinue the current antibiotics and begin intravenous cefazolin
The correct course of action is to de-escalate the broad-spectrum empiric antibiotic regimen to targeted therapy with an anti-staphylococcal beta-lactam. The patient's blood cultures are positive for Staphylococcus aureus that is susceptible to oxacillin, meaning it is methicillin-susceptible S. aureus (MSSA). For the treatment of MSSA bacteremia, narrow-spectrum beta-lactam antibiotics (such as cefazolin, nafcillin, or oxacillin) are the gold standard. They possess superior, more rapid bactericidal activity against MSSA compared to vancomycin, leading to improved clearance of bacteremia and significantly reduced mortality. Good antimicrobial stewardship dictates switching to the most effective, narrowest-spectrum agent once susceptibility data is available.

Step-by-Step Solution

1
Interpret the blood culture and susceptibility data.
The blood cultures grew Staphylococcus aureus that is susceptible to oxacillin, identifying the organism as methicillin-susceptible Staphylococcus aureus (MSSA).
Accurate interpretation of oxacillin susceptibility is required to distinguish MSSA from MRSA, which dictates definitive therapy.
2
Determine the definitive treatment of choice for MSSA bacteremia.
Intravenous anti-staphylococcal beta-lactams, such as cefazolin, nafcillin, or oxacillin, are the gold standard treatments for MSSA.
Clinical trials and guidelines show that beta-lactams are more rapidly bactericidal and are associated with significantly lower mortality and failure rates compared to vancomycin for MSSA bacteremia.
3
Apply antimicrobial stewardship principles to modify the empiric regimen.
The empiric broad-spectrum regimen of vancomycin and piperacillin-tazobactam must be discontinued and de-escalated to intravenous cefazolin.
Continuing broad-spectrum agents or inferior treatments exposes the patient to unnecessary drug toxicity, increases the risk of antimicrobial resistance, and worsens clinical outcomes.

Key Concept

De-escalation of empiric antibiotics to targeted therapy; specifically, the superiority of anti-staphylococcal beta-lactams (e.g., cefazolin, nafcillin) over vancomycin for methicillin-susceptible Staphylococcus aureus (MSSA) bacteremia.
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Topics

Infectious Diseases

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