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Difficulty: MediumInfectious Diseases

A 38-year-old woman is hospitalized for a 1-week history of severe lower back pain, generalized weakness, and recurrent chills. Her medical history is significant for intravenous drug use. Her temperature is 39.2∘C39.2^\circ C (102.6∘F102.6^\circ F), blood pressure is 112/68 mm Hg112/68 \text{ mm Hg}, and pulse is 115/min115/\text{min}. Physical examination reveals a new III/VI holosystolic murmur loudest at the cardiac apex. There is pronounced point tenderness over the lower lumbar spine, but no focal neurologic deficits are present. Laboratory studies show a leukocyte count of 15,200/μL15,200/\mu L. Magnetic resonance imaging of the spine demonstrates abnormal signal intensity in the L4-L5 intervertebral disc and adjacent vertebral bodies, consistent with discitis and osteomyelitis. Blood cultures are drawn, and empiric therapy with intravenous vancomycin and cefepime is initiated. On hospital day 3, the patient's fever has improved. A transesophageal echocardiogram reveals a 1.4-cm vegetation on the mitral valve. Blood cultures are finalized and grow *Staphylococcus aureus* that is resistant to penicillin but susceptible to oxacillin, cefazolin, daptomycin, and vancomycin. Which of the following is the most appropriate next step in pharmacotherapy?

  1. Discontinue the current regimen and initiate intravenous cefazolinAnswer
  2. B
    Discontinue cefepime and continue intravenous vancomycin
  3. C
    Continue both intravenous vancomycin and cefepime
  4. D
    Discontinue the current regimen and initiate intravenous daptomycin
  5. E
    Discontinue the current regimen and initiate oral linezolid

Answer

Discontinue the current regimen and initiate intravenous cefazolin
For severe, deep-seated infections caused by methicillin-susceptible *Staphylococcus aureus* (MSSA), such as left-sided infective endocarditis and vertebral osteomyelitis, targeted beta-lactam antibiotics (e.g., cefazolin, nafcillin, or oxacillin) are the definitive treatment of choice. Clinical data consistently demonstrate that these beta-lactams are superior to vancomycin for MSSA, leading to more rapid clearance of bacteremia and lower rates of treatment failure and mortality. Therefore, the principles of antimicrobial stewardship and optimal clinical care dictate prompt de-escalation from empiric broad-spectrum coverage to a narrow-spectrum beta-lactam once susceptibilities confirm MSSA.

Step-by-Step Solution

1
Interpret the blood culture susceptibilities.
The organism is Staphylococcus aureus susceptible to oxacillin.
Oxacillin susceptibility defines the isolate as methicillin-susceptible Staphylococcus aureus (MSSA).
2
Determine the optimal antibiotic class for MSSA bacteremia, endocarditis, and osteomyelitis.
Narrow-spectrum, anti-staphylococcal beta-lactams are the treatment of choice.
Clinical evidence consistently shows that anti-staphylococcal penicillins (nafcillin, oxacillin) or first-generation cephalosporins (cefazolin) have superior efficacy, lower mortality rates, and lower relapse rates compared to vancomycin for MSSA infections.
3
Select the appropriate management step based on principles of antibiotic stewardship.
De-escalate the empiric broad-spectrum regimen to intravenous cefazolin.
De-escalation improves patient outcomes by utilizing the most effective targeted therapy while minimizing unnecessary broad-spectrum exposure (reducing toxicity and resistance).

Key Concept

Targeted antimicrobial therapy (de-escalation) for methicillin-susceptible Staphylococcus aureus (MSSA) bacteremia and endocarditis.
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Topics

Infectious Diseases

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