What is the most common organism responsible for infective endocarditis (IE) in a patient who underwent prosthetic valve repair within the last month?

Most Common Organism in Early Prosthetic Valve Endocarditis

B - Staphylococcus epidermidis is the most common organism causing infective endocarditis within one month of prosthetic valve surgery.

Microbiology Based on Timing After Surgery

Staphylococcus epidermidis dominates early prosthetic valve endocarditis (PVE), defined as occurring within 60 days of surgery, according to the American College of Cardiology. 1 This patient at one month post-surgery falls squarely into this early-onset category where coagulase-negative staphylococci, particularly S. epidermidis, are the predominant pathogens. 2

Early-Onset PVE (≤60 days post-surgery):

• S. epidermidis is the most common organism in this timeframe 1
• S. aureus is also common but typically second to S. epidermidis 2, 3
• Healthcare-acquired infection patterns predominate 2
• Infection occurs at the host-prosthesis sewing ring interface 4

Intermediate-Onset PVE (60-365 days):

• Coagulase-negative Staphylococcus remains the most common organism 2
• Mix of healthcare-acquired and community-acquired infections 2

Late-Onset PVE (>1 year):

• Microbial spectrum resembles native valve endocarditis 2
• Streptococci become more common 5, 6
• S. aureus and S. epidermidis remain important but less dominant 2

Why Not the Other Options

Staphylococcus aureus (Option A) is indeed a major pathogen in early PVE and is the most common organism in healthcare-associated IE overall 2, but it is specifically noted as most common in early-onset PVE alongside—not instead of—coagulase-negative staphylococci. 3 In the specific context of prosthetic valves within the first 60 days, S. epidermidis predominates. 1

Streptococcus viridans (Option C) is the most common organism in native valve endocarditis 2 and becomes more prominent in late-onset PVE (>1 year post-surgery) 5, 6, but is relatively uncommon in early prosthetic valve infections. 6

Clinical Implications

The clinical course of S. epidermidis native valve endocarditis is typically indolent with satisfactory response to therapy 2, but prosthetic valve involvement carries significantly higher mortality. 2 This patient requires:

• Empiric therapy with vancomycin plus gentamicin, plus cefepime or an antipseudomonal carbapenem for early PVE 3
• Transesophageal echocardiography to detect vegetations and complications, as transthoracic echo sensitivity is only 40-63% 2, 1
• Consideration for early surgical intervention, as prosthetic valve endocarditis has surgical rates approaching 50% and significantly lower mortality with surgery (23%) versus medical therapy alone (56%) 2
• Treatment duration of 6 weeks after blood cultures clear 3

Important Pitfall

Do not dismiss S. epidermidis as a contaminant in this clinical context. While S. epidermidis is a common blood culture contaminant, multiple positive blood cultures in a patient with a prosthetic valve within 60 days of surgery should be considered a major Duke criterion and treated as true infection until proven otherwise. 1