Can Aerococcus viridans positive blood cultures in an elderly, potentially immunocompromised patient with a history of recurrent infections be considered a contaminant?

Aerococcus viridans: Not a Contaminant in This Clinical Context

Aerococcus viridans should NOT be considered a contaminant in an elderly, potentially immunocompromised patient with recurrent infections—this organism is a recognized pathogen that requires treatment, particularly in this high-risk population. 1, 2

Why Aerococcus viridans is a True Pathogen Here

Clinical Significance in High-Risk Populations

• Aerococcus viridans is an established human pathogen causing bacteremia, endocarditis, urinary tract infections, meningitis, and arthritis, particularly in elderly and immunocompromised patients 1, 2, 3

• The organism is frequently misidentified or dismissed as a contaminant because it grows fastidiously and resembles streptococci or staphylococci on culture, leading to delayed or missed diagnoses 1, 3, 4

• In immunocompromised patients, A. viridans should be suspected as an opportunistic pathogen and treated according to susceptibility results 2

Distinguishing True Infection from Contamination

The CDC guidelines provide clear criteria for determining true bacteremia versus contamination 5:

• A recognized pathogen (like A. viridans) cultured from one or more blood cultures meets criteria for laboratory-confirmed bacteremia when not related to infection at another site 5

• For organisms that can be skin contaminants, two or more positive blood cultures drawn on separate occasions are required to confirm true bacteremia 5

• A. viridans is NOT listed among common skin contaminants (which include coagulase-negative staphylococci, diphtheroids, Bacillus species, Propionibacterium species, and micrococci) 5

Key Diagnostic Considerations

Number of positive cultures matters:

• If A. viridans grew from ≥2 blood culture sets drawn at different times, this strongly indicates true bacteremia rather than contamination 5
• The American Heart Association criteria for endocarditis specifically recognize viridans streptococci (which A. viridans resembles) as typical organisms when isolated from ≥2 blood cultures 5

Clinical context is critical:

• The presence of fever, signs of sepsis (hypotension, tachypnea, delirium), or other clinical manifestations of infection supports true bacteremia 5
• Elderly patients and those with immunocompromising conditions have higher risk of true bacteremia when cultures are positive 5
• History of recurrent infections increases the likelihood that any positive culture represents true infection 5

Aggressive Nature of A. viridans Infections

A. viridans endocarditis is particularly virulent:

• Requires surgical intervention in 40% of reported cases (4 of 10) 6
• Associated with mortality in approximately 11% of cases (1 of 9) 6
• Variable time to diagnosis ranging from days to seven months, emphasizing the need for prompt recognition 6

The organism can cause severe manifestations:

• Documented cases include post-operative bacteremia in cancer patients 2
• Urinary tract infections with bacteremia in elderly nursing home residents 3
• Native valve endocarditis requiring urgent valve replacement 6
• Even cutaneous vasculitis presentations 4

Recommended Clinical Approach

Immediate actions:

• Do NOT dismiss this as contamination—treat as true bacteremia pending further evaluation 1, 2
• Obtain at least 2 additional sets of peripheral blood cultures from separate venipunctures to confirm persistent bacteremia 7
• Perform transesophageal echocardiography to evaluate for endocarditis, given A. viridans' propensity for valvular infection 5, 6

Antimicrobial therapy:

• Initiate treatment according to susceptibility results—A. viridans can be multidrug-resistant, including resistance to penicillin, ampicillin, and piperacillin-tazobactam 2
• Vancomycin has been successfully used in documented cases of resistant A. viridans bacteremia 2
• Treatment duration should be 14 days for uncomplicated bacteremia or at least 6 weeks if endocarditis is confirmed 8

Follow-up monitoring:

• Repeat blood cultures 48-72 hours after initiating therapy to document clearance 9
• Monitor for metastatic complications including new fever, neurological symptoms, or organ dysfunction 9

Critical Pitfalls to Avoid

• Do not confuse A. viridans with streptococci or dismiss it as a contaminant—this delays appropriate therapy and worsens outcomes 1, 3, 4
• Do not rely on a single positive culture—obtain multiple sets to confirm true bacteremia versus contamination 7
• Do not use empiric vancomycin without confirming susceptibility—some strains may be resistant to commonly used empiric regimens 2
• Do not overlook endocarditis—echocardiography should be performed when symptoms persist, given the aggressive nature of A. viridans endocarditis 6