Treatment of Aerococcus viridans Infections
Penicillin or ampicillin remains the first-line treatment for Aerococcus viridans infections, with cephalosporins (particularly cefotaxime or ceftriaxone) serving as effective alternatives for penicillin-allergic patients. 1, 2
Bacterial Classification
Aerococcus viridans is a Gram-positive, catalase-negative, oxidase-negative, microaerophilic coccus that is fastidious and often misidentified as viridans group streptococci due to similar morphology and growth characteristics 1. This organism requires complex nutritional requirements and may be dismissed as a contaminant, leading to delayed or missed diagnoses 1, 3.
Antimicrobial Treatment Recommendations
For Serious Infections (Bacteremia, Endocarditis)
Primary therapy:
- Penicillin G 18-24 million units IV daily (divided doses or continuous infusion) for 4-6 weeks 4
- Ampicillin 2 g IV every 4 hours as an alternative to penicillin 5
- Consider adding gentamicin 3 mg/kg/day IV (in 2-3 divided doses or single daily dose) for the first 2 weeks for synergy, particularly in endocarditis cases 5, 4
For penicillin-allergic patients:
- Cefotaxime (dose adjusted for severity) has demonstrated successful treatment in documented cases 2
- Ceftriaxone 1-2 g IV daily is a reasonable alternative 5
- Vancomycin 30 mg/kg/day IV in 2 divided doses for patients with severe penicillin allergy (anaphylaxis, angioedema, urticaria) 5
For Urinary Tract Infections
Primary therapy:
- Levofloxacin 500-750 mg IV/PO daily for 10-14 days has shown clinical success 6
- Ampicillin or amoxicillin at standard UTI dosing (500 mg PO/IV every 8 hours) 5
- Nitrofurantoin 100 mg PO every 6 hours may be considered for uncomplicated lower UTI 5
Critical Clinical Considerations
Diagnostic Pitfalls
The fastidious nature of A. viridans means it may be overlooked or misidentified 1, 3. Matrix-assisted laser desorption ionization time-of-flight mass spectrometry (MALDI-TOF) is essential for accurate identification and should be utilized when available 1. Blood cultures may take several days to become positive, and the organism is frequently confused with streptococcal species 1, 3.
Disease Severity
A. viridans endocarditis demonstrates particularly aggressive behavior, requiring surgical valve replacement in approximately 40% of reported cases (4 of 10 cases) with mortality occurring in approximately 11% (1 of 9 cases) 3. The time to diagnosis ranges from a few days to seven months, emphasizing the need for high clinical suspicion 3.
Treatment Duration
- Endocarditis: 4-6 weeks of IV therapy 4, 3
- 4 weeks for symptoms <3 months duration or aortic valve involvement
- 6 weeks for symptoms >3 months duration or mitral valve involvement
- Bacteremia/septicemia: 14 days minimum 1
- Uncomplicated UTI: 10-14 days 6
Monitoring Requirements
Given the virulent nature of A. viridans endocarditis, echocardiography should be performed promptly when symptoms persist despite initial therapy 3. Serial blood cultures are necessary to document clearance of bacteremia 1, 3.
Antibiotic Resistance Patterns
Penicillin resistance has been documented in A. viridans isolates 1. Antimicrobial susceptibility testing must guide definitive therapy, particularly when empiric treatment fails or in critically ill patients 1. The case report of penicillin-resistant A. viridans successfully treated with vancomycin demonstrates the importance of tailoring therapy to susceptibility results 1.