What is the appropriate treatment for a patient with Aerococcus viridans positive blood cultures, potentially immunocompromised or with a history of recurrent infections?

Treatment of Aerococcus viridans Bacteremia

For Aerococcus viridans positive blood cultures, initiate vancomycin 40 mg/kg/day IV divided every 8-12 hours (targeting trough 15-20 μg/mL) as first-line therapy, particularly in immunocompromised patients or those with recurrent infections, and treat for 14 days for uncomplicated bacteremia or 4-6 weeks if endocarditis is confirmed. 1, 2

Initial Diagnostic Approach

• Obtain at least 2 sets of blood cultures from separate venipuncture sites before initiating antibiotics to confirm bacteremia and rule out contamination, as Aerococcus species are fastidious organisms often confused with viridans streptococci or dismissed as contaminants. 3, 2

• Perform urgent echocardiography (transthoracic initially, followed by transesophageal if indicated) to evaluate for infective endocarditis, as Aerococcus viridans is a recognized cause of endocarditis similar to viridans streptococci. 3, 4

• Assess for urinary tract source by obtaining urinalysis and urine culture, as the urinary tract is the most common source of Aerococcus bacteremia (50% of cases), particularly in elderly patients with urologic abnormalities. 2, 4

• Evaluate for specific risk factors including advanced age (median 74 years), recurrent urinary tract infections, urinary incontinence, indwelling catheters, renal stones, or recent urologic instrumentation. 4

Empiric Antibiotic Selection

• Start vancomycin immediately as empiric therapy because penicillin resistance in Aerococcus viridans has been documented, with some isolates showing high-level resistance to penicillin, erythromycin, clindamycin, and ceftriaxone. 1, 2, 5

• Add piperacillin-tazobactam 3.375g IV every 6 hours initially if the patient is critically ill or septic, then de-escalate to vancomycin monotherapy once Aerococcus is confirmed and susceptibilities are available. 2

• Avoid empiric fluoroquinolone monotherapy as quinolones are less effective against gram-positive cocci and may predispose to treatment failure, though levofloxacin has been used successfully in documented susceptible cases. 6, 7

Definitive Therapy Based on Susceptibilities

• Continue vancomycin if the isolate demonstrates penicillin resistance (MIC >0.12 μg/mL), which occurs in a significant proportion of clinical isolates, especially in immunocompromised patients. 1, 5

• Consider switching to high-dose ampicillin 200-300 mg/kg/day IV divided every 4-6 hours if susceptibility testing confirms penicillin susceptibility, as beta-lactams remain the preferred agents for susceptible strains. 8, 1

• Add gentamicin 1 mg/kg IV every 8 hours for synergy if endocarditis is confirmed, as vancomycin has been reported effective for viridans-like organisms only in combination with an aminoglycoside for endocardial infections. 3, 1

Treatment Duration

• Treat uncomplicated bacteremia for 14 days as documented in successful case reports of Aerococcus viridans septicemia. 2, 4

• Extend therapy to 4-6 weeks for confirmed infective endocarditis following standard endocarditis treatment guidelines, as Aerococcus endocarditis carries significant morbidity with valve involvement. 3, 4

• Consider intensive care unit admission and vasopressor support as 45-50% of Aerococcus bacteremia cases require ICU-level care due to septic shock. 4

Monitoring and Follow-up

• Monitor vancomycin trough levels to maintain therapeutic concentrations of 15-20 μg/mL in severe infections while avoiding nephrotoxicity, particularly in elderly patients with baseline renal impairment. 8, 1

• Obtain repeat blood cultures 48-72 hours after initiating therapy to document clearance of bacteremia, as persistent positive cultures may indicate endocarditis or undrained focus. 3

• Perform serial echocardiography if endocarditis is suspected but initial transthoracic echocardiogram is negative, as 45% of Aerococcus bacteremia cases have suspected endocarditis and transesophageal echocardiography may be required for definitive diagnosis. 3, 4

• Evaluate for embolic complications including stroke, as septic emboli can occur even with normal echocardiography findings. 4

Special Considerations for Immunocompromised Patients

• Maintain high clinical suspicion in granulocytopenic patients as Aerococcus viridans can be a significant pathogen in immunocompromised hosts, with documented cases in patients with neutropenia. 5

• Do not dismiss positive cultures as contaminants in symptomatic immunocompromised patients, as the fastidious nature and tetrad arrangement on Gram stain often leads to misidentification or dismissal as contamination. 2, 5

• Consider broader differential diagnosis as Aerococcus viridans can present atypically, including as cutaneous vasculitis, in addition to typical bacteremia presentations. 9

Common Pitfalls to Avoid

• Do not delay treatment while awaiting definitive identification, as matrix-assisted laser desorption ionization time-of-flight mass spectrometry (MALDI-TOF) may take 3 days for accurate speciation. 2

• Do not assume penicillin susceptibility based on similarity to viridans streptococci, as resistance patterns differ significantly and penicillin-resistant strains are well-documented. 5

• Do not overlook polymicrobial bacteremia, as 35% of Aerococcus bacteremia cases have concurrent growth of other organisms including coagulase-negative staphylococci, Enterococcus faecalis, or Proteus mirabilis. 4

• Do not initiate empirical antibiotics for undefined fever without obtaining blood cultures first, as this practice is a major cause of culture-negative infections and delays appropriate diagnosis. 3