What is the recommended treatment for Strep mitis (Streptococcus mitis) endocarditis?

Treatment of Streptococcus mitis Endocarditis

For Streptococcus mitis endocarditis, the recommended treatment is intravenous penicillin G, ampicillin, or ceftriaxone for 4 weeks, with the addition of gentamicin for the first 2 weeks in cases with penicillin MIC >0.1 μg/mL. 1

Treatment Algorithm Based on Penicillin Susceptibility

For Penicillin-Susceptible Strains (MIC ≤0.125 mg/L):

• Standard 4-week regimen:

  • Penicillin G: 12-18 million U/day IV in 4-6 doses or continuously for 4 weeks 1
  • OR Ampicillin: 100-200 mg/kg/day IV in 4-6 doses for 4 weeks 1
  • OR Ceftriaxone: 2 g/day IV or IM in 1 dose for 4 weeks 1

• Alternative 2-week regimen (only for uncomplicated native valve endocarditis with normal renal function):

  • Penicillin G, ampicillin, or ceftriaxone as above for 2 weeks
  • PLUS Gentamicin: 3 mg/kg/day IV or IM in 1 dose for 2 weeks 1

For Penicillin-Resistant Strains (MIC >0.5 μg/mL) or Abiotrophia/Granulicatella species:

• Use the same regimen as for enterococcal endocarditis:
  • Penicillin G, ampicillin, or ceftriaxone for 4-6 weeks
  • PLUS Gentamicin for the entire course of therapy 1

For Beta-Lactam Allergic Patients:

• Vancomycin: 30 mg/kg/day IV in 2 doses for 4 weeks
• Current practice includes addition of gentamicin for a 4-week course 1

Special Considerations for Prosthetic Valve Endocarditis

• Extended therapy duration required:
  • For penicillin-susceptible strains: 6 weeks of penicillin, ampicillin, or ceftriaxone PLUS gentamicin for first 2 weeks 1
  • For strains with MIC >0.1 μg/mL or Abiotrophia/Granulicatella: 6 weeks of penicillin, ampicillin, or ceftriaxone PLUS gentamicin for the entire 6 weeks 1
  • For beta-lactam intolerant patients: 6 weeks of vancomycin PLUS gentamicin for first 2 weeks 1

Monitoring and Safety Considerations

• Weekly monitoring of vancomycin and gentamicin blood concentrations is recommended 1
• Regular renal function tests should be performed when using potentially nephrotoxic antibiotics 1
• Gentamicin can be given once daily in patients with normal renal function when treating susceptible streptococci 1
• Patients >65 years or with impaired renal or vestibulocochlear function may benefit from regimens without aminoglycosides 1

Important Clinical Pearls

• S. mitis is part of the viridans streptococci group and has increasing rates of penicillin resistance (>30% of S. mitis strains show intermediate or full resistance) 1
• Bactericidal drug combinations are preferred over monotherapy for tolerant organisms 1
• Home treatment for part of the antibiotic course may be considered for carefully selected patients with good home healthcare access 1
• Consultation with an infectious disease specialist should be considered for all patients with infective endocarditis, especially those with resistant organisms 1
• Treatment duration is based on the first day of effective antibiotic therapy (when blood cultures become negative), not the day of surgery if valve replacement is needed 1

Potential Pitfalls

• Short-term therapy (2 weeks) should not be used for complicated cases, prosthetic valve infections, or penicillin-resistant strains 1
• Failure to determine the minimal inhibitory concentration (MIC) of penicillin for S. mitis isolates may lead to treatment failure 2
• Underestimating the need for prolonged therapy (6 weeks) in prosthetic valve endocarditis can result in incomplete sterilization of infected heart valves 1
• Not recognizing tolerance to antibiotics (where bacteria are inhibited but not killed) can lead to treatment failure 1