Table 1.

In vitro activities of selected antimicrobial agents against viridans group streptococci isolated from blood cultures of neutropenic cancer patients

Antimicrobial agentMIC range (mg/liter)MIC50(mg/liter)aMIC90(mg/liter)b% Resistant (breakpoint)c
Penicillin≤0.016–40.031250.52 (≥4)
Amoxicillin≤0.016–≥40.031250.57 (≥2)
Cefotaxime≤0.016–20.031250.254 (≥2)
Chloramphenicold 0.25–161.524 (≥16)
Erythromycin≤0.06–≥32≤0.06432 (≥1)
Clindamycin≤0.06–≥32≤0.060.59 (≥1)
Tetracycline0.25–≥320.5≥3239 (≥8)
Cotrimoxazoled ≤0.002–≥328≥3268 (≥4)
Ciprofloxacin0.25–42423 (≥4)
Sparfloxacin0.125–10.250.50 (≥8)
Vancomycin0.125– (≥2)
Quinupristin-dalfopristin0.5–2110 (≥4)
  • a MIC50, MIC at which 50% of the isolates are inhibited.

  • b MIC90, MIC at which 90% of the isolates are inhibited.

  • c MIC breakpoints for resistance are those recently defined by NCCLS for susceptibility testing of viridans group streptococci to all agents except those for which no breakpoints have been established. For amoxicillin, cotrimoxazole, and sparfloxacin, the NCCLS resistance breakpoints for S. pneumoniae were applied. For ciprofloxacin, the NCCLS resistance breakpoint for nonfastidious organisms was applied; for quinupristin-dalfopristin, the resistance breakpoint suggested by the manufacturer for S. pneumoniaewas used. No NCCLS-approved breakpoints are currently available for teicoplanin.

  • d Susceptibility to antimicrobial agents was tested by Etest.