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Very high resistance to amoxicillin in Streptococcus pneumoniae: an epidemiological fact or a technical issue?

M-T Camacho1, J Casal, L Aguilar

  • 1Spanish Reference Pneumococcal Laboratory, Centro Nacional de Microbiología, Instituto de Salud Carlos III, Ctra. Majadahonda-Pozuelo Km. 2, 28220 Majadahonda, Madrid.

Journal of Chemotherapy (Florence, Italy)
|November 30, 2006
PubMed

Insights

Reproducibility of high amoxicillin minimum inhibitory concentrations (MIC(AMX)) in Streptococcus pneumoniae is low. Retesting revealed significant shifts in MIC(AMX) values, especially with microdilution methods.

Area of Science:

  • Microbiology
  • Antimicrobial Resistance
  • Clinical Diagnostics

Background:

  • High amoxicillin minimum inhibitory concentrations (MIC(AMX)) in Streptococcus pneumoniae can impact treatment efficacy.
  • Accurate determination of MIC(AMX) is crucial for guiding antibiotic therapy and monitoring resistance trends.

Purpose of the Study:

  • To assess the reproducibility of high amoxicillin minimum inhibitory concentrations (MIC(AMX)) for Streptococcus pneumoniae.
  • To compare the reproducibility of agar dilution and microdilution methods for determining MIC(AMX).

Main Methods:

  • Retesting of Streptococcus pneumoniae isolates with previously determined high MIC(AMX) (16 µg/ml and 8 µg/ml) using both agar dilution and microdilution methods.
  • Testing followed NCCLS guidelines with standard and 2 µg/ml dilution steps.
  • Multiple retests (10 times) were performed to evaluate reproducibility.

Main Results:

  • A significant shift towards higher MIC(AMX) values was observed with microdilution compared to agar dilution (p<0.05).
  • Routine MIC(AMX) of 16 µg/ml was not confirmed by agar dilution in any retested strain.
  • Microdilution confirmed the high MIC(AMX) in 6 strains, with modal values of 10 µg/ml and 14 µg/ml using 2 µg/ml steps.

Conclusions:

  • There is low reproducibility for the highest MIC(AMX) values in Streptococcus pneumoniae.
  • Microdilution methods may show a higher tendency towards elevated MIC(AMX) values compared to agar dilution.
  • Standardized methods and careful interpretation are essential for reliable MIC(AMX) determination.

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