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Biosensor for Detection of Antibiotic Resistant Staphylococcus Bacteria
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Beta-lactam stability in frozen microdilution PASCO MIC panels using strains with known resistance mechanisms as

S Valdezate1, J Martínez-Beltrán, L de Rafael

  • 1Servicio de Microbiología, Hospital Ramón y Cajal, Madrid, Spain.

Diagnostic Microbiology and Infectious Disease
|October 1, 1996
PubMed

Insights

Antibiotic stability in frozen susceptibility panels varies by drug and storage temperature. Amoxicillin/clavulanate and piperacillin/tazobactam were stable at -70°C, while carbapenems degraded faster at -20°C, especially with resistant strains.

Area of Science:

  • Microbiology
  • Clinical Pharmacy
  • Pharmaceutical Science

Background:

  • Accurate antibiotic susceptibility testing (AST) is crucial for effective treatment.
  • Frozen microdilution panels are widely used for AST.
  • Understanding the long-term stability of antibiotics in these panels is essential for reliable results.

Purpose of the Study:

  • To evaluate the stability of key antibiotics in frozen microdilution susceptibility panels.
  • To determine the impact of storage temperature (-20°C and -70°C) and time (16 weeks) on antibiotic bioactivity.
  • To assess the influence of bacterial resistance mechanisms on antibiotic stability.

Main Methods:

  • Five antibiotics (amoxicillin/clavulanate, piperacillin/tazobactam, cefepime, imipenem, meropenem) were tested.
  • Frozen microdilution panels were stored at -20°C and -70°C for 16 weeks.
  • Minimum inhibitory concentration (MIC) values were determined using ATCC strains and strains with known resistance mechanisms.
  • Results were compared to standard agar dilution methods.

Main Results:

  • High agreement (97.7%) between panel and agar dilution methods at purchase.
  • Amoxicillin/clavulanate and piperacillin/tazobactam remained stable at -70°C throughout the study.
  • Carbapenems (imipenem, meropenem) showed decreased activity from week 12 at -70°C, and earlier at -20°C, particularly with resistant strains.
  • Cefepime demonstrated excellent stability at both temperatures.
  • Deterioration was more pronounced with resistant strains at -20°C.

Conclusions:

  • Antibiotic stability in frozen susceptibility panels is drug- and temperature-dependent.
  • Resistant strains are more sensitive indicators of antibiotic bioactivity loss during storage.
  • Standardization of quality control using resistant strains is recommended for routine AST panel stability studies.
  • -70°C storage offers better long-term stability for certain antibiotic combinations.

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