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Short-term storage of six penicillins and cephalothin in microdilution trays for antimicrobial susceptibility tests
Abstract:
To perform microdilution antimicrobial susceptibility tests efficiently, broth dilutions of the antimicrobial drugs should be prepared in large batches and stored frozen until needed. Studies were carried out to document the stability of ampicillin, benzylpenicillin, carbenicillin, methicillin, oxacillin, nafcillin, and cephalothin in microdilution trays during storage at -20 and -60 C. No significant deterioration was noted after 21 days of storage, as detected by bioassay and by replicate minimal inhibitory concentration determinations with control organisms. No significant differences were noted moreover between trays stored for three weeks at -20 C and those stored at -60 C. The microdilution technique was found to be a highly reproducible method for quantitative determination of antimicrobial susceptibility.
Insights
Prepare antimicrobial drug dilutions in bulk and freeze for efficient antimicrobial susceptibility testing. Storing these dilutions for up to 21 days at -20°C or -60°C showed no significant drug deterioration.
Area of Science:
- Microbiology
- Pharmacology
- Clinical Laboratory Science
Background:
- Efficient antimicrobial susceptibility testing (AST) requires reliable preparation of antimicrobial drug dilutions.
- Broth microdilution is a common method for AST, but requires stable drug preparations.
- Investigating the stability of commonly used antibiotics in frozen microdilution trays is crucial for laboratory efficiency.
Purpose of the Study:
- To assess the stability of several beta-lactam antibiotics in frozen microdilution trays.
- To determine the optimal storage temperature for long-term preservation of antibiotic dilutions.
- To validate the reproducibility of the microdilution method for AST.
Main Methods:
- Seven beta-lactam antibiotics (ampicillin, benzylpenicillin, carbenicillin, methicillin, oxacillin, nafcillin, cephalothin) were prepared in broth dilutions.
- Diluted antibiotic trays were stored at -20°C and -60°C for 21 days.
- Antibiotic stability was evaluated using bioassays and replicate minimal inhibitory concentration (MIC) determinations with standard microbial strains.
Main Results:
- No significant deterioration of ampicillin, benzylpenicillin, carbenicillin, methicillin, oxacillin, nafcillin, and cephalothin was observed after 21 days of frozen storage.
- Storage temperature (-20°C vs. -60°C) did not significantly impact antibiotic stability over the 21-day period.
- The microdilution technique demonstrated high reproducibility for quantitative antimicrobial susceptibility determination.
Conclusions:
- Frozen broth microdilution trays containing ampicillin, benzylpenicillin, carbenicillin, methicillin, oxacillin, nafcillin, and cephalothin are stable for at least 21 days at both -20°C and -60°C.
- This stability allows for efficient batch preparation of antimicrobial dilutions for routine AST.
- The microdilution method is a reliable and reproducible technique for quantitative AST.