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Evaluation of two methods for overcoming the antibiotic carry-over effect
R H Eng1, S M Smith, C E Cherubin
1Infectious Disease Section/Medical Service, Veterans Administration Medical Center, East Orange, New Jersey 07019.
Summary
The antibiotic carry-over effect can lead to falsely low Minimum Bactericidal Concentration (MBC) results. Spreading bacterial samples on agar plates or washing them before plating eliminates this issue, improving accuracy for critical infections.
Area of Science:
- Microbiology
- Clinical Laboratory Science
- Pharmacology
Background:
- The antibiotic carry-over effect is a known issue in determining Minimum Bactericidal Concentration (MBC).
- This effect occurs when residual antibiotics in a transferred sample inhibit bacterial growth on agar plates.
- It leads to inaccurate, falsely low MBC values, potentially impacting treatment decisions.
Purpose of the Study:
- To identify and address the antibiotic carry-over effect in MBC determination.
- To propose and validate methods for eliminating this phenomenon in clinical laboratory settings.
- To ensure accurate MBC testing for patients with severe infections.
Main Methods:
- Investigated the impact of antibiotic carry-over on MBC results.
- Evaluated two methods to eliminate the carry-over effect: wide streaking of the inoculum and centrifugation/resuspension in antibiotic-free media.
- Compared the efficacy and practicality of these methods against the standard MBC determination technique.
Main Results:
- The antibiotic carry-over effect was confirmed to cause falsely low MBC values.
- Both wide streaking and centrifugation/resuspension effectively eliminated the antibiotic carry-over effect.
- These methods, while requiring more effort, are feasible for clinical laboratories.
Conclusions:
- Accurate MBC determination is crucial, especially for infections like endocarditis and meningitis.
- Implementing wide streaking or centrifugation/resuspension methods is highly recommended to overcome antibiotic carry-over.
- These validated techniques enhance the reliability of MBC testing in clinical practice.