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Effects of procedural variations on the activity of aminoglycosides in vitro
Abstract:
Am J Clin Pathol 63:438-445, 1975. The effects of procedural variations on the activities of gentamicin, tobramycin, sisomicin, kanamycin, and amikacin in vitro were evaluated using 134 clinical isolates. In broth dilution studies, a change in assay medium from brain-heart infusion broth to Mueller-Hinton broth resulted in significant changes in minimal inhibitory concentrations in 36% (242 of 670) of assays. A change in the bacterial population size utilized in broth dilution studies resulted in significant changes in minimal inhibitory concentrations in 34% (155 of 456) of assays. These variations in activities appeared to depend more on the organism tested than on the particular aminoglycoside used; with strains of Staphylococcus aureus, Proteus, and Providenica being most affected. For all five aminoglycosides, minimal inhibitory concentrations determined by broth dilution, regardless of medium, showed poor correlation with zone sizes obtained by the Bauer-Kirby technic. These results suggest that unless some standard assay procedure for activity of aminoglycosides is adopted, meaningful comparison of results within and among laboratories will not be possible.
Insights
Procedural variations significantly alter aminoglycoside antibiotic activity testing. Standardizing methods is crucial for reliable in vitro susceptibility results and inter-laboratory comparisons.
Area of Science:
- Clinical Microbiology
- Antimicrobial Susceptibility Testing
Background:
- Aminoglycosides are critical antibiotics for treating serious bacterial infections.
- Standardized in vitro testing is essential for guiding clinical therapy.
Purpose of the Study:
- To evaluate the impact of procedural variations on the in vitro activity of five aminoglycosides.
- To assess the reliability of current susceptibility testing methods for aminoglycosides.
Main Methods:
- Broth dilution studies were performed using 134 clinical bacterial isolates.
- Assays varied medium composition (brain-heart infusion vs. Mueller-Hinton broth) and bacterial inoculum size.
- Minimal inhibitory concentrations (MICs) were compared with zone sizes from the Bauer-Kirby method.
Main Results:
- Changes in assay medium significantly altered MICs in 36% of tests.
- Variations in bacterial inoculum size significantly altered MICs in 34% of tests.
- Broth dilution MICs poorly correlated with Bauer-Kirby zone sizes for all tested aminoglycosides.
Conclusions:
- Procedural variations in susceptibility testing significantly affect aminoglycoside activity.
- Organism-specific differences were noted in susceptibility to these variations.
- Standardization of aminoglycoside susceptibility testing protocols is urgently needed for consistent results.