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Susceptibility testing of linezolid by two standard methods
1Department of Medical Microbiology, Royal Free Hospital and University College Medical School, London, UK.
Summary
Methodological differences in susceptibility testing significantly impact linezolid results. While zone sizes varied, minimum inhibitory concentrations (MICs) were largely consistent, except for pneumococci, necessitating breakpoint adjustments for accurate linezolid susceptibility testing.
Area of Science:
- Clinical Microbiology
- Antimicrobial Susceptibility Testing
- Pharmaceutical Sciences
Background:
- Linezolid is a crucial antibiotic for treating infections caused by resistant gram-positive cocci.
- Standardized susceptibility testing methods are essential for guiding appropriate antimicrobial therapy.
- Variations in laboratory methodologies can potentially influence antimicrobial susceptibility test (AST) results.
Purpose of the Study:
- To compare linezolid susceptibility testing parameters using two distinct methodologies: National Committee for Clinical Laboratory Standards (NCCLS) and British Society for Antimicrobial Chemotherapy (BSAC).
- To evaluate the impact of these methodological differences on Minimum Inhibitory Concentrations (MICs) and disk diffusion zone sizes.
- To identify potential discrepancies and their implications for clinical interpretation of linezolid susceptibility.
Main Methods:
- Susceptibility testing of 198 gram-positive cocci strains against linezolid.
- Comparison of MICs and disk inhibition zones obtained via NCCLS and BSAC guidelines.
- Analysis of results, considering species-specific variations and incubation conditions (e.g., CO2 incubation).
Main Results:
- Disk inhibition zones were consistently larger (4-5 mm) when using the BSAC method compared to NCCLS.
- MIC values for linezolid did not significantly differ between the two methods across most strains.
- An exception was observed for pneumococci, which showed increased sensitivity (lower MICs) with the BSAC method, attributed to CO2 incubation effects on linezolid activity.
Conclusions:
- Methodological variations in susceptibility testing, particularly incubation conditions, can influence linezolid's apparent activity against specific pathogens like pneumococci.
- Current breakpoints for linezolid may require re-evaluation or adjustment when employing testing methods other than NCCLS standards.
- Standardization or clear guidelines are needed to ensure consistent and reliable linezolid susceptibility testing globally.