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Effect of plate size and location of disc on zone diameter in the disc antimicrobial susceptibility test
Abstract:
An established standard antimicrobial disc susceptibility test recommends the use of a 150-mm petri dish. Many workers substitute 100-mm plates and use various types of mechanical dispensers. A comparison of zone diameters was made by testing Staphylococcus aureus and Escherichia coli against penicillin and chloramphenicol with the use of these different sized plates and dispensers. Zone diameters were consistently smaller at the edges of plates than in the center (0.4 to 1.2 mm). Significant small differences of up to 1.2 mm or no differences were observed when the mean diameters from these various plates were compared. All of the zone diameter measurements fell within accepted values for analytical variability in this procedure. There is no evidence that any individual combination of plates, dispensers, or disc locations provides greater precision in zone diameter measurement. More difficulty was encountered in measuring zones at the periphery of small plates. This suggests that busy clinical laboratory workers might not produce work of comparable quality with the smaller plates.
Insights
Smaller petri dishes (100-mm) and various dispensers are often used in antimicrobial susceptibility testing. However, these may lead to difficulties in zone diameter measurement, potentially impacting clinical laboratory quality.
Area of Science:
- Clinical Microbiology
- Antimicrobial Resistance
- Laboratory Diagnostics
Background:
- The standard antimicrobial disc susceptibility test utilizes 150-mm petri dishes.
- Many laboratories opt for 100-mm plates and diverse mechanical dispensers for practical reasons.
- The impact of these substitutions on test accuracy and precision remains a concern.
Purpose of the Study:
- To compare zone diameters obtained using standard 150-mm plates versus smaller 100-mm plates with various dispensers.
- To evaluate the precision and analytical variability of antimicrobial disc susceptibility testing with different plate sizes and dispensers.
- To assess the potential impact on clinical laboratory quality when using non-standard equipment.
Main Methods:
- Antimicrobial disc susceptibility testing was performed on Staphylococcus aureus and Escherichia coli.
- Tests were conducted using both 150-mm and 100-mm petri dishes with different mechanical dispensers.
- Zone diameters were measured and compared between the various experimental conditions.
Main Results:
- Zone diameters were consistently smaller at the edges compared to the center of plates (0.4 to 1.2 mm difference).
- Mean zone diameters showed minor differences (up to 1.2 mm) or no significant differences across plate sizes and dispensers.
- All measurements fell within accepted analytical variability for the procedure; however, measuring zones at the periphery of smaller plates was more difficult.
Conclusions:
- No specific combination of plates, dispensers, or disc locations demonstrated superior precision in zone diameter measurement.
- The use of smaller plates may introduce practical challenges in zone measurement, potentially affecting the quality of work in busy clinical laboratories.
- Further evaluation is needed to ensure the reliability and comparability of results when deviating from standard antimicrobial susceptibility testing protocols.