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The relationship of disc position to zone-of-inhibition size in the disc-agar diffusion test
Abstract:
One hundred sixteen clinical isolates of Enterobacter sp., Escherichia coli, Klebsiella sp., Proteus mirabilis, Staphylococcus aureus and S. epidermidis were tested against cefoperazone, cefoxitin, cefamandole and piperacillin by disc-agar diffusion. Zones-of-inhibition of discs dropped near the center of the plate were compared with those dropped near the periphery. In 28% of the comparisons the inner and outer zones differed by 2 mm or more. Of those, the inner zone was larger 2.5 times more often than the outer zone. Results suggest that the chances of obtaining a susceptible result are greater when discs are placed near the center as opposed to the periphery of the plate.
Insights
Disc placement impacts antibiotic susceptibility testing results. Placing antibiotic discs closer to the center of the agar plate increases the likelihood of accurate susceptibility results for clinical isolates.
Area of Science:
- Clinical microbiology
- Antimicrobial susceptibility testing
- Bacterial resistance
Background:
- Accurate antimicrobial susceptibility testing (AST) is crucial for guiding effective antibiotic therapy.
- Standardized methods for AST are essential to ensure reliable and reproducible results.
- The physical placement of antibiotic discs on agar plates may influence zone-of-inhibition measurements.
Purpose of the Study:
- To evaluate the effect of antibiotic disc placement (center vs. periphery) on zone-of-inhibition measurements in disc-agar diffusion assays.
- To determine if disc location influences the interpretation of bacterial susceptibility to antibiotics.
Main Methods:
- Disc-agar diffusion method was employed.
- One hundred sixteen clinical isolates including Enterobacter sp., Escherichia coli, Klebsiella sp., Proteus mirabilis, Staphylococcus aureus, and S. epidermidis were tested.
- Zones-of-inhibition were measured for discs placed centrally versus peripherally on the agar plate.
Main Results:
- A significant difference (≥2 mm) in zone-of-inhibition was observed in 28% of comparisons between central and peripheral disc placements.
- The inner (central) zones were larger than the outer (peripheral) zones 2.5 times more frequently.
- Central disc placement appears to yield larger zones of inhibition.
Conclusions:
- Antibiotic disc placement significantly affects zone-of-inhibition measurements in disc-agar diffusion.
- Central disc placement is more likely to result in a susceptible interpretation compared to peripheral placement.
- Standardization of disc placement is recommended for consistent and reliable AST results.