Related Experiment Video
Updated: Aug 5, 2026
Antimicrobial Characterization of Advanced Materials for Bioengineering Applications
Published on: August 4, 2018
Diffusion of trimethoprim and sulfamethoxazole from susceptibility disks into agar medium
Abstract:
The standard practice of using a single susceptibility disk for the antimicrobial combination Septra (trimethoprim/sulfamethoxazole) has been further justified by a direct measurement of the diffusion rates of each compound through agar medium. [(14)C]trimethoprim and [(35)S]sulfamethoxazole, singly and in combination, were applied to blank susceptibility disks which were incubated on 4-inch (10.16-cm) agar plates (Mueller-Hinton medium) at 37 C. The migration of each compound from the disk and diffusion through agar were measured with time by determining the radioactivity in concentric zones extending from the origin. The two compounds diffuse with similar rates, maintaining approximately a 1:20 concentration ratio which is approximately the ratio of trimethoprim to sulfamethoxazole observed in plasma during treatment. The diffusion rate is independent of the presence of the other compound; greater than 95% of the radioactivity is transferred from the disk to the agar in 24 h.
Insights
This study measured the diffusion rates of trimethoprim and sulfamethoxazole. Their similar diffusion rates in agar support using a single disk for antimicrobial susceptibility testing of Septra.
Area of Science:
- Microbiology
- Pharmacology
- Clinical Laboratory Science
Background:
- Antimicrobial susceptibility testing (AST) is crucial for guiding antibiotic therapy.
- Septra (trimethoprim/sulfamethoxazole) is a commonly used antibiotic combination.
- Standard AST for Septra often uses a single susceptibility disk.
Purpose of the Study:
- To directly measure and compare the diffusion rates of trimethoprim and sulfamethoxazole through agar.
- To provide scientific justification for the standard practice of using a single disk for Septra AST.
Main Methods:
- Radiolabeled trimethoprim and sulfamethoxazole were applied to susceptibility disks.
- Disks were incubated on Mueller-Hinton agar plates at 37°C.
- Radioactivity was measured in concentric zones to determine compound migration and diffusion over time.
Main Results:
- Trimethoprim and sulfamethoxazole exhibited similar diffusion rates in agar.
- The compounds maintained an approximate 1:20 concentration ratio, mirroring in-vivo plasma ratios.
- Diffusion rates were independent of the presence of the other compound.
- Over 95% of radioactivity transferred from disk to agar within 24 hours.
Conclusions:
- The similar diffusion rates of trimethoprim and sulfamethoxazole support the use of a single disk for Septra susceptibility testing.
- This finding validates current laboratory practices for this important antimicrobial combination.
