Assessment of formulas for calculating critical concentration by the agar diffusion method

Area of Science:

• Microbiology
• Pharmacology
• Clinical Chemistry

Background:

• Accurate determination of critical antibiotic concentrations is essential for effective antimicrobial susceptibility testing.
• Existing methods for calculating critical concentrations, such as those based on Fick's law by Cooper and Woodman, have limitations.
• The agar diffusion method is a common technique for assessing antibiotic activity.

Purpose of the Study:

• To compare the accuracy of two formulas, Cooper and Woodman and Vesterdal, in calculating critical antibiotic concentrations using the agar diffusion method.
• To evaluate the proximity of calculated critical concentrations to the Minimum Inhibitory Concentration (MIC) determined by agar dilution.
• To assess the influence of method parameters on critical concentration determination.

Main Methods:

• Agar diffusion method with antibiotic-charged disks was employed.
• Critical concentrations were calculated using Cooper and Woodman's and Vesterdal's formulas.
• Results were compared against Minimum Inhibitory Concentration (MIC) values obtained via agar dilution.
• Ninety-one bacterial strains and two cephalosporins (cefotaxime, ceftriaxone) were analyzed.

Main Results:

• The Cooper and Woodman formula yielded critical concentrations higher than the MIC.
• The Vesterdal formula produced critical concentrations closer to the MIC.
• Determined critical concentrations were found to be independent of method parameters, such as dilution.

Conclusions:

• The Vesterdal formula provides a more accurate estimation of critical antibiotic concentrations compared to the Cooper and Woodman formula when using the agar diffusion method.
• This finding supports the use of the Vesterdal formula for more reliable antimicrobial susceptibility testing.
• The critical concentration is a robust measure, unaffected by variations in experimental setup like dilution differences.