Modeling the Effect of Natural Substances and Antibiotics Concentrations on the Variation of Inhibition Zone Diameter in Disc Diffusion Susceptibility Test

Aims: The aim of this study was to propose a model describing the relationship between the antimicrobial concentration and the inhibition zone diameter during antimicrobial susceptibility testing.

Place and Duration of Study: Department of Microbiology of the University of Yaoundé I between June 2016 and December 2016 (6 months).

Methodology: Water, ethanol extracts and essential oils from plants as well as antibiotics were tested at different concentrations against five bacteria including both Gram+ and Gram-. The inhibition diameters obtained were plotted against the quantity of substance loaded on the disc. Data were divided into two groups and one was used for model construction (test data) while the other was combined to literature data and used for validation. The model construction begun with CurveExpert 1.4 software in order to search for the most suitable equation family fitting the data. The model family selected was modified accordingly to reduce the number of parameters.

Results: We propose a model where the inhibition diameter (Id) = aXb where “a” and “b” are parameters estimated using nonlinear regression, X is the quantity of antimicrobial deposited on the disc. Validation of the model on new set data gave R2 values ranging between 0.96 and 0.98. Moreover the proposed model described three inhibition patterns already discussed theoretically in literature.

Conclusion: This work proposes for the first time a simple and direct mathematical relationship between the antimicrobial concentration and inhibition diameter in the disc diffusion susceptibility test. This may be used in comparing the level of activity of antimicrobials and contribute in classifying natural substances according to their activity pattern.