Comparative Evaluation of Methods Commonly Used to Determine Antimicrobial Susceptibility to Plant Extracts and Phenolic Compounds

A comparison was made to evaluate the ability of the most commonly used qualitative agar diffusion methods and a quantitative broth dilution assay to determine the antimicrobial activity of a plant extract and a variety of phenolic compounds. A disc and well diffusion technique and a microtiter broth microdilution (MBM) assay were used as antimicrobial susceptibility tests of a plant extract and several phenolic compounds against 7 bacterial species. In both the well and disc diffusion assays, the level of reproducibility was poor and a linear or logarithmic relationship did not exist between inhibition zone size and the concentration of the agents. The MBM method produced the most consistent results and allowed the determination of the relative sensitivities of each species and the relative antimicrobial activities of each agent. This study demonstrated that when a diffusion method is used, multiple concentrations of the agent must be assayed to ensure that a relationship exists between the concentration of the agent and inhibition zone size. When a relationship does not exist, antimicrobial activity should be determined by a quantitative dilution technique.