Modeling development of inhibition zones in an agar diffusion bioassay

A two-temperature agar diffusion bioassay is commonly used to quantify the concentration of nisin using Micrococcus luteus as the indicator microorganism. A finite element computational model based on Fick's second law of diffusion was used to predict the radius of the inhibition zone in this diffusion bioassay. The model developed was used to calculate nisin concentration profiles as a function of time and position within the agar. The minimum inhibitory concentration (MIC) of nisin against M. luteus was determined experimentally. The critical time (T(c)) for growth of M. luteus within the agar diffusion bioassay was experimentally determined using incubation studies with nisin. The radius of the inhibition zone was predicted from the computational model as the location where the predicted nisin concentration at T(c) was equal to MIC. The MIC was experimentally determined to be 0.156 μg mL(−1), and T(c) was determined to be 7 h. Good agreement (R(2) = 0.984) was obtained between model-predicted and experimentally determined inhibition zone radii.