A COMPARISON OF METHODS FOR THE MEASUREMENT OF BACTERIOCIN ACTIVITY

Agar diffusion (spot and well) and photometric (tube or microtiter) assays, both in the form of critical dilution and quantitative assays, were compared for 4 bacteriocins (enterocin 1146, lactococcin 140, leucocin F10 and nisin) against 2 indicator strains each. In the agar well diffusion assay (AWDA) a linear relationship existed between response (diameter or area of the zone of inhibition) and the logarithm of the dose while a non-linear equation was used to model the sigmoidal dose/response curve in photometric assays (PA). The dose/response curves were used to define titers of the standard solutions in arbitrary units and to develop quantitative assays for all the bacteriocins. With the exception of lactococcin 140, the PA provided estimates which were more reproducible than those obtained with the AWDA. The quantitative assays compared favourably with the classical critical dilution assays for bacteriocins, eliminating the need to dilute to extinction to estimate the titers of bacteriocin solutions, providing a continuous scale for activity and allowing the calculation of confidence intervals.