THE USE OF AUTOMATED TUBIDIMETRIC DATA FOR THE CONSTRUCTION OF KINETIC-MODELS

An automated tubidimetric instrument (Bioscreen) was used to observe t he growth response of Listeria monocytogenes to combinations of temper ature (15-30-degrees-C), hydrogen-ion (0.1-21.9 mum) (equivalent pH 4. 66-7.0) and NaCl concentration (0.5-9.5% w/v). Compared to traditional plate count techniques, the technique allowed many more data points t o be captured and replicates to be used, with less expenditure of effo rt. Optical density curves were filtered (smoothed) to minimize the ef fect of signal noise and the mean signal from uninoculated wells was s ubtracted to minimize the effect of signal draft. A novel procedure fo r fitting growth curves to optical density data has been developed. Th e procedure involves the use of the logistic function and a calibratio n equation for fitting, in a single step, in the dimension of optical density. This approach allowed the four parameters of the logistic equ ation to be derived at each set of the experimental conditions. A quad ratic response surface was then fitted to the curve parameters using t emperature, NaCl and hydrogen-ion concentration as three independent v ariables. Predicted time to 1000-fold increase in cell numbers compare d well to predictions from predictive microbial growth equations gener ated in other laboratories using traditional plate counting. We propos e that this technique should be further evaluated as a method for gene rating data for modeling the kinetics of microbial growth.