Modelling the combined temperature and salt (NaCl) limits for growth of a pathogenic Escherichia coli strain using nonlinear logistic regression

A broth-based method is used to determine if exponential phase Escherichia coli R31, an STEC, is able to grow within 50 days under various combination s of sub-optimal temperatures and salt concentrations. From these data, the growth limits for combinations of temperature (7.7-37.0 degreesC) and wate r activity (0.943-0.987; NaCl as humectant) are defined and modelled using a nonlinear logistic regression model. That form of model is able to predic t the combinations of salt concentration/water activity and temperature tha t will prevent the growth of E. coli R31 with selected levels of confidence . The model fitted the data with an approximate concordance rate of 97.3%. The minimum water activity that permitted growth occurred in the range 25-3 0 degreesC, the temperature range which optimises cell yield. At temperatur es below this range the minimum water activity which allowed growth increas ed with decreasing temperature. (C) 2000 Elsevier Science B.V. All rights r eserved.