Development of a dynamic continuous-discrete-continuous model describing the lag phase of individual bacterial cells

Aims: A previous model for adaptation and growth of individual bacterial cells was not dynamic in the lag phase, and could not be used to perform simulations of growth under non-isothermal conditions. The aim of the present study was to advance this model by adding a continuous adaptation step, prior to the discrete step, to form a continuous-discrete-continuous (CDC) model. Methods and Results: The revised model uses four parameters: N-0, intial population; N-max, maximum population; p(0), mean initial individual cell physiological state; SDp0, standard deviation of the distribution of individual physiological states. A truncated normal distribution was used to generate tables of distributions to allow fitting of the CDC model to viable count data for Listeria monocytogenes grown at 5 degreesC to 35 degreesC. The p(0) values increased with increasing SDp0 and were, on average, greater than the corresponding population physiological states (h(0)); p(0) and h(0) were equivalent for individual cells. Conclusions: The CDC model has improved the ability to simulate the behaviour of individual bacterial cells by using a physiological state parameter and a distribution function to handle inter-cell variability. The stages of development of this model indicate the importance of physiological state parameters over the population lag concept, and provide a potential approach for making growth models more mechanistic by incorporating actual physiological events. Significance and Impact of the Study: Individual cell behaviour is important in modelling bacterial growth in foods. The CDC model provides a means of improving existing growth models, and increases the value of mathematical modelling to the food industry.