DMFit is an application to fit bacterial curves where a linear phase is preceded and followed by a stationary phase. It can be used to view a graphical representation of microbiological growth/survival data and fit a growth/survival model to the data. Bacterial curves can be fitted to two different types of models: the model of Baranyi and Roberts and the trilinear model, biphasic models and linear models.
Time vs. log counts (or ln counts) should be loaded into the data points table, on the left side of the screen. The dataset can be typed directly or copied from another application (e.g. Excel spreadsheet, textfile) and pasted. When bacterial concentrations are lower than the detection threshold, '-0.01' can be used as a code for not detected. Although displayed on the graph, this point will not used in the fitting. Having loaded data into the data table, the points will be fitted according to the models displayed in the results panel combobox. When the linear model is selected, regression coefficients are calculated with a linear regression subroutine. For any other model, an initial estimate of the growth parameters and then use this as a starting point for estimating the parameters by a non linear least square method. If the fitting algorithm does not converge to a solution, the points will not be fitted.
When the program finds estimates for the parameters, the estimated parameters and their standard errors are shown in the results panel along with goodness of fit data: 'R-square' (adjusted R-square) and 'SE of Fit' (standard error of fit) .The units of the estimates relate to the units of the data in the 'data points table. For example, if the bacterial counts are in log10 cfu/g and the Time in hours, the maximum growth/death rate will be in log10 cfu/g/h. If the counts are in ln cfu/g and the time in hours, the maximum rate will be in ln cfu/g/h (the rate expressed in this unit is also called 'specific maximum rate').