Determining Doubling Time

As the number of cells in a microbial culture increases, turbidity increases. In this experiment students calculate doubling time and growth rate constant using absorbance as the measure of growth. Turbidity is caused by suspended cells in the growth medium scattering light, and may be measured using a colorimeter. Absorbance increases as the cell concentration increases giving a convenient, rapid and accurate method of measuring cell growth rates. Absorbance is plotted against time on a graph, which enables students to determine both doubling time and growth rate constant for the microorganism. Doubling of absorbance indicates doubling of the number of cells and the time taken for this to occur can be read from the graph.