How does a bacterium grow during its cell cycle?

Rod-shaped bacteria such as Escherichia coli and Bacillus subtilis appear to extend continuously in length between divisions. However, the kinetics of growth of the individual cell in the steady state is still unknown. A brief, critical account of the main approaches used to determine the pattern of surface extension is given. In general, these approaches are of three types. Firstly, attempts have been made to relate average cell size to growth rate of the culture and to determine possible stages in the cell cycle at which the rate of length extension might change. Secondly, comparisons have been made between the measured length distribution of cells and theoretical distributions, based on three primary hypotheses (linear, bilinear and exponential growth). Thirdly, the principle of Collins and Richmond, involving the calculation of growth rate from the length distributions of extant, separating and new-born cells, is described. It is emphasized that there is a strong element of variation in size at different stages of the cell cycle. This variation imposes severe limitations on models which utilize only average cellular dimensions. We conclude that the Collins-Richmond principle affords the most powerful approach to the analysis of bacterial growth kinetics. However, we propose that the method be modified to permit calculation of separate rates of growth of cells between discernible events in the cell cycle, as well as simply between birth and division.