Timing the Start of Division in E. coli: a Single-Cell Study

Abstract

Precise determination of morphology dynamics during growth and division of bacterial cells is restricted by optical resolution and micron size of the object. We have developed a method for high precision cell edge detection in a phase-contrast image allowing continuous follow up of the cell contour with about 30 nm accuracy. This approach is used to analyze the entire life cycle of single E. coli cells and provides a detailed morphological characterization of the cell division process. We show that initiation of the envelope constriction occurs much earlier than the appearance of a visible constriction, and is also manifested in a break in the length dynamics corresponding to the addition of new poles formation. We use simple rescaling of variables to provide a global view of the entire cell population. In particular, the data for the dynamics of the constriction width for all the cells in the population collapses to the vicinity of the function predicted by our theoretical model. Some of the parameters that describe cell division obey certain quantitative relations. In addition, we have developed an algorithm for analysis of the spatial distribution of the division initiator protein, tubulin-like FtsZ, in fluorescent images of single cells. With this algorithm, profile and positional dynamics of the FtsZ constriction ring were analyzed, revealing a time gap between the ring maturation and the start of constriction. This gap is presumably required for assembly of the other division proteins forming the divisome. This information provides new constraints on the possible molecular mechanisms involved in the formation of both the divisome and the cell septum.