Asymmetric Cell Growth and Nucleoid Displacement in Escherichia coli

Abstract

Single celled growth of Escherichia coli is typically considered as an example symmetric division, based on the sizes of daughter cells and precision of center finding of the septum. Here, we investigate the symmetry of membrane addition in the mid-plane and DNA segregation using a video-microscopy approach. We find the membrane expansion dynamics to be asymmetric based on mid-cell photobleaching landmarks in FM4-64, used to stain the membrane. The apparent growth bias of E. coli does not correspond to the age of the pole. We find the membrane growth asymmetry is correlated to nucleoid displacement, consistent with ideas of coupling of cell growth and nucleoid positioning. The mobility of the actin-homolog MreB also correlates with membrane growth asymmetry, based on fluorescence recovery after photobleaching (FRAP) measurements of a YFP-fusion. These correlations suggest the small asymmetry of membrane addition observed could potentially drive nucleoid segregation and MreB mobility asymmetry in E. coli.\n\nIMPORTANCEAsymmetry in bacterial cell division is seen in its most simple form in the binomial event of protein segregation that results in noise in equal segregation and depends on the protein copy number. In the case of specific proteins this can also affect growth, and result in differentiation. However, during the asexual division of Escherichia coli single cells are thought to grow symmetrically and divide equally. We find a slight but consistent asymmetry in growth based on quantitative morphometry of cell pole displacement in time-series of growing E. coli cells. Increased cell wall extension in one half of the cell over the other appears to explain the asymmetry in the displacement of cell poles. Interestingly the growth asymmetry is not correlated to the age of the pole (old and new). This observed asymmetry appears to correlate with the asymmetry of nucleoid segregation, resulting in the nucleoids finding the midpoints of the respective daughter cells, and formation of the septum at the geometric centre of the elongated cell just prior to division. The mobility dynamics of the cytoskeletal protein MreB, which organizes the cell membrane, is are more dynamic where membrane growth is faster. Thus we propose a linkage between this observed growth asymmetry and that of MreB dynamics.