Efficiency and Robustness to Nonoptimal Temperatures of Nucleoid Exclusion Processes in Escherichia coli

Abstract

Prokaryotic organisms, such as Escherichia coli, lack internal wall. Nevertheless, they have internal spatial organization, which is a critical requirement for the proper functioning of several cellular processes. One of the internal structures that is crucial for the emergence and maintenance of such organization is the nucleoid. With a higher density than the cytoplasm, the nucleoid is able to segregate different macromolecules — such as protein aggregates and chemotaxis clusters — to the cell poles, while also placing structures, such as the Z-ring, between recently replicated nucleoids during cell division. In this regard, for a cell population to thrive in fluctuating environments, these cellular processes need not only to be efficient in optimal conditions, but also robust to nonoptimal conditions. Here, we study the efficiency of the processes of segregation of protein aggregates, polarization of chemotaxis network, and Z-ring positioning at midcell and relate it with the nucleoid’s morphology. In order to evaluate their robustness to nonoptimal conditions, we also study the effects of temperature shifts on the nucleoid(s) morphology, and how this then affects the efficiency of these biophysical processes. For this, we collected confocal microscopy images of populations of cells with fluorescently tagged protein aggregates, protein clusters composing the chemotaxis networks, and FtsZ-proteins composing Z-rings, at different temperatures. In addition, we also stained the nucleoids of these cells. From the analysis of the data collected from the images, we found that, for each temperature condition, the spatial distribution of the cellular components observed is consistent with the nucleoid’s volume exclusion effect. Furthermore, we found that the nucleoid’s length along the major cell axis is correlated with the kurtosis of the spatial distribution of protein aggregates and chemotaxis clusters along that axis. Similarly, the distribution of distances between replicated nucleoids (prior to cell division) along the major cell axis is correlated with the kurtosis of the spatial distribution of Z-rings along that axis. Finally, we found a negative correlation between the efficiency of these spatial placement processes at optimal temperatures and their robustness to nonoptimal temperatures, indicating a trade-off between these features. Overall, these results suggest that the robustness of the morphological features of the nucleoid to temperature shifts contribute to the adaptability of E. coli to non-optimal temperatures.