Two concurrent processes—a cell-related interdivision cycle and a chromosome cycle—compete to determine cell division.

Escherichia coli has long been used as a model organism due to the extensive experimental characterization of its pathways and molecular components. Take chemotaxis as an example, which allows bacteria to sense and swim in response to chemicals, such as nutrients and toxins. Many of the pathway's remarkable sensing and signaling properties are now concisely summarized in terms of design (or engineering) principles. More recently, new approaches from information theory and stochastic thermodynamics have begun to address how pathways process environmental stimuli and what the limiting factors are. However, to fully capitalize on these theoretical advances, a closer connection with single-cell experiments will be required. IntroductionAll living organisms from animals to unicellular bacteria live under constant evolutionary pressure. To stay ahead in the game of evolution, organisms need to process noisy information, allowing them to make survival decisions quickly. However, to process information and move organisms also require energy. Thus the final behavior of any organism has to be an outcome which produces strong advantages under likely occurring environments. Chemotaxis of Escherichia coli is particularly well understood in terms of its molecular components, allowing this bacterium to migrate towards food and away from toxins [1][2][3][4][5]. Indeed, an ever increasing amount of studies has highlighted several design principles, i.e. engineering blue prints, ensuring exquisite sensitivity, efficiency, robustness, and wide dynamic range at all levels of the pathway.This review focuses on recent findings in E. coli chemotaxis, in particular on how molecular mechanisms give rise to information processing, its associated thermodynamic cost, and the resulting swimming behavior. What new design principles will be discovered next? Classical view of Escherichia coli chemotaxisEscherichia coli is a Gram-negative bacterium inhabiting soil, as well as the animal and human gastrointestinal tracts. Inside the host, it contributes to the digestion of food and enhances resistance against pathogens [6]. This bacterium has a relatively simple chemotactic pathway (Fig. 1 1A). External stimuli are processed at the receptor level, where receptors sense and memorize chemical concentrations from the past by their adapted methylation level (Fig. 1A, see red box for 'sensing module'). The receptor-signaling activity can be monitored experimentally by tagging the CheY and CheZ proteins with a fluorescence-resonance-energy-transfer (FRET) reporter pair to follow their phosphorylation-dependent interaction. To swim cells are equipped with 5-8 flagellar rotary motors (Fig. 1A, blue box for 'motility module'), each of which rotates either clockwise (CW) or counterclockwise (CCW). Taking together these flagella determine cell movement, given either by a 'run' or a random reorientation in a 'tumble' [7][8][9]. The phosphorylated protein CheY-p links sensing and motility (Fig. 1A). In absence of any chemical gradient E. coli per...

Understanding the classic problem of how single E. coli cells coordinate cell division with genome replication would open the way to addressing cellcycle progression at the single-cell level. Recent studies produced new data, but the contrast in their conclusions and proposed mechanisms makes the emerging picture fragmented and unclear. Here, we re-evaluate available data and models, including generalizations based on the same assumptions. We show that although they provide useful insights, none of the proposed models captures all correlation patterns observed in data. We conclude that the assumption that replication is the bottleneck process for cell division is too restrictive. Instead, we propose that two concurrent cycles responsible for division and initiation of DNA replication set the time of cell division. This framework allows us to select a nearly constant added size per origin between subsequent initiations as the most likely mechanism setting initiation of replication.