Quantitative investigation of bacterial chemotaxis at the single-cell level

Living cells sense and respond to constantly changing environmental conditions. Depending on the type of stimuli, the cell may response by altering gene expression pattern, secreting molecules, or migrating to a different environment. Directed movement of cells in response to chemical stimuli is called chemotaxis. In bacterial chemotaxis, small extracellular molecules bind receptor proteins embedded in the cell membrane, which then transmit the signal inside the cell through a cascade of proteinprotein interactions. This chain of events influences the behavior of motor proteins that drive the rotation of helical filaments called flagella. Individual cells of the gut-dwelling bacteria Escherichia coli (E. coli) have many such flagella, whose collective action results in the swimming behavior of the cell. A recent study found that in absence of chemical stimuli, fluctuations in the protein cascade can cause non-Poissonian switching behavior in the flagellar motor (2). A corollary was that extension of such behavior to the whole-cell swimming level would have implications for E. coli’s foraging strategy. However, existence of such behavior at the swimming cell level could not be predicted a priori, since the mapping from single flagellum behavior to the swimming behavior of a multi-flagellated cell is complex and poorly understood (3, 4). Here we characterize the chemotactic behavior of swimming E. coli cells using a novel optical trap-based measurement technique. This technique allows us to trap individual cells and monitor their swimming behavior over long time periods with high temporal resolution. We find that swimming cells exhibit non-Poissonian switching statistics between different swimming states, in a manner similar to the rotational direction-switching behavior seen in individual flagella. Furthermore, we develop a data analysis routine that allows us to characterize higher order swimming features such as reversal of swimming direction and existence of multiple swimming speeds. When stimulated with a step-increase in chemo-attractants, E. coli cells initially respond by reducing the frequency of swimming direction change. Over time, however, cells return to their pre-stimulus behavior despite the increased chemo-attractant concentration in the environment. This process is called chemotactic adaptation. Adaptation allows cells to maintain chemotactic sensitivity over a wide range of background chemical concentrations. iii We study chemotactic adaptation of E. coli at the individual cell level using our optical trapping method. Chemical stimuli were delivered from the chemical gradient established in a custom-made laminar flow device. We observe two striking features of individual cell’s adaptation and their dependence on stimulus strength. We also observe asymmetry between responses to positive and negative stimuli. Existing evidence and theoretical models suggest that the observed features of single-cell adaptation and their dependence on stimulus strength may be explained in terms of interactions of neighboring receptor proteins in large clusters. Further experiments using various mutant strains of E. coli would shed light on the molecular-level mechanisms of the observed behavior