Quantitative investigation of bacterial chemotaxis at the single-cell level

Bacterial Chemotaxis;Optical Trapping;Signal Transduction

Min, Taejin

关键词: Bacterial Chemotaxis;    Optical Trapping;    Signal Transduction;   

Others  :  https://www.ideals.illinois.edu/bitstream/handle/2142/31056/Min_Taejin.pdf?sequence=1&isAllowed=y

美国|英语

来源: The Illinois Digital Environment for Access to Learning and Scholarship

【 摘 要 】

Living cells sense and respond to constantly changing environmental conditions. Dependingon the type of stimuli, the cell may response by altering gene expression pattern, secretingmolecules, or migrating to a different environment. Directed movement of cells in response tochemical stimuli is called chemotaxis.In bacterial chemotaxis, small extracellular molecules bind receptor proteins embedded in thecell membrane, which then transmit the signal inside the cell through a cascade of proteinproteininteractions. This chain of events influences the behavior of motor proteins that drive therotation of helical filaments called flagella. Individual cells of the gut-dwelling bacteriaEscherichia coli (E. coli) have many such flagella, whose collective action results in theswimming 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 flagellarmotor (2). A corollary was that extension of such behavior to the whole-cell swimming levelwould have implications for E. coli’s foraging strategy. However, existence of such behavior atthe swimming cell level could not be predicted a priori, since the mapping from single flagellumbehavior 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 noveloptical trap-based measurement technique. This technique allows us to trap individual cells andmonitor their swimming behavior over long time periods with high temporal resolution. We findthat swimming cells exhibit non-Poissonian switching statistics between different swimmingstates, in a manner similar to the rotational direction-switching behavior seen in individualflagella. Furthermore, we develop a data analysis routine that allows us to characterize higherorder swimming features such as reversal of swimming direction and existence of multipleswimming speeds.When stimulated with a step-increase in chemo-attractants, E. coli cells initially respond byreducing the frequency of swimming direction change. Over time, however, cells return to theirpre-stimulus behavior despite the increased chemo-attractant concentration in the environment.This process is called chemotactic adaptation. Adaptation allows cells to maintain chemotacticsensitivity over a wide range of background chemical concentrations.iiiWe study chemotactic adaptation of E. coli at the individual cell level using our opticaltrapping method. Chemical stimuli were delivered from the chemical gradient established in acustom-made laminar flow device. We observe two striking features of individual cell’sadaptation and their dependence on stimulus strength. We also observe asymmetry betweenresponses to positive and negative stimuli. Existing evidence and theoretical models suggest thatthe observed features of single-cell adaptation and their dependence on stimulus strength may beexplained in terms of interactions of neighboring receptor proteins in large clusters. Furtherexperiments using various mutant strains of E. coli would shed light on the molecular-levelmechanisms of the observed behavior.