Energy Taxis in Escherichia Coli on the Single-Cell Level

Abstract

Energy taxis allows bacteria to respond to external stresses that affect its electron transport system, and to navigate to the environment optimal for energy generation. For example, in an oxygen gradient bacteria accumulate in a sharply defined band at the oxygen concentration that maximizes their proton motive force (PMF) which is coupled to electron transport (Zhulin, 1996). Examples of energy taxis include aerotaxis, taxis to alternative electron acceptors, and taxis to respiratory substrates. In E. coli, energy sensing is mediated by Tar and Aer receptors: Tar senses changes in PMF, while Aer is likely a redox sensor (Edwards, 2007). It is therefore expected that in the spatial gradient of oxygen or alternative electron acceptors changes in bacterial swimming behavior, e.g. tumble bias, will be correlated with PMF, but this has never been demonstrated directly.We study taxis to nitrate - an alternative electron acceptor for E. coli - in bacteria under anaerobic conditions at the single cell level, using an optical trap (Min, 2009). We move bacteria in a defined gradient of nitrate inside the flow chamber, and measure resulting changes in both tumble bias and flagella rotation frequency, which is proportional to PMF under low viscous load that corresponds to free swimming bacteria (Gabel, 2003).Our preliminary results show that under anaerobic conditions E. coli respond to changes in nitrate concentration, which is reflected in both tumble bias and flagella rotation. Thus running response or decrease in tumble bias corresponds to the increase in flagella rotation frequency and vice versa. While changes in tumble bias are transient and are followed by adaptation, nitrate concentration change have long-lasting effect on flagella rotation which is expected if PMF depends on the absolute concentration of nitrate.Using our method we can measure preferred concentration of nitrate for individual bacteria at which their PMF is maximized and how it depends on other factors such as concentration of electron donor, energetic state of the bacteria and mode of metabolism.References J. C. Edwards et al. Mol Microbiol (2007). I. B. Zhulin et al. J Bacteriol (1996). T. L. Min et al. Nature Methods (2009). C. V Gabel & H. C. Berg. PNAS (2003).