Experimental evolution partially restores functionality of bacterial chemotaxis network with reduced number of components.

The chemotaxis signaling pathway, which enables bacteria to follow chemical gradients in their environment, is highly conserved among motile bacteria. It is assumed that Escherichia coli contains the minimal and non-redundant set of protein activities that are necessary for bacterial chemotaxis and nearly universally conserved among bacterial chemotaxis pathways. These include stimulus sensing, signal transduction towards the flagellar motor, and adaptation-based temporal comparisons of the environment. In this study, we show that functionality of the chemotaxis signaling pathway lacking some of its proteins can be partially regained by subjecting E. coli strains to experimental evolution under selection for chemotactic spreading in porous medium. While the core signaling components are indeed essential for the pathway function, the absence of auxiliary pathway proteins required for adaptation and desensitization could be compensated by specific sets of mutations affecting the other pathway components. Further characterization of the evolved strain lacking the adaptation enzyme CheR suggested that this strain utilizes an alternative mechanism of biased drift in chemical gradients, which does not rely on short-term adaptation that is normally considered a prerequisite for bacterial chemotaxis. Although the efficiency of this alternative mechanism remains below the one that can be achieved by the original memory-based chemotaxis strategy of E. coli, it can mediate chemotaxis not only in porous medium but also in liquid. Thus, even short-term experimental evolution of microorganisms can result in the appearance of behavioral strategies that are qualitatively different from those used by parental organisms.