Important steps in our understanding of the bio–molecular mechanism of the bacterial chemotaxis allow for building an abstract informational–cybernetic model. The aim of this paper is to show that a mathematical model of complexity based on local (but not overall) shared information (soft assembly) allows for setting the appropriate constraints on both the connections among the bacterium’s subystems as well as between the bacterium and the external environment (the case of Escherichia coli is considered here). The conclusion is that a necessary trade–off between intrinsic order and variability allows a good modelisation of the bacterium’s behaviour.