Physics of microbial taxis and behaviours in response to various physical stimuli.

The physical environment exerts a profound influence on microbial life. The directional movement of cells in response to their physical environment is understood as taxis, which has been studied in biology as chemotaxis, phototaxis, gravitaxis and so forth. These taxis are induced by physiological, physical or both factors. Nevertheless, the physical picture of cellular responses to the environment at the individual cell scale remains unclear. In this review paper, we therefore aim to provide an overview of the recent progress in the physical depiction of microbial taxis and their behaviour in various environments. Four types of responses are discussed: responses to (i) gravity, (ii) light, (iii) surface and (iv) flow. For each response, a simple physical picture is presented that describes the mechanism of behaviours. Knowledge of physical mechanisms will help us to understand the behaviour of cells in various environments at both the individual and population levels. Finally, the future challenges and prospects in this research field are discussed.This article is part of the theme issue 'Biological fluid dynamics: emerging directions'.