A Hitchhiker's guide through advances and conceptual changes in chemotaxis

Chemotaxis is a basic recognition process, governed by protein network that translates molecular-based information on the surrounding environment into a guided motional response of the recipient cell or organism. This process is prevalent from bacteria to human beings. Some of the chemotaxis systems - like that of the bacterium Escherichia coli - are well established; others - like that of mammalian sperm cells - are at their relatively early stages of research. In contrast to mammalian sperm chemotaxis, where studies have so far been limited to the phenomenological level primarily, the model of bacterial chemotaxis is known down to the angstrom resolution. Despite this difference in depth of understanding, many fundamental questions are open not only in the new but also in the old chemotaxis fields of research, and recent advances in them are raising additional intriguing questions. This review summarizes some of these surprises and previously unasked or overlooked questions, and as such it offers a guided tour through conceptual changes in chemotaxis.