ABSTRACT
Single-celled organisms, such as Bacteria, can face enormous changes in their external environment over very short periods of time. If we consider Escherichia coli and its life cycle, we can see how significant these changes are. In its normal habitat of the colon, the E. coli cell is kept at a more or less constant temperature of 37°C, is surrounded by other microbes and nutrients (in the form of partly or completely digested food), and is in a low-oxygen environment. After excretion, and before the cell re-enters the digestive system of the same or a different host, E. coli is suddenly thrust into a colder, well oxygenated, more aqueous, and nutrient-poor situation. To be able to cope with all these changes in lifestyle, the cell must quickly turn off some genes and turn on others. At the gene level the switches are repressor and activator proteins (SECTION F4 on the lac and trp operons), while at the protein level enzymes can be switched on or off by the presence or absence of metabolites (see SECTION F4 on the role of allolactose). These cytoplasmic responses have come into effect due to changes outside the cell, and the way in which an indicator that the outside world has changed is carried from the cell wall and to the genes or proteins that might be involved in a response is called signal transduction.
