Bacteria need to sense changes in environmental conditions and respond rapidly to ensure their survival by adjusting their metabolic processes to take advantage of the current situation. Bacteria have a wide range of regulatory responses to adaptive opportunities, for example, when nutrients become available transport and processing machinery is produced to utilize them.

Sensory systems mediate gene expression during host invasion by pathogens by detecting conditions characteristic of the host environment and thus production of toxins and other virulence factors is elicited. Stress conditions such as antibiotics, heavy metals, starvation, exposure to UV radiation, elicit changes in gene expression that lead to effective coping mechanisms. The machinery responsible for these adaptive responses, stimulus detection, signal processing and production of output responses, is common to all cell sensory systems.

This recognition and processing of environmental signals through signal transduction pathways utilizes reversible protein phosphorylation.