Bacteria respond to changes in internal and external pH by adjusting the activity and synthesis of proteins associated with many different processes, including proton translocation, amino acid degradation, adaptation to acidic or basic conditions and virulence. While, for many of these examples, the physiological and biological consequence of the pH-induced response is clear, the mechanism by which the transcription/translation machinery is signalled is not. These examples are discussed along with several others in which the function of the gene or protein remains a mystery.