Creatures often suffer changes in conditions of their surroundings from the womb to the tomb through the climate alterations and their own movements, and may have various strategies in place to overcome such environmental changes. Among the conditions they encounter, the pH of environments surrounding creatures varies dramatically during lifespans, especially in microorganisms. In humans, the skin covering the body prevents acidification inside the body, and cells are surrounded by body fluid whose pH is kept at 7.4. Nevertheless, pH surrounding cells is often acidified, especially diseased areas, such as cancer nests, inflammatory loci, and infarction areas. Life sciences at the molecular level have been focused on studies in near neutral pH conditions because neutral pH is favourable for many creatures. For a deeper understanding of life, it is essential to clarify molecular mechanisms of how organisms and cells keep their activities when their surroundings are acidified, in the “acidic world”.

This book summarises the current achievements concerning how cellular functions are kept in acidic environments for a wide range of living organisms, from bacteria to humans. Research results obtained to date allow the prediction that all creatures have different sets of genes and use them under different pH conditions. However, we only know little about the “acidic world”, and a huge number of uncovered questions are waiting to be resolved. Their elucidation would help to open up a new way of understanding life.