Water relations in single cells

The intracellular water content is an important factor affecting the growth and survival of single cells of microorganisms under adverse environmental conditions. Certain types of bacteria, yeasts, filamentous fungi and algae are capable of growth in environments with water activities below 0.9 and even as low as 0.6, and are the most osmotolerant living organisms known. The two most important factors that determine such extreme osmotolerance are: (1) the resistance of the enzymes in a cell to the solutes present, and (2) the cell’s ability to maintain within itself particular solutes, which are compatible with continued activity of intracellular enzymes, at levels sufficient to balance the external osmotic pressure and thus avoid dehydration. The levels of such compatible solutes are metabolically controlled and include polyols in yeasts, glutamic acid in the least osmotolerant bacteria, y-aminobutyric acid and proline in the more osmotolerant bacteria and potassium in specifically halophilic bacteria. In contrast, under certain conditions osmoregulatory mechanisms in microorganisms may reduce rather than maintain the water content of the cell. For example, during the morphogenic changes that accompany the formation of endospores by some bacteria, a special form of osmoregulation occurs in which a newly synthesized electronegative polymer (‘peptidoglycan’) in the outer region of the spore brings about and maintains, rather than avoids, dehydration of the central core. Indeed, spore heat resistance can be predictably modified experimentally by osmotic manipulation. The core dehydration mechanism is probably implicated in the enormous resistance of endospores to heat. It may also be involved in the exceptional dormancy and longevity of such cells, and suggests a principle that may operate in other dormant biological systems.