An overview of the role and diversity of compatible solutes in Bacteria and Archaea.

The accumulation of compatible solutes is a prerequisite for the adaptation of microorganisms to osmotic stress imposed by salt or organic solutes. Two types of strategies exist to cope with high external solute concentrations; one strategy is found in the extremely halophilic Archaea of the family Halobacteriaceae and the Bacteria of the order Haloanaerobiales involving the accumulation of inorganic ions. The other strategy of osmoadaptation involves the accumulation of specific organic solutes and is found in the vast majority of microorganisms. The organic osmolytes range from sugars, polyols, amino acids and their respective derivatives, ectoines and betaines. The diversity of these organic solutes has increased in the past few years as more organisms, especially thermophilic and hyperthermophilic Bacteria and Archaea, have been examined. The term compatible solute can also be applied to solutes that protect macromolecules and cells against stresses such as high temperature, desiccation and freezing. The mechanisms by which compatible solutes protect enzymes, cell components and cells are still a long way from being thoroughly elucidated, but there is a growing interest in the utilization of these solutes to protect macromolecules and cells from heating, freezing and desiccation.