Adaptation to Changing Osmolanty

Abstract

Microorganisms lack systems for active water transport; therefore, their cellular water content and turgor is governed by osmosis and is strongly affected by the osmolarity of the environment. In some microorganisms, dedicated water channels, the aquaporins, mediate accelerated water fluxes in both directions when the external osmolarity changes. The survival and growth of Bacillus subtilis in osmotically changing habitats depends on highly integrated cellular adaptation reactions that are either part of the SigB-controlled general stress regulon or specific to osmotic stress. The specific stress reactions of many Bacillus spp. comprise the synthesis and uptake of certain organic osmolytes, in particular proline, glycine betaine, and ectoine, under hyperosmotic conditions and their expulsion under hypoosmotic circumstances. The genetic mechanism by which B. subtilis distinguishes between exogenously provided and endogenously synthesized proline is currently unknown. Accumulation of compatible solutes under high osmolarity conditions is not only common in the microbial world (Archaea and Bacteria) but is also characteristic of fungal, plant, animal, and even human cells. Glycine betaine (N,N,N-trimethyl glycine) is one of the most potent compatible solutes found in nature. The high degree of sequence identity of the OpuB and OpuC systems and the close proximity of their structural genes in the B. subtilis genome argue that these two loci evolved through a gene duplication event. Both the specific osmostress reactions and the induction of the SigB-dependent general stress response are likely to play important physiological roles for the effective adaptation of B. subtilis to changing osmolarity in its natural habitats.