Abstract
Bacteria respond to changes in medium osmolarity by varying the concentrations of specific solutes in order to maintain constant turgor. The primary response of Lactobacillus plantarum to an osmotic upshock involves the accumulation of compatible solutes such as glycine betaine, proline, and glutamate. We have studied the osmotic regulation of glycine betaine transport in L. plantarum by measuring the overall and unidirectional rates of glycine betaine uptake and exit at osmostasis, and under conditions of osmotic upshock and downshock. At steady state conditions, a basal flux of glycine betaine (but no net uptake or efflux) is observed that amounts to about 20% of the rate of "activated"' uptake (uptake at high osmolarity). No direct exchange of 14C-labeled glycine betaine in the medium for unlabeled glycine betaine in the cytoplasm was observed in glucose metabolizing and resting cells, indicating that a separate glycine betaine efflux system is responsible for the exit of glycine betaine. Upon osmotic upshock, the uptake system for glycine betaine is rapidly activated (within seconds), whereas the basal efflux is inhibited. These two responses account for a rapid accumulation of glycine betaine until osmostasis is reached. Upon osmotic downshock, glycine betaine is rapidly released by the cells in a process that has two kinetic components, i.e. one with a half-life of less than 2 s which is unaffected by the metabolic status of the cells, the other with a half-life of 4-5 min in glucose-metabolizing cells which is dependent on internal pH or a related parameter. We speculate that the former activity corresponds to a stretch-activated channel, whereas the latter may be facilitated by a carrier protein. Glycine betaine uptake is strongly inhibited immediately after an osmotic downshock, but slowly recovers in time. These studies demonstrate that in L. plantarum osmostasis is maintained through positive and negative regulation of both glycine betaine uptake and efflux, of which activation of uptake upon osmotic upshock and activation of a "channel-like" activity upon osmotic downshock are quantitatively most important.
Highlights
In bacteria the intracellular concentration of compatible solutes is regulated by the osmolarity of the environment, which involves changes in transport activities as well as synthesis and/or degradation of these compounds [1]
In patch clamp experiments with giant liposomes fused with membranes of E. coli, gadolinium (Gd3ϩ) inhibited the mechanosensitive channel activities, about 10-fold higher concentrations of Gd3ϩ were needed to inhibit the bacterial channels than the eukaryotic ones [11]
When bacteria are faced with changes in external osmolarity, they respond by raising or lowering the cytoplasmic pools of specific molecules, termed compatible solutes
Summary
In bacteria the intracellular concentration of compatible solutes is regulated by the osmolarity of the environment, which involves changes in transport activities as well as synthesis and/or degradation of these compounds [1]. Upon moderate osmotic downshock (from 0.5– 0.8 to 0.2– 0.3 M NaCl or KCl), bacteria release compatible solutes such as Kϩ, trehalose, glutamate, proline, and glycine betaine, whereas other low molecular weight compounds are retained by the cells (4 –7). Mechanosensitive channels that respond to changes in turgor pressure are thought to be involved in the specific release of solutes upon osmotic downshock [4, 6, 11] The activity of these channels has been shown in patch clamp experiments using giant azolectin liposomes that were fused with bacterial membranes or spheroplasts. A number of inhibitors of mechanosensitive channels have been described in eukaryotic cells (16 –18) Some of these inhibitors have been used to modulate channel activity in bacteria, of which only gadolinium was found to inhibit the release of specific solutes upon osmotic downshock [6, 11]. This allows us to follow the inward and outward fluxes simultaneously, under osmostasis
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
