Is the accumulation of osmoprotectant the unique mechanism involved in bacterial osmoprotection?

Sucrose, trehalose, maltose, cellobiose, gentiobiose, turanose and palatinose are very unusual osmoprotectants for Sinorhizobium meliloti, because these compounds, unlike other bacterial osmoprotectants, do not accumulate as cytosolic osmolytes in salt-stressed S. meliloti cells. Rather, these compounds were catabolized during early exponential growth, and contributed to enhance the cytosolic levels of the two endogenously-synthesized osmolytes: glutamate and the dipeptide N-acetylglutaminylglutamine amide (NAGGN). Furthermore, all of the disaccharides that acted as powerful osmoprotectants shared the same uptake routes in S. meliloti. Here, we show that these disaccharides, in fact, belong to a new family of non-accumulated sinorhizobial osmoprotectants and that two mechanisms of osmoprotection coexist in S. meliloti.