Escherichia coli cells exhibited osmotolerance and grew well even in hyperosmotic medium containing 1.2 M NaCI (or 2.0 M sorbitol) if they had been previously exposed to moderate osmotic stress for a short time (30min). Untreated cells showed repressed growth. This acquirement of osmotolerance against hyperosmolarity was dependent on the concentration of the osmoticum in the osmotic stress conditioning. The most effective conditioning occurred when almost the same moderate osmolarity of electrolytes (about 0.4-0.6 M NaCI or KCl) and non-electrolytes (about 0.8-1.2 M sorbitol or sucrose) was used as osmoticum. The acquired osmotolerance could be inhibited markedly by chloramphenicol or rifampicin, but not by nalidixic acid. Experiments with 35S-methionine incorporation suggest that the synthesis of several proteins might be related to the acquiring of osmotolerance.Osmotolerance is essential to survival for bacterial cells exposed to higher osmotic strengths in their habitat environment. "Ordinary" non-halophilic bacteria as well as halophilic bacteria are thought to develop a variety of strategies for hyperosmolarity. Many species are known to accumulate high concentrations of compatible solutes, such as potassium, proline and betaine, in cells when grown in a hyperosmotic medium (3,6,7,9). These bacteria are thought to maintain intracellular concentrations of such compatible solutes at levels at least as high as those of external concentrations. Among these bacteria, Escherichia coli has been studied extensively with respect to the accumulation of compatible solutes (2,4,10,(14)(15)(16)20) and osmoregulation concerning gene expression response to the hyperosmolarity of the culture medium (1,5,7,(17)(18)(19)23). However, there are only a few * Address
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