SORBITOL PROMOTES GROWTH OF ZYMOMONAS-MOBILIS IN ENVIRONMENTS WITH HIGH-CONCENTRATIONS OF SUGAR - EVIDENCE FOR A PHYSIOLOGICAL-FUNCTION OF GLUCOSE-FRUCTOSE OXIDOREDUCTASE IN OSMOPROTECTION

The gram negative ethanologenic bacterium Zymomonas mobilis is able to grow in media containing high concentrations of glucose or other suga rs. A novel compatible solute for bacteria, sorbitol, which enhances g rowth of Z. mobilis at glucose concentrations exceeding 0.83 M (15%), is described. Added sorbitol was accumulated intracellularly up to 1 M to counteract high external glucose concentrations (up to 1.66 M or 3 0%). Accumulation of sorbitol was triggered by a glucose upshift (e.g. , from 0.33 to 1.27 M or 6 to 23%) and was prevented by the uncoupler CCCP (carbonyl cyanide m-chlorophenylhydrazone; 100 mu M). The sorbito l transport system followed Michaelis-Menten kinetics, with an apparen t K-m of 33 mM and a V-max of 11.2 nmol.min(-1).mg(-1) (dry mass). Sor bitol was produced by the cells themselves and was accumulated when gr owing on sucrose (1 M or 36%) by the action of the periplasmic enzyme glucose-fructose oxidoreductase, which converts glucose and fructose t o gluconolactone and sorbitol. Thus, Z. mobilis can form and accumulat e the compatible solute sorbitol from a natural carbon source, sucrose , in order to overcome osmotic stress in high-sugar media. No other ma jor compatible solute (betaine, proline, glutamate, or trehalose) was detected.