Sorbitol promotes growth of Zymomonas mobilis in environments with high concentrations of sugar: evidence for a physiological function of glucose-fructose oxidoreductase in osmoprotection

Abstract

The gram-negative ethanologenic bacterium Zymomonas mobilis is able to grow in media containing high concentrations of glucose or other sugars. A novel compatible solute for bacteria, sorbitol, which enhances growth 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 30o). 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 tIM). The sorbitol transport system followed Michaelis-Menten kinetics, with an apparent Km of 34 mM and a Vm.. of 11.2 nmol -min-' mg-' (dry mass). Sorbitol was produced by the cells themselves and was accumulated when growing on sucrose (1 M or 36%) by the action of the periplasmic enzyme glucose-fructose oxidoreductase, which converts glucose and fructose to gluconolactone and sorbitol. Thus, Z. mobilis can form and accumulate the compatible solute sorbitol from a natural carbon source, sucrose, in order to overcome osmotic stress in high-sugar media. No other major compatible solute (betaine, proline, glutamate, or trehalose) was detected.The gram-negative, strictly fermentative and ethanologenic bacterium Zymomonas mobilis tolerates high concentrations of both ethanol (up to 13% [24]) and sugar (16,28) in the growth medium. An early report stated that Z. mobilis, (then called Termobacterium mobile), which had been isolated from pulque

Activity of glucose-fructose oxidoreductase in fresh and permeabilised cells of Zymomonas mobilis grown in different glucose concentrations

“…The localisation of GFOR in the periplasm of Z. mobilis was later observed by Loos et al (6) and confirmed by Aldrich et al (2). Afterwards, Loos et al (7) described the osmoprotective role of sorbitol for Z. mobilis, suggesting a physiological function of GFOR.…”

Synthesis of sorbitol by Zymomonas mobilis under high osmotic pressure

“…Loss et al (14) reported in their study that GFOR, present in the periplasm space of the bacterium, restrict sorbitol formation to conditions of osmotic stress. Because of its extraordinarily low affinity to fructose (Km 400 mM to 1M), sorbitol is formed at appreciable rates only when cells encounter high fructose concentrations.…”

Sugars as source indicators of biogenic organic carbon in aerosols collected above the Howland Experimental Forest, Maine

“…Polyols are also the major soluble carbohydrates in lichens (Dahlman et al, 2003), often found on the bark of trees, branches and leaves. Bacteria can also form and accumulate polyols (e.g., sorbitol) in order to overcome osmotic stress (Loos et al, 1994). Levoglucosan, with minor mannosan, galactosan and 1,6-anhydro-β-D-glucofuranose (levoglucosan isomer) are the primary thermal alteration products produced during biomass combustion of cellulose and hemicellulose and, therefore, are key tracers for smoke particulate matter from burning biomass .…”