Strains of xylose utilizing Zymomonas with improved xylose utilization and ethanol production during fermentation in stress conditions were obtained using an adaptation method. The adaptation involved continuously growing xylose utilizing Zymomonas in media containing high sugars, acetic acid, ammonia, and ethanol.
1. An isolated improved xylose-utilizing Zymomonas strain obtained by a method comprising:
a) providing xylose-utilizing Zymomonas cells;
b) continuously growing the xylose-utilizing Zymomonas cells of (a) in a feeding growth medium comprising xylose, whereby a culture comprising ethanol is produced;
c) adding to the culture made in (b) an amount of ammonia and acetic acid or ammonium acetate, whereby a stress culture comprising ethanol and ammonium acetate is produced;
d) continuously growing the stress culture made in (c), whereby improved xylose-utilizing Zymomonas cells are produced, wherein the improvement in the cells is increased conversion of xylose to ethanol in the presence of ammonium acetate compared to the Zymomonas cells of (a);
e) isolating one or more cells from the improved culture made in (d); and
f) growing the one or more improved cells to make the strain.
2. An isolated improved xylose-utilizing Zymomonas strain of claim 1 , wherein at least 12% more xylose is consumed as compared to the corresponding non-adapted strain.
3. An isolated improved xylose-utilizing Zymomonas strain of claim 2 , wherein at least 20% more xylose is consumed as compared to the corresponding non-adapted strain.
4. An isolated improved xylose-utilizing Zymomonas strain of claim 3 , wherein at least 30% more xylose is consumed as compared to the corresponding non-adapted strain.
5. An isolated improved xylose-utilizing Zymomonas strain of claim 1 that consumes at least 80% of the xylose in a medium containing 60 g/L xylose, 80 g/L glucose, and 9.54 g/L acetate, with the pH adjusted to 5.8, when starting at a cell density of 0.10 OD 600 nm, as compared to the corresponding non-adapted strain consumption of about 18% of the available xylose.
6. The isolated improved xylose-utilizing Zymomonas strain of claim 5 , wherein xylose consumption is about 85% of the xylose in the medium.
7. A method for producing ethanol comprising:
a) providing an isolated improved xylose-utilizing Zymomonas strain of any one of claim 1 , 2 , 3 , 4 , 5 or 6 ;
b) culturing the strain of a) in a fermentation medium wherein ethanol is produced; and
c) isolating the ethanol made in step (b).