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. A method for producing an improved xylose-utilizing Zymomonas strain 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. The method according to claim 1 , wherein in (c) the addition results in an ammonium acetate concentration of at least 24 mM.
3. The method according to claim 1 , wherein the concentration of ammonium acetate is increased at least one time during step (d).
4. The method of claim 1 , wherein the xylose in the medium in part (b) is at a concentration of at least 50 g/l.
5. A method according to claim 1 wherein in (b) the ethanol is at least 18 g/L.