300 • Impacts of Applied Genetics — Micro-Organisms, Plants, and Animals 
clucer, and its unique biochemical position, Zymo- 
monas has not been studied extensively/ 
The organism most often studied is Zymomonas 
mobilis, which can produce up to 1.9 moles of 
ethanol per mole of glucose. Recent studies reported 
from Australia, have established the Z. mobilis can 
ferment high concentrations of glucose rapidly to 
ethanol in both batch and continuous culture with 
higher specific glucose uptakes rates for glucose and 
ethanol production rates than for yeasts currently 
used in alcohol fermentations in Australia.® ® 
For example, several kinetic parameters for a Z. 
mobilis fermentation were compared with Saccha- 
romyces carlsbergensis^^' specially selected for its 
sugar and alcohol tolerance. “ Both specific ethanol 
productivity and specific glucose uptake rate are sev- 
eral times greater for Z. mobilis. This result is mainly 
due to lower levels of biomass formation and glucose 
consumption. The lower biomass produced would 
seem to be a consequence of the lower energy avail- 
able for growth with Zymomonas than with yeasts— 
the Enter-Doudoroff pathway producing only 1 mole 
of adenosine triphosphate (ATP) per mole of glucose, 
compared to glycolysis with 2 moles ATP per mole 
glucose. In none of the first three examples can etha- 
nol be produced from pentose sugar. 
The fourth approach utilizes a mixed culture of 
Clostridia, which will utilize cellulose and hemicellu- 
lose, hexoses, and pentoses for ethanol production. 
The application of genetics for 
improving microbial strains 
In the previous sections, the process steps have 
been identified that are particularly sensitive to the 
quality of the microbial strains. The following are im- 
provements of microbial characteristics that are 
either now possible or might be so in the future and 
that will have an impact on the overall economics of 
the process. The effect of new genetic techniques re- 
quiring future research is similar for all micro-orga- 
nisms in two ways. 
1. Manipulations could be attempted today with 
less effort and greater chance of success if tools 
like cell fusion and recombinant DNA (rDNA) 
techniques were available for all of the mi- 
crobes of interest. 
'Gibbs, et al., op. cit. 
»K. J. Lee, D. E. Tribe, and P. L. Rogers, "Ethanol Production by Zymo- 
monas mobilis in Continuous Culture at High Glucose Concentrations," Bio- 
technology Lett . 421-426, 1979. 
®P. L. Rogers, K. J. Lee, and D. E. Tribe, Biotechnol. Lett. 1:165-170, 1979. 
'“Ibid. 
”D. Rose. Proc. Bichem. 1 1(2), 1976, pp. 10-12. 
2. Manipulations require further knowledge in a 
specific area or the development of an entirely 
new genetic system in ethanol producing mi- 
crobes— e.g., there is no genetic system for the 
thermophilic anaerobic bacteria. Knowledge on 
how to genetically alter ethanol tolerance of 
both bacteria and yeast is lacking. 
The economics of the fermentation of a substrate 
into alcohol is primarily controlled by three factors: 
1. Ethanol yield.— The amount of product pro- 
duced per unit of substrate determines the ma- 
jor raw materials cost of the fermentation. 
2. Final ethanol concentration.— The cost of separat- 
ing the ethanol from the fermentation broth is a 
function of the ethanol concentration in that 
broth. 
3. Productivity.— The amount of ethanol produced 
per liter of fermenter \olume per hour deter- 
mines the capital cost of the fermentation step, 
once the type of fermenter and the annual out- 
put have been chosen. Productivity is not inde- 
pendent of the final ethanol concentration, and 
so an optimum compromise between these vari- 
ables must be chosen. 
The impact of genetics on ethanol yield 
Most microbes that are chosen for making ethanol 
already produce nearly the theoretical maximum 
yield. In these cases little improvement can he made. 
The yield may he lower when the microbe has 
been chosen for its other technical advantages such 
as ability to degrade cellulose, bower yield of a 
microbial end product, like ethanol, can result fi’om 
the diversion of substrate to cell mass or to an alter- 
native product. Both of these faults can he readily at- 
tacked. A number of cell changes (e.g., leaky mem- 
branes) can cause the microbe to waste energ^v, re- 
quiring it to metabolize more suhstrati’ into alcohol 
to make the same cell mass. Where the thermo- 
dynamics and redox balance of the fermentation 
allow, unwanted waste jiroducts can he eliminated 
by mutation of the relevant pathways. Only limited 
work has been done on this type of research w ith in- 
dustrially sigificant bacteria. 
The impact of genetics €tn final idhantil 
concentration 
This is amenable to genetic manipulation, both em- 
pirical and planned. An impro\ement in ethanol tol- 
erance decreased both separation costs and ferment- 
er capital cost (through increased productiv ilyl 
When traditional distillation is used, the ellei t on 
