Intracellular consumption of carbon sources can be deduced from real-time oxygen measurements as follows:
C6H12O6 (glucose)+6O2 (oxygen) ? 6 CO2 (carbon dioxide)+ 6H2O (water)
With continuous oxygen-uptake rate (OUR) data and frequent glucose off-line measurements, a whole picture of the metabolic status of a cell growing and expressing a target protein in a bioprocess can be acquired.
Interpretation of OUR is given in the Table. With a high specific feed rate using glucose only, around 40% of the sugar is used by cell catabolism, while the ratio of catabolic activity to anabolic activity decreases during low specific feed rates. When a feed contains glucose and complex components, a rather constant ratio (60%) of catabolic to anabolic activity is observed regardless of the carbon-limitation condition being relaxed or stringent.
It should be noted that the ratio of catabolic to anabolic activity is dependent on other factors besides source fed and carbon limitation. A change in the process temperature or pH, or the addition of product inducer may also alter this ratio. Thus, online registration of oxygen-consumption demand provides indicative information to allow a better understanding of the process under development. It also provides an opportunity for retrospective data comparison, which is necessary for process optimization and scale-up.
Automatic Feed Start
The availability of automatic process control systems for monitoring metabolic changes can be used to increase the quality of development data as well as to develop high-yielding production runs. An example given here is the control of feed start during fed-batch, high cell-density microbial fermentation.
Traditionally, while implementing a fed-batch process, initiation of feed is done by visually observing the rise in the dissolved oxygen tension (DOT) signal combined with a steep decrease in stirrer speed (if DOT-stirrer control is applied), a process that can be labor intensive. The use of reliable oxygen-consumption rates to trigger feed allows well-characterized and controlled process operations as well as determination of the state of the organism’s metabolic profile.