2.3 Cell Yield and Stoichiometric Coefficients

Consider the experimental cell (Pseudomonas lindneri) growth data shown in Fig 2-1a, originally reported by Bauchop and Elsden. The experiment consisted of inoculating five test tubes containing growth medium with the bacterium. Each of the test tubes contained different concentrations of carbon source - in this case glucose at levels from about 4 mM to 36 mM. The cultures were incubated anaerobically (i.e. in absence of oxygen) at growth temperature ( 30° C) for two days or until growth ceases. The resulting cells were filtered, dried and weighed. This mass of bacteria obtained is plotted against the starting glucose concentration. The important observation illustrated by the data is the straight line relationship between carbon source concentration (reactant in chemical parlance) and the cell concentration ( product ).

The slope of the line represents the amount of cells obtained per unit amount of glucose consumed.

If we convert the above to mass basis,

The above value is often called cell yield, growth yield, or yield coefficient. If you examine the growth reaction stated in the previous section, the slope (in mass units) we calculated above can be equated as follows.

In the above the numerator term contains the amount of cell created and the denominator contains the amount of substrate consumed. In other words, the measurements reported by Bauschop and Elsden enable us to calculate the stoichiometric coefficient, a. That is,

Let us consider another set of data shown in Figure 2-1b. The cell yield depends on growth conditions. You will note that under anaerobic conditions, slope (also yield) is 58.2 g (mol substrate)-1, or 0.32 g cell (g substrate)-1 . Similarly under anaerobic conditions, yield is 22 g (mol substrate)-1, or 0.21 g cell (g substrate)-1 . Invariably, the yield under anaerobic conditions will be smaller than at aerobic conditions because the cell derives significantly more metabolic energy under aerobic conditions. It is also important to note that not all cells can grow both aerobically and anaerobically.

From a practical viewpoint, an aerobic organism is preferred. This is because, the amount of product protein produced is proportional to cell amount. Higher biosynthesis is possible with aerobic cultures than with anaerobic ones.