4.4 Factors Affecting Growth Rate
Nutrients in the medium, pH, temperature, dissolved oxygen concentration and other cultivation environmental conditions all affect growth rate. Temperature and pH dependence are illustrated in Fig 4-3 a and b. In Figure 4-3 a the maximum growth rate is observed at 39° C for E. coli. Product formation kinetics (for example insulin), product yield (YP/S), cell yield (YX/S) are also affected by temperature. In general, cell yield decreases with temperature while similar defining relationships for product has not been reported. It is important to note that the optimum temperature for growth may be different from that for product formation.
Figure 4-3 a Effect of Temperature on Growth Rate of E. coli. Maximum growth rate is at 39° C. Plot is given as a function of inverse absolute temperature. The declining line from 39°C to 21° and then to 13° C suggest that the growth rate constant behaves somewhat similar to chemical reaction rate constant.
Figure 4-3b Effect of pH on Growth rate. Typical pH ranges over which reasonable growth can be expected is about 1 to 2 units. With adaptation, broader ranges can be achieved.
Optimum pH values for growth range from 4 to 7 for bacteria, from 4 to 7 for yeast and 6.2 to 7.2 for animal cells. Optimum pH for product formation may be different from that for growth. Many bacteria produce a different mix of products when pH is altered. For example, Clostridium butylicum produces acetic and butyric acids at near neutral pH while butanol, acetone and ethanol are produced under acidic pH (biological equivalent of Le Chatelier's Principle!). However, in the case of a recombinant cell expressing a recombinant protein, pH usually affects kinetics of recombinant protein generation rather than the product mix. Hybridomas are known to produce antibodies at a higher rate at pH 6.2 than at 7.2. Because of the difference in conditions for growth and product formation, optimization is often necessary.
Oxygen is an important substrate for aerobic organisms. Since metabolic energy production by cells is directly related to oxygen uptake rate (also called respiration rate), oxygen concentration is very strongly coupled to growth rate. As illustrated in Fig 4 - 4 , growth
Figure 4-4 Growth Rate depends on dissolved oxygen concentration. The critical dissolved oxygen concentration refers to value of DO below which growth rate is lower than the maximum value.
rate sharply rises to its maximum value with dissolved oxygen concentration. The relationship is similar to the behavior we discussed. See also Figure 4-1. The concentration at which maximum growth rate is attained is often referred to as critical oxygen concentration, . This value is typically less than 0.5 mg L-1 for bacteria and yeast, and about 1 to 2 mg L-1 for animal and insect cells. Note that these values are significantly lower than air saturation value of 6.7 mg L-1 at 37 C.