Growth Phases in Batch Culture

Yeast cells grow in the exponential phase. The cell mass concentrations are given in Table 4.4. Calculate the specific growth rate, 

The time course curve, or growth curve, for a batch culture usually consists of six phases, namely the lag, accelerating, exponential growth, decelerating, stationary, and declining phases. 

During the lag phase, the cells inoculated into a new medium self-adjust to the new environment and begin to synthesize the enzymes and components necessary for growth. The number of cells does not increase during this period. The duration of the lag phase depends on the type of cells, the age and number of inoculated cells, their adaptability to the new culture conditions, and other factors. For example, if cells already growing in the exponential growth phase are inoculated into 

After the exponential growth phase, the cell growth is limited by the availability of nutrients and the accumulation of waste products of metabolism. Consequently, the growth rate gradually decreases, and this phase is called the decelerating phase. 

Finally, growth stops in the stationary phase. In some cases the rate of cell growth is limited by the supply of oxygen to the medium. When the stationary phase cells begin to die and destroy themselves (by lysis) in the declining phase, the result is a decrease in the cell concentration. 

The rate of cell growth is influenced by temperature, pH, composition of medium, the rate of air supply, and other factors. In the case that all other conditions are kept constant, the specific growth rate may be affected by the concentration of a certain specific substrate (the limiting substrate). The simplest empirical expression for the effect of the substrate concentration on the specific growth rate is the following Monod equation, which is similar in form to the Michaelis-Menten equation for enzyme reactions  

---

Secondary metabolism is a form of differentiation, but cells grown in vitro are rapidly dividing, undifferentiated cells. Only at the end of the growth phase of batch-cultured cells may some form of differentiation occur, connected with the production of secondary metabolites. A plant produces a wide variety of secondary metabolites, all with different, mostly unknown functions. In in vitro cultured cells those compounds which defend the plant against microorganisms, namely, phytoalexins, are often easily formed. For example. Cinchona cell cultures produce large amounts of anthraquinones, but the alkaloids of interest, the quinolines, are produced in trace amounts only. Similarly Papaver cell cultures produce sanguinarine and closely related alkaloids, but no morphinane alkaloids. 

Figure 20.6 Different phases of a cell s growth cycle in batch culture...

Figure 5.17 Growth of a propene utilising Mycobacterium sp in batch culture exhibiting typical growth kinetics 1) lag phase 2) acceleration 3) exponential phase 4) deceleration 5) stationary phase 6) decline.

Mason, C. A., and F.ngli, T. (1993). Dynamics of microbial growth in the decelerating and stationary phase of batch culture. In Starvation in Bacteria (KjeUeberg, S., ed.). Plenum, New York. 

Cell lysis generally becomes important when viability is low. Examples include continuous culture (with or without cell retention) at low dilution rates and the late stationary and death phases of batch culture. Cell lysis is also important in stirred reactors with very high agitation rates. In order to quantify cell growth parameters under these conditions, the lysed cells must be accounted for. One way to do this is to measure the amount of the cytosolic enzyme lactate dehydrogenase (LDH) released into the medium (see Chapter 2, section 2.5). A procedure for measuring LDH activity is described in Chapter 4, section 4.7. 

The models just discussed have a number of serious deficiencies, despite their being able to provide reasonable descriptions of certain phenomena in a limited number of cases. A most notable deficiency is their inability to predict a lag phase in batch growth. Consideration of this led Ramkrishna (R3) to develop new models in which microbial cultures are endowed with a certain amount of biochemical structure although these models are distributed rather than segregated, so that phenomena associated with reproduction cannot be treated, the models do predict qualitatively a number of phenomena that cannot be touched by unstructured models. 

Production of enzymes degrading plant cell walls has been studied using media containing cellobiose or ammonium ions as limiting nutrients. Pectin lyase was primarily cell-associated during exponential growth in batch culture but accumulated in the supernatant during the stationary phase. 

Radiorespirometry was employed to study carbon metabolism during the growth of Streptomyces coelicolor A3(2) in a minimal medium, which permitted the production of 199 as the sole detectable secondary metabolite. A switch in the pattern of carbon metabolism from the Embden-Myerhof-Parnas pathway to the pentose phosphate pathway occurred during the period of slower growth in batch culture which immediately preceded entry into the stationary phase. This coincided with the period of production of 199. It was proposed that the biosynthesis of 199 is supported by the generation of NADPH during the latter part of the growth [220]. 

---