GENERAL MICROBIOLOGY - T.P. Pyrog - 2004

6. MICROBIAL GROWTH

6.4. GROWTH PHYSIOLOGY

6.4.7. Growth in Continuous Culture

The continuous cultivation method involves the continuous supply of fresh nutrient medium into a vessel (fermenter, cultivator) where Bacteria are grown, while simultaneously withdrawing the culture liquid containing Bacterial Cells and metabolic products at an exact corresponding rate. This cultivation approach closely approximates an ideal scenario where cells remain in the exponential growth phase for extended periods under constant Substrate Concentration and other unchanging parameters. Let us examine continuous cultivation under chemostat and turbidostat regimes.

Chemostat growth. In this regime, bacterial growth is controlled by the substrate concentration. By maintaining a constant concentration of one of the essential substrates (such as a nitrogen or carbon source) through the Regulation of the dilution rate (the feeding rate of the nutrient medium), the culture growth can be stabilized. The primary parameter of continuous culture is the dilution rate D:

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where f is the flow rate of the nutrient medium, L/h; V is the working volume of the cultivator, L.

Thus, the value D represents the fraction of the liquid volume replaced per hour. The rate of biomass change in the cultivator dX/dt is determined by the equation

where μΧ is the biomass increase, and DX represents losses due to Cell washout.

If the growth rate μ and the dilution rate D are equal, the losses from cell washout and the biomass production balance each other out, meaning the net change is zero and the biomass X remains constant. The culture thus exists in a state of dynamic equilibrium.

Culture growth in a chemostat is controlled by substrate concentration. The Stability of the system relies on this growth rate limitation by a single essential substrate (the limiting factor). The dependence of the growth rate on the substrate concentration с_к is described by a saturation curve (Fig. 6.10). The substrate concentration at which the growth rate reaches half of its maximum value is designated as K₁. This parameter is one of the most crucial characteristics defining Bacterial growth in a chemostat.

Fig. 6.10. Dependence of growth rate on substrate concentration

Turbidostat growth. The operating principle of a turbidostat is based on regulating the medium flow rate According to the density (turbidity) of the population. A turbidity sensor controls the inflow of the nutrient medium via a feedback control system. Turbidostat control can also be based on alternative Methods for measuring biomass or metabolic products generated during growth. Examples include the pH-stat method for controlling flow rate, or the auxostat (oxystat), which regulates flow rate based on oxygen consumption.

Principal differences between batch and continuous cultures:

a batch culture can be viewed as a closed system (somewhat analogous to a multicellular Organism) that progresses through several developmental phases (lag phase, exponential phase, etc.). Environmental conditions vary significantly across these phases. Automatic regulation in a batch culture is virtually impossible;

a continuous culture is an open system striving to achieve dynamic equilibrium. The time factor is largely eliminated, and uniform environmental conditions are maintained for the organisms. It is readily amenable to automated control.