MICROBIOLOGY Textbook - 2012

CHAPTER 8. MICROBIAL GROWTH

8.4. CONTINUOUS CULTIVATION OF MICROORGANISMS

In the mid-1950s, the method of continuous CULTIVATION OF MICROORGANISMS, also known as continuous culture, was developed and theoretically justified. The core Principle of the method is that fresh nutrient medium is continuously fed into a fermenter of constant volume, while spent medium containing the proliferated Cells is simultaneously withdrawn at the exact same rate. As a result, the microorganisms are maintained under constant conditions regarding the availability of nutrients and Metabolic waste products.

The ratio of the flow rate (F) (the rate at which the nutrient medium enters the fermenter) to the culture volume (V) is termed the dilution rate D:

Class="center">

The dilution rate expresses the volume of nutrient medium replaced in the fermenter per hour.

Assuming that under continuous culture conditions the instantaneous increase in biomass (μX) is balanced by the washout of biomass with the culture medium (DX), the dynamic equilibrium in the fermenter can be described by the following relationship:

and consequently:

or

Equation [8.19] expresses the fundamental principle of continuous cultivation: the equality between the specific growth rate of the culture and the dilution rate of the nutrient medium in the fermenter. This relationship must always be satisfied in single-stage continuous cultivation of microorganisms.

Continuous cultivation is carried out as follows. A fermenter of a specific volume is filled with nutrient medium, which is sterilized, cooled to an optimal Temperature, inoculated, and incubated under batch conditions until the culture reaches the exponential growth phase. Afterward, the supply of nutrient medium into the fermenter is initiated at a predetermined flow rate F.

Two MAIN TYPES OF devices are used for the continuous cultivation of microorganisms: the chemostat, developed by Monod, and the turbidostat, developed by Bryson. These two devices differ in the mechanism used to control culture growth.

A chemostat consists of a medium reservoir, a metering pump, a culture vessel (into which fresh nutrient medium flows at a constant rate), and a collection vessel that receives the culture medium containing the cells. The dilution rate of the nutrient medium (D) is set in advance, and a specific Cell concentration is established in the fermenter accordingly. The chemostat has proven to be a very popular continuous cultivation system because it is a relatively simple, reliable system capable of operating across a wide range of dilution rates—from critical (D_c) to limiting (D_p). If the dilution rate exceeds the critical value, the culture will be washed out of the vessel; conversely, if it becomes too low and reaches the limiting threshold, Microbial growth will slow down and the culture will enter the stationary phase. Between these two extremes, the culture can proliferate at any given dilution rate.

A turbidostat is a continuous cultivation system in which a specific cell concentration is preset, and the dilution rate is adjusted accordingly within the culture vessel. Culture growth in a turbidostat is controlled by a photocell that detects Changes in the turbidity of the medium. The signal from the photocell regulates the inflow of nutrient medium into the vessel via a control system. The turbidostat is technically more complex than the chemostat. Moreover, it operates efficiently at high dilution rates close to μ_max, since the density of the bacterial suspension at such rates is low, allowing the device to respond precisely to changes in turbidity. At low dilution rates, the turbidity of the medium increases significantly, causing the instrument to lose sensitivity.

Continuous culture systems can be single-stage or multi-stage, feature simple or complex media feeds, and be open or closed, homogeneous or heterogeneous.

Continuous cultivation is utilized to obtain microbial biomass or their metabolic products. The continuous method for producing fodder and baker's Yeast has been developed and is widely implemented. Continuous Fermentation is also employed to produce Antibiotics, Vitamins, Hormones, and other pharmaceutical substances. Multi-stage continuous fermentation systems are used in the industrial production of alcohol, beer, glycerin, acetone, butanol, and organic acids.