Continuous Culture of Micro-Organisms (With Diagram)

After reading this article you will learn about the continuous culture of micro-organisms.

The typical bacterial growth curve includes three transitional periods between growth phases. This means that not all the cells are in identical physiological conditions. Time is required for some cells to catch up with others. In terms of physiological conditions, the growth curve includes young, actively metabolizing cells on one hand, and the cells in the process of dying on the other, with cells in between these extremes.

The effect of chemical substances and physical conditions on the organisms is not identical in all phases of growth. To study the metabolism of an organism for experimental research or industrial processes, it is often desirable to maintain a microbial population in the logarithmic phase of growth in a constant environment. This is accomplished by a technique called continuous culture of micro-organisms.

A variety of different pieces of apparatus has been developed to grow micro-organisms in continuous culture. The chemostat is a continuous system cultivator in which the medium is thoroughly mixed to obtain maximum homogeneity. The fresh nutrient medium flow into the culture vessel from a reservoir of sterile medium at a defined and constant rate.

The volume in the culture vessel is kept constant by a device that allows the culture medium, together with accumulated waste products and older and dead cells, to leave the culture vessel at the same rate (Fig. 18.25). The level of growth is controlled by maintaining a fixed, limiting concentration of a particular nutrient in the medium.

The remaining constituents essential for growth of the selected organism are added in the medium in excess of requirement. The growth-limiting nutrient is added into the medium at a concentration below that required for maximum growth in a batch culture, i.e. a closed vessel. Fig. 18.26 illustrates the effect of a single limiting nutrient on the final and total cell crop of a bacterial culture.

The turbidostat is another continuous culture apparatus. In a turbidostat the system includes an optical-sensing device which measures the absorbency of the culture density (turbidity) in the growth vessel.

Changes in turbidity retard (or increase) passage of light through the culture. These changes activate mechanisms that control the flow of nutrient into, and the flow of waste out of the main culture vessel (Fig. 18.27).

Chemostat and turbidostat are usually operated at different dilution rates. The dilution rate is the ratio inflowing amount of nutrient medium per hour to the volume of the culture. In the chemostat, maximum stability is attained within a range of dilution rates over which cell density changes only slightly with changes in dilution rates, i.e. at low dilution rates.

In contrast, in the turbidostat, maximum sensitivity and stability are achieved at high dilution rates, within a range over which culture density changes rapidly with dilution rate.

The dialysis technique is another device which maintains the culture in the logarithmic phase for a slightly longer period (Fig. 18.28). In this device fresh nutrient are always available to the culture, and waste products are continually removed. However, the population pressure is the factor that ultimately limits growth.

Continuous culture systems offer valuable advantages. They provide a constant source of cells in the logarithmic phase of growth for the study of physiology and genetics of the organisms. Secondly, these systems allow the cells to be grown continuously in limiting concentrations of the nutrient.

Such growth gives valuable information on the catabolism of the limiting substrate. The system can be combined with selective enrichment to isolate an organism which can utilize any particular type of compound as a nutrient. This is very important in getting rid of a common industrial waste product or a poisonous pollutant.