Continuous culture Synchronous cultures and Batch culture:

Continuous culture of microorganisms:

The bacterial growth curve includes three transitional periods between growth phases. This means that not all the cells are identical physiological conditions. In terms of physiological conditions, the growth curve includes young, metabolising cells on one hand, and the cells in the process of dying on the other, with cells in between these extremes. To study the metabolism of an organism for experimental research of industrial processes, it is often desirable to maintain a population in the logarithmic phase of growth in a constant environment. This is the accomplished technique called the continuous culture of the microorganism. Time is required for some cells to catch up with others. The effect of chemical and physical conditions on the organisms is not identical in all phases of growth.

It is a flow system of constant volume to which fresh medium is added continuously and spent culture medium is removed continuously at a constant rate. The most common type of continuous culture device used is a chemostat which contains both the population density and the growth rate of the culture. This system depends on two factors:

The concentration of an essential nutrient such as carbon or nitrogen source within culture vessels.

Dilution rate i.e the rate at of which fresh medium is being added to the culture medium divided by the volume of the culture vessels.

The growth rate can be controlled by the adjusting the dilution rate of growth.At a low dilution rate, the cells reach at high density because they are leaving the culture vessel at a very slow rate. Moreover, they have time to use the substrate almost completely.Therefore the substrate concentration is maintained at a low level within a vessel resulting a slow growth rate.On the other hand at high dilution rate the cell density is low because the cells are leaving the vessels at the high rate, moreover, they do not have enough time to utilize the substrate resulting high substrate concentration within the vessel.

The turbidostatic is another continuous culture apparatus. In a turbidostatic, the system includes an optical sensing device measures the absorbance of the culture density (turbidity) In the growth vessel. Changes in turbidity retard passage of light through the culture. That changes activate mechanisms that control the flow of nutrient into, and the flow of waste out of, the main culture vessel.

Chemostat and turbidostatic are usually operated at different dilution rates. The dilution rate is the ratio of an inflowing amount of nutrient 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 dilution rates. Ie . at low dilution rates. In contrast, in the turbidite, maximum sensitivity and stability are achieved at high dilution rates, a range over which culture density changes rapidly with dilution rate.

The dialysis technique is another device which maintains the culture in logarithmic phase for a slightly longer period. In this device, fresh nutrients are always available to the culture, and 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 the growth for the study of physiology and genetics of the organisms. Secondly, these systems allow the cells to be grown 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.

Synchronous cultures :

Synchronous cultures are composed of the population of cells that are the same stage of their life cycle. All the cells in the culture will divide at the same time, will grow for a generation time, and all will divide again at the same time. Thus the entire population is kept uniform with respect to growth and division. It is not possible to analyze a single bacterial cell to obtain the information about growth behavior. Ie. Organisation, differentiation and macromolecular synthesis. Synchronous culture provides the entire cell crop in the same stage of growth. Measurement made on such cultures are equivalent to the measurement made individual cells.

Synchronous cultures of bacteria can be obtained by a number of techniques. Two fundamentally different experiment approaches have employed. In the first approach, a Synchronous population of the cells can be sorted out according to age or size by physical separation of cells. In methods of the second type, a culture is induced by manipulating the physical environment or chemical composition of the medium to obtain asynchronously dividing the population. The techniques based on selection are preferable to those based on induction since induction is likely to introduce distortions in physiologic state of the cells.

Batch culture:

Batch culture is defined as the growth occurring in a volume of culture medium ie. Continuously being attached by the action of the growing organism until it is no longer suitable for growth. In the early stage of exponential growth in batch culture, the condition may remain relatively constant but in later stages, drastic changes in the chemical composition of culture medium usually occur as a result of cell multiplication. In both experimental research and industrial processes, it is often desirable to maintain a bacterial population growing in the exponential phase.

References

Arvind, Keshari K. and Kamal K Adhikari. A Textbook of Biology. Vidyarthi Pustak Bhander.

Michael J.Pleczar JR, Chan E.C.S. and Noel R. Krieg. Microbiology. Tata Mc GrawHill, 1993.

Powar. and Daginawala. General Microbiology.

Rangaswami and Bagyaraj D.J. Agricultural Microbiology.