Therefore, study of organisms from such heterogeneously growing population does not provide reliable information on the behavior of individual cells. This can be however, obtained by study of populations from synchronous cultures.
Nature of synchronous growth -
Since all cells in the synchronously growing culture divide at a time, they yield a zig zag pattern of growth curve.
For some time, there will be no increase in cell number and then suddenly the cell number doubles. This is because all cells divide at a time.
The synchrony in the growth hardly remains for 2 - 3 generation. Then the culture looses synchrony and cells start dividing In heterogeneous manner.
All cells do not divide at same time. The rate, at which cell division takes place, depends on the age of the cell. Young cells are smaller in size as compared to old cells.
Cells with intermediate size grow more rapidly. Again very large cells grow slowly.
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| synchronous growth of bacteria. The step like growth pattern indicates that all the cells of the population divide at about the same time. |
Methods of synchronization of growth
Various practical manipulations are applied to obtain synchronous growth from a heterogeneous population.
1. Physical selection method
2. Blological selection method
1. Physical selection method -
These methods involve separation of the cells on the basis of their size. Selection of cells with uniform size and then their synchronous culture.This is based on the fact that cells with same size occur in same state of cell division and hence their use provides synchronized growth.
Cells with identical size are selected by various use provides methods:
- By use of membrane filter.
- By use of density gradient centrifugation.
a. Selection of cells by membrane filter -
Use of membrane filter with specific pore size will help in separating cells with uniform diameter. Such cells are then used for obtaining synchronous growth.Another method for obtaining cells with uniform size by use of membrane filter is Helmstetter Cummings technique.
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| Helmstetter Cummings method for obtaining synchronous growth. |
When heterogeneous culture is fltered through cellulose nitrate membrane filter, some cells get adhered to the filter.
The filter is then inverted and fresh broth is allowed to low through.
Initial flow removes loosely adhered bacteria from the filter. Then, the later flow will contain only these cells which have just separated after division from adhered cells.
Therefore these cells will be in the same state of division. Hence, culture arising from these cells will be growing as synchronized culture.
b. Use of density gradient centrifugation -
Another method of separation of cells having identical size is the use of centrifugation technique. Thus, centrifugation of a culture allows one to separate cells with identical size. They can yield synchronous growth upon inoculation to a fresh medium.2. Blological selection methods -
Characteristic physiological properties of organisms can also be useful in obtaining synchronised growth. Two methods based on these parameters are widely used.a). Cyclic temperature shifft method
b). Use of limiting growth substance.
a). Use of cyclle temperature shift method -
The method is commonly used for obtaining synchronized growth of mesophilic bacteria, where they are incubated for some time at 37°C and 20°C alternately.
The fundamental property of the bacteria is that they can initiate cycle of new chromosome replication and cell division only at a speciflc temperature ( at 37°C). and not at lower temperature, i.e. at 20°C.
But., once the cycle has initiated, it can continue till it is completed. Therefore, cells which have been transferred to 20°C and kept for some time, all cells will continue to complete their division cycle, but will not start new cycle of cell division.
Thus, all cells are brought to same state of cell division. These organisms will therefore give synchronized growth on further incubation.
b) Use of limiting growth substances -
Supply of limniting growth substances in the medium controls the growth rate of fastidious organisms.The organisms can be made to grow synchronously by supplying them with fresh supply of this limiting growth substance in the nutrient medium after they have entered stationary phase of growth. b.
In stationary growth phase, bacteria stop growth because of non availability of limiting growth substance. Hence, all cells may occur in same state of physiology in this growth phase. The transfer of these cells to a fresh medium containing limiting growth substance will allow them to divide at a time and hence will give synchronous growth.
In stationary growth phase, bacteria stop growth because of non availability of limiting growth substance. Hence, all cells may occur in same state of physiology in this growth phase. The transfer of these cells to a fresh medium containing limiting growth substance will allow them to divide at a time and hence will give synchronous growth.
