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After reading this article you will learn about the reproduction of bacteria and its processes.
Reproduction of Bacteria:
All growing cells must divide. Most bacteria multiply by transverse, binary fission, in which a single cell divides in two after developing a transverse cell wall. Within a short period, often as short as 20 minutes, a bacterium can create a complete duplicate of itself, which in turn is capable of duplicating. The life cycle of a bacterium consists of the time between one cell division and the next.
Obviously, the degree of morphological changes within this life cycle is slight, when one compares it with that of a eukaryote that develops from a fertilizer egg to a complex organism.
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However, complex biochemical events do occur, and the process requires the synthesis of the millions of parts. These parts are organized into the various homo- and heteropolymers, which make up the sub-cellular organelles, and are the functional units of the cell.
As bacteria are extremely small, there are numerous technical difficulties in observing and interpreting the intricate cytological events that take place during the process of reproduction.
However, with the help of special procedures for chemical analysis and new microscopic techniques, particularly electron microscope examinations of ultrathin sections and time-lapse photography, the following developments are postulated.
1. Transfer of nutrients from the medium into the cell by a selective process.
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2. Conversion of the nutrients by the enzyme system of the cell into protoplasmic material characteristic of the particular organism.
3. Increase in the amount of nuclear material.
4. Cell elongation (more evident in bacilli).
5. Organization of the contents of the cell to distribute the material between two cells.
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6. Invagination of the cytoplasmic membrane, followed by the formation of a transverse cell wall.
7. Separation of daughter cells.
The daughter cells may stick together to form a pair, which after another cell division in the same plane would give rise to a chain of four cells. If the second cell division is at right angles to the plane of the first division, the result would be four cells in a flat configuration.
Repeated divisions at right angles to each other result in planar sheets or “windowpanes” of cells, as produced by Lampromedia. When division occurs in a third plane, cubical packets of cells result, as produced by Sarcina.
Processes of Reproduction in Bacteria:
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1. Growth Rate and Generation Time of Bacteria:
The most common process of reproduction in bacteria is by binary fission, in which one cell divides to produce two cells. Thus the increase in population is by geometric progression:
1 → 2 → 4 → 8 → 16 → 32
The time required for the cell to divide or for the population to double is known as the generation time. Not all micro-organisms have the same generation time; for some it may be 15 minutes and for others it may be many hours, as shown in Table 18.3. Similarly, the generation time is not the same for a particular organism under all conditions.
It is strongly dependent upon the nutrients in the medium and on environmental conditions of growth. Micro-organisms are capable of growing over a wide range of physical conditions, and of utilizing many different nutrients, but maximum growth requires certain specific conditions for a given species.
Under optimal conditions the generation time determines the rate of growth of a microbial culture. The growth increment can be quantitatively analysed by the determination of generation time.
2. Mathematical Expression of Growth of Bacteria:
To calculate the generation time of individual micro-organisms the following experimental data are required:
1. The number of organisms present at the beginning.
2. The number of organisms present at the end of a given time interval.
3. The time interval.
The relationship of cell numbers and generations can be expressed in a series of equations. Starting with a single cell, the total population B at the end of a given time period would be expressed as
B = 1 × 2n
where 2n is the bacterial population after n generations. However, under practical conditions several thousands of bacteria are introduced into medium at zero time and not one, so the formula now becomes
Bn = B0× 2n
where, B0 = number of organisms at zero time.
Bn= number of organisms after n generations.
n = number of generations.
Solving the equation for n, we have
log Bn = log B0 + n log 2
n = logBn – logB0/log 2
Thus we can calculate the number of generations if we know the initial population Bo and the population Bn after time t. The generation time G is equal to t (the time which elapsed between B0 and Bn) divided by the number of generations n, or
G = t/n =t log2/log Bn – log B0
An alternative method is used to describe bacterial growth in mathematical terms when the culture is undergoing balanced growth. The rate of increase in bacteria at any particular time is proportional to the number or mass or bacteria present at that time.
The constant of proportionality is an index of the rate of growth and is called the exponential growth rate constant (K). It is defined as number of doublings in unit time, and is usually expressed as the number of doubling in an hour.
It is calculated from the following equation:
Bn = B0× 2Kt
Bn = Population at time t.
B0 = Population at time zero
Taking the logarithms
log Bn = log B0 + Kt log 2, and
Solving the equation for K
K = log Bn – log B0/t log 2
The exponential growth rate constant is therefore reciprocal to generation time i.e.
G = 1/K
For example, generation time E. coli are 30 minutes, i.e. 1/2 hour.
1/2 = 1/K
K = 2 doublings per hour
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