AGM101 - Bacteria: Reproduction

Bacteria: Reproduction - Growth

Bacteria - Reproduction

Since bacteria are single celled, a single cell division is enough to get reproduced into two daughter bacteria. The cell division of bacteria is referred as transverse binary fission. A single cell is divided into two by forming transverse septum (cross wall). The binary fission is an asexual reproduction.

The process of binary fission is as follows:

The cell became elongated from its original size and approximately twice length
DNA replication takes place and DNA moves towards polar region
Septum or cross wall (septum) occurs from the mesosome region
New cell wall formation occurs
After completion, the cells were separated out

Ex. Bacillus, E. coli, Streptococcus

Other methods of reproduction occur in bacteria:

Budding : A small bud developed at one polar region of the cell, elongated and finally developed to new cell. Ex. Rhodopseudomonas acidophila

Fragmentation : Filamentous bacteria (actinomycetes) fragment into small rods and each rod gives raise to new cells. Ex. Nocardia

Conidiospores and sporangiospores : The filamentous bacteria produce spores in the hyphal tips and released to the environment, referred as conidiospores and sporangiospores. If the spores are produced in sac, referred as sporangiospores and without sac are referred as conidiospores.

Bacterial Growth

In microbiology, growth refers the increase number of cells. The bacterial cell is a synthetic machine that able to duplicate itself. More than 2000 chemical processes were occurred during one cell become two.

Step wise reactions occur during the growth of bacteria are as follows :

The macromolecules such as DNA, RNA, protein, polysaccharides, lipids polymerization
Assembly of macromolecules
Cell organelles like cell wall, membrane, ribosome, etc. formation

Growth rate: Increase of bacterial cell number per unit time is referred as growth rate

Generation: The interval of formation of daughter cells from parental cell is called as generation

Generation time: Time required to form two daughter cells from a single cell is called as generation time. This is also called as doubling time. E. coli has the doubling time of 20 min and Rhizobium has 2 h.

Growth curve of bacteria

Assuming a single bacterium has been inoculated to a liquid medium and the bacterium starts multiplying by binary fission gives increase of population. If we plot a graph of log₁₀ value of population against time, the graph will be of as follows:

This curve is referred as growth curve of bacteria. The curve can be divided in to 4 phases. In the beginning, the growth (increasing the population) will not occur leads to straight line (called lag phase), then the cells start multiply gives linear line (log or exponential phase) followed by straight line (called stationary phase) and decline linear line (death phase or decline phase).

Lag phase : When a bacterium is introduced into a medium, it will not multiply immediately. After a period of time only the bacterium will start multiply. This period is called as lag phase.

During this period, the cell increases its size
It will synthesis new protoplasm, enzymes, co enzymes etc
The bacterium will adjust the physiological environment
The cells are metabolically active
At the end of lag phase, each cell will start reproduction

Log / exponential phase : After lag phase, the cell start multiply steadily by binary fission at constant rate. The portion of curve is linear and have constant generation or doubling time.

All the cells are uniform chemical composition, metabolic activity and physiological characters
At this time, the generation time will be of constant
The cells will be sensitive to chemical inhibitors
This is the ideal stage to study the metabolic activity of the bacterium

Stationary phase : By this time, the essential nutrients of the medium get depleted and some toxic substances produced by the organism leads to slow down the cell division. At this time no net increase or decrease of cells in the medium occurs. This stage is called as stationary phase.

During this stage, cell division rate and cell death rate is uniform
Death due to low nutrients content, pH change, toxic wastes, reduced oxygen occur
Cells are very small and having low number of ribosomes
Some cases, cell do not die and at the same time do not multiply
Some bacteria produce antibiotics at this stage as secondary metabolites for competitiveness

Death phase : In this phase, few cells are alive and few died. The death rate is higher than the growth rate leads to decline the population. This phase is referred as death phase or decline phase.

The cell lysis occur during this stage
Some species can die very rapidly and some can persists for months

Note : These growth curve and the terms lag, log, stationary and decline phases are used only for the population or group of bacterial cells not for individual cells.

Mathematical expression of bacterial growth

The increase in cell number in an exponentially growing bacterial culture approaches a geometric progression of the number 2. As one cell divides to become two cells, we express this as 2^0 to 2^1. As two cells become four, we express this as 2^1 to 2^2, and so on. A fixed relationship exists between the initial number of cells in a culture and the number present after a period of exponential growth, and this relationship can be expressed mathematically as

N = N0 x 2^n

where N is the final cell number, N0 is the initial cell number, and n is the number of generations during the period of exponential growth. The generation time (g) of the exponentially growing population is t/n, where t is the duration of exponential growth expressed in days, hours, or minutes. From a knowledge of the initial and final cell numbers in an exponentially growing cell population, it is possible to calculate n, and from n and knowledge of t, the generation time, g.

Study state of growth of bacteria: Maintaining the bacterial population in a particular stage / phase of the cycle is referred as study state of growth. The growth curve which discussed earlier is meant for the bacterial culture in a constant media referred as batch culture. In this type of culture, the maintenance of bacteria at particular stage is not possible. For achieving the study state growth, the constant population should be maintained in the medium. This can be achieved by adding new medium and removing few cells at frequent time interval. This type of culture is referred as continuous culture.

The bacterium at late log phase is much interested for many industrial processes. So, maintaining the bacteria at log phase can be achieved by two systems called chemostat and turbidostat.

Chemostat : Maintaining the population of bacteria at particular stage using an important nutrient source as growth control is referred as chemostat. Ex. In a medium, if the glucose concentration is maintained by subsequent addition at particular dilution rate, so that it will be taken up by the organism immediately and at the same time a portion of the culture will be removed subsequently. This condition will maintain the bacterial growth at particular stage especially in log phase. This kind of devise is called chemostat.

Turbidostat: An electrical devise will monitor the cell density (turbidity) and at appropriate turbidity the nutrient will be added ( as discussed above – glucose) at a dilution rate to maintain the constant population. If the cell density is higher, the instrument will add more quantity of solution to reduce or dilute the population and vice versa. This kind of device is referred as turbidostat.

Both the devices were highly useful in the continuous culture of bacteria.

Synchronous growth of bacteria : All the cells of a bacterial culture in same stage of their growth cycle is referred as synchronous growth. When bacterial growth which we discussed earlier as growth curve, if observed under microscope, will have different stages of its reproduction ie, cell elongation, septum formation, divided cells etc. So, The cells and cell divisions will not be uniform. But, in synchronous growth culture, the bacterial cells will be of uniform stage. The synchronous growth of bacteria can be achieved by growing the cells at suboptimal temperature (below the optimum temperature). E. coli needs 37°C for its normal growth and if grown at 28°C, the synchronous growth will occur. This type of culture will be useful in molecular biology especially the gene cloning and transformation studies.

Diauxic growth of bacteria: Diauxic growth is any cell growth characterized by cellular growth in two phases, and can be illustrated with a diauxic growth curve. Diauxic growth, meaning double growth, is caused by the presence of two sugars (Example glucose and lactose) on a culture growth media, one of which is easier (glucose) for the target bacterium to metabolize. The preferred sugar is consumed first, which leads to rapid growth, followed by a lag phase. During the lag phase the cellular machinery used to metabolize the second sugar (lactose) is activated and subsequently the second sugar is metabolized. This can also occur when the bacterium in a closed batch culture consumes most of its nutrients and is entering the stationary phase when new nutrients are suddenly added to the growth media. The bacterium enters a lag phase where it tries to ingest the food. Once the food starts being utilized, it enters a new log phase showing a second peak on the growth curve.

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