Growth curve of bacteria is a standard curve which consists of four distinct phases like **log**, **lag**, **stationary** and **death phase** which shows a **Sigmoid growth**. The Growth of bacteria and other organisms is simply referred to as the increase in cell number, cell size and cell mass. The growth of organism influences by many factors like temperature, pH, oxygen requirement, nutrients availability, moisture content etc. Bacteria are the prokaryotic or unicellular organism that most frequently grows by the binary fission.

Growth curve of Bacteria, there are some important measures that have to be taken:

- There should be a suitable culture medium for the growth of the microorganism.
- After utilizing the nutrients of the culture medium, cell division occurs in a logarithmic pattern i.e the growth of bacteria is always expressed in the logarithmic unit, not arithmetic.

Logarithmic growth means, the graph is **non-linear** and there will be **exponential** increase i.e 1, 2, 2^{2 } whereas arithmetic growth means the linear and uniform growth pattern.

- The distinct growth phases are only obtained by the incubation of bacteria at different time intervals.

### Content: Growth Curve of Bacteria

- Growth of Bacteria by Binary Fission
- Mathematical Expression of Bacterial Growth
- Phases of Growth Curve

### Growth of Bacteria by Binary Fission

Binary fission is a type of vegetative reproduction of bacteria which involves the following steps:

- The bacterial cells contain a single copy of DNA.
- Then the process of replication occurs by which the bacterial cell will form two copies of DNA.
- After the replication, each DNA moves towards the corner of the bacterial cell.
- Then, the cell wall protrudes into the cytoplasm and form a transverse septum.
- This transverse septum will divide or separates the cell into two identical daughter cells.

## Mathematical Expression of Bacterial Growth

The growth pattern of bacteria is in “**Geometric fashion**” i.e. a bacterial cell first divides into two, then four, then six and so on ( x, x^{2},x^{4},x^{6}……x^{n}).

Where **n**: number of generations

To mathematically express the growth of bacteria, there is a relationship that exists between the initial number of cells present in the log phase and the final number of cells present after the log phase in the cell culture which can be expressed as:

**N=N _{0} X 2^{n} ** …………… (Equation 1)

The final number of cells: **N**

The initial number of cells: **N _{0} **

Number of generations: **n**

Taking log at both the sides in equation 1

Log_{10} N= Log_{10 }N_{0 }+ n Log_{10} 2

n= Log_{10} N- Log_{10} N_{0}/ 2

n= Log_{10} N- Log_{10} N_{0}/ 0.301

n=3.3 (Log_{10} N- Log_{10} N_{0})

**Important terms to remember**:

**Generation time**: It defines the total time that is required by the bacteria to double its population. Generation time denotes by ‘g’. Mathematically it is expressed as:

**g =t/n**

where **t**: Time is taken by the individual bacterium for the cell division into two.

And **n**: Number of generations.

**Growth rate**: It defines the number of generation of bacteria per hour. Growth rate denotes by ‘R’. Mathematically it is expressed as:

**R=n/t**

Where,** n**: Number of generation

And **t**: cell division of bacteria per hour.

### Phases of the Growth Curve

The different phases of the bacteria during its growth cycle at the given time interval refer to the growth curve of bacteria. The growth curve of bacteria can be obtained by the following protocol:

- First, take the sample of bacteria.
- Then inoculate it into the fresh culture medium (nutrient broth) that contains all the nutrients for the growth.
- Then incubate the bacteria for at time intervals and determine the count of bacterial cells at that given time interval by the use of a spectrophotometer.
- After noting down the readings a standard graph is prepared between the numbers of bacterial cell vs time interval
- And in this way, a standard growth curve is obtained.

There are distinct four phases of the growth curve of bacteria.

- Lag phase
- Log phase
- Stationary phase
- Death phase

#### Lag Phase

This phase is the “**Adaptation** or **Adjustment phase**”, where microorganisms adapt themselves to the new environment of the growth medium. Microorganisms retain in this phase for a short period of time (1hr- several days). In this, there is no increase in cell number. The microorganisms only grow in size. In the lag phase, microorganisms release some metabolites:

- For adaptation.
- To restore the spent material.

#### Log Phase

This phase also refers to “**Exponential** or **Logarithmic phase**”. This phase shows an intense metabolic activity of microorganisms by producing primary metabolites. These metabolites enhance the growth rate i.e. an increase in cell number as well as cell size. It is a period of rapid growth. In this phase, microorganisms are resistant to adverse conditions:

- Antibiotic
- Radiation

*Here, the number of cells produced > the number of cells dying.*

#### Stationary Phase

This phase also refers to “**Stagnant phase**”. During this phase, population size is in a state of equilibrium. Cell division begins to slow down. Overall, there is no increase in cell number and cell mass. Factors that slow down the process of cell division can be due to:

- Limited nutrient availability
- Accumulation of toxic product
- An acidic PH of media
- Low oxygen availability

*Here, the number of cells produced = the number of cells dying.*

#### Death Phase

- This phase also refers to “
**Decline phase**”, which is the last stage of the growth curve. - In this, the population size decreases at the logarithmic rate.
- Death phase is characterized by the loosing of cell division ability of microorganisms.

*Here, the number of cells produced < the number of cells dying.*