Brief notes on Microbial Growth and Its Application

In the terms of individual’s growth, when an infant grows he becomes a child, teenager, an adolescent, adult and ultimately, an old person, this hold true for all animals and even plants, the growth occurs in various phases and during that time the individual increases in size as well as adds, to its functional attributes.

MICROBIAL GROWTH

Growth is defined as irreversible increase in size or cellular components of an organism. That can occur as a result of reproduction, as a result of arrival of new generation. Growth in the microbes can be measured in a variety of ways. Some common methods to measure growth of the microbes are given in the following.

Cell counting

Bacterial growth can be accurately counted by using Petroff-Hauser counting chamber. The chamber includes a glass slide, a cover slip, which is framed and kept 1/50 mm above the slide, so that bacterial suspension is present in each ruled square of the slide.

The area of the square is 1/400 mm², the glass cover slip is emplaced 1/50 mm above the slide and hence, the volume over the square is 1/20000 mm³ or l /20,000,000 cm³. If in one square, an average of 5 bacteria is seen, then there are 5 X 20,000,000 or 10⁸ bacteria per ml.

Cell mass measurement

Cell mass is directly proportional to the cell number. This can be done by centrifugation of known volume of culture and then, weighing the pellet obtained.

This is called the fresh weight and the dry weight of cells is obtained by drying the pellets at 90-110°C overnight on pre-weighed, dried filter papers.

When the growth in a liquid medium, the culture of the fungal mats can be obtained by suction filtration on dried, pre-weighed filter papers then the mycelia are dried at 90-110° C overnight and followed by obtaining the final weight. The difference in the weight will give the amount of growth of the fungal mycelia.

Turbidity measurement

The cell mass and number is obtained by using optical density method, particularly in the case of bacteria. Turbidity is developed in a liquid medium due to presence of cells, which makes colony appearance in the liquid broth. After inoculation, as cell mass and number increase; the nutrient medium becomes opaque and turbid.

This turbidity can be measured with a photometer that detects amount of unscattered light this can be expressed in photometer units.

To keep the microorganisms in a constant environment for longer period, continuous culture method is adopted. This essentially requires a flow of constant volume of the inoculums to which a fresh medium is added continuously and from which continued removal of the medium with culture can occur. When such a system is in equilibrium, cell number and nutrient status remain constant. Also, the system remains in dynamic steady state.

Growth measurement by chemo stat

Chemo stat is used for maintaining the organisms in continuous culture, it regulates growth rate of the organisms by stabilizing the concentration of the nutrients in the medium.

The growth rate and growth yield can be controlled independently on each by adjusting the dilution rate and by varying the concentration of the nutrients present in a limiting amount.

The high dilution of the medium does not allow the organisms to grow fast; a large amount of it may die of starvation due to no availability of the nutrients required for the maintenance of the cell metabolism.

The number of cells per ml. is called cell density which is controlled in the chemo stat. If the concentration of the nutrients in the incoming medium is raised with the dilution rate remaining consent, the microbial density will increase although the growth rate will remain the same.

vi) Growth in a batch culture

A batch culture is that in which growth of the microbes occurs in a limited volume of the liquid medium. During growth in the liquid medium of unicellular organisms, increase in the cell number is logarithmic for some time. When logarithmic growth of the cell number in a population is plotted against time, a straight line is obtained to suggest that there is an equal percentage increase in the number of cells during a constant time.