Microbial Growth and Measurement
Growth may be defined as an increase in the cellular constituents of an organism. In microbiology, the term growth generally refers to an increase in the number of cells in a population and this is frequently called as population growth. AN individual microbial cell may also increase in size and this is called as cell growth. There is an important distinction between the multicellular and unicellular organisms. Growth in multicellular organisms leads to increase in the size of the organism, whereas growth in unicellular organisms leads to increase in the number of individuals in the population.
Measurement of Microbial Growth
Microbial growth can be determined by observing an increase in cell mass (biomas) or cell number because growth to an increase in both. Several different techniques are available for this purpose, and selection of the technique depends upon the particular microorganisms involved and t he requirement of the problem. The most commonly employed techniques for microbial growth measurement are discussed below.
A. Measurement of Cell mass
Increase in the mass of cells in a population can be measured by wet weight, dry weight, total volume, chemical analysis and by measuring turbidity.
1. Wet weight
To determine the wet weight of cultured cells in a liquid medium they must be filtered or centrifuged. The cells are then washed by resuspenending in distilled water, and the residual medium is removed by filteration or centrifugation. The wet weight of packed centrifuged cells is then determined. This method is often employed to estimate the growth of bacteria taken for lipid analysis or enzyme isolation.
2. Dry Weight
This technique is commonly used to determine the growth of fungi and bacteria. In case of fungi, the mycelial mat is separated from the medium, washed with distilled water, filtered or centrifuged again. The cells are then dried in an oven at 50-60⁰ C for about 24 hr, cooled in a desiccator and finally weighted. Usually, this procedure of heating followed by weighting is repeated until a constant weight is obtained. This techniques is useful for measuring the growth of fungi but it is not suitable for bacteria as they weigh so little that it may be necessary to grow them in bulk in order to collect them in sufficient quantity.
3. Determination of total volume
In this method, a standard volume (e.g. 10ml) of the culture is placed in a test tube, called Hopkins tube, having a narrow, hollow, cylindrical column by centrifugation at a standard speed and specified time. Their total volume is read on the graduated scale, and from knowledge of the average volume of the individual cells, an estimation of numbers is possible.
4. Chemical analysis of cellular constituent
Mass can also be determined by estimating the amount of a cellular constituent through chemical procedures. The most widely quantitated constituted is total cell nitrogen, and it has been established that cells contain about 14% nitrogen. Therefore, analysis of total nitrogen present in the given sample will be approximate biomass. Other chemical methods involve determination of free amino groups, nucleic acids (DNA and RNA) or the phosphorus of these acids, and so on.