GROWTH OF MICROBES

Growth is defined as an orderly increase in cellular components. Microorganisms grow in a variety of physical and chemical environments. A number of methods are available for microbial growth. The choice depends upon the measurement, objectives and on available techniques usefulness. In some cases of industrial fermentation which contain complex media, indirect methods for estimation need to be used however, no matter what method is used, considerable care is required in interpreting the results. Bacterial growth can be measured either by colony counting or cell counting, weighing cell i.e. cell mass measurement or by cell activity.

Cell mass is directly proportional to cell number. This can be obtained after centrifugation of a known volume of culture and weighing the pellet obtained. This is called fresh weight but dry weight of cells is obtained by drying of the pelleted cells at 90-110 degrees centigrade overnight.
The cell mass and number is also obtained by using optical density method. Turbidity is developed in the liquid medium due to the presense of cells which make cloudy appearence to the eyes. The more the sample, more cells are present, hence more light is scattered. Turbidity can be measured with a photometer or a spectrometer device that detect the amount of unscattered light recorded in photometer unit.

If the concentration of the cell in the sample is high, light scattered away from the detecting unit by one to one cell can be rescattered back by another. Hence, the one to one correspondense between cell number and turbidity does not follow linearity. Secondly, dead cells also interfere during measurement. Hence this method is reasonable accurate only for measurement of microbial growth till early log phase.