2.5.10 Indirect methods

2.5.10 Indirect methods

Measurement of cell mass

During population growth, there is increase in the total cell mass, as well as cell numbers. By determination of microbial dry weight, the mass and numbers can be determined. Cells growing in liquid medium are collected by centrifugation, washed, dried in an oven and weighed. This is useful for measuring the growth of fungi, as bacteria weigh very little, and large volumes have to be centrifuged.

Turbidimetry

This technique depends on the fact that microbial cells scatter light striking them. Because microbial cells in a population are of roughly constant size, the amount of scattering is proportional to the concentration of cells present.

When the concentration of bacteria reaches about 107 / ml the medium appears slightly cloudy or turbid. Further increase in conc. / density results in greater turbidity and less light is transmitted through the medium. The extent of light scattering can be measured by a spectrophotometer and is almost linearly related to the bacteria concentration at low absorbance levels. Thus bacterial growth can be easily measured by turbidimetry.

Last modified: Monday, 7 November 2011, 7:02 AM