Question

enumeration of microorganisms--direct and indirect method

Answer 1

It is necessary to enumerate microorganisms to determine the rates of microbial growth and death as microbial growth is essentially synonymous with microbial reproduction.

A variety of methods have been developed for the enumeration of microbes by measuring cell numbers, cell mass, or cell constituents that are proportional to cell number.  The four general approaches used for estimating the sizes of microbial populations are direct and indirect counts of cells and direct and indirect measurements of microbial biomass.

1. Direct Count of Cells - Direct Count Using a Counting Chamber; Direct Count Using Fluorescent Dyes.

2.  Indirect Count of Cells - Viable Count method; The Most Probable Number (MPN).

3.  Direct Measurement of Microbial Biomass - Cell mass is determined directly by weighing whole cells; biomass can be correlated with cell numbers by reference to a standard curve. Wet weight or dry weight of bacteria may be used for estimation of cell numbers.

4.  Indirect Measurement of Microbial Biomass - Microbial biomass is estimated by measuring relatively constant biochemical components of microbial cells, such as protein, ATP, lipopolysaccharides, peptidoglycan, and chlorophyll; biomass can also be indirectly estimated by measured turbidity that can then be correlated with cell numbers by reference to a standard curve.

Answer 2

The measurement of bacterial cells per unit of sample is known as bacterial enumeration. It is important in Microbial ecology and Epidemiology. There are two ways of counting bacterial cells. They are:

Direct method

It involves counting the actual cells or colonies.

1. Direct and viable method involves serial dilution of original sample. Enumeration is done on the basis of standard plate count.

2. The direct and total method involves staining the cells with fluorescent dyes to make them visible in raw samples. Staining and microscopy are used for enumeration.

Indirect method

In this the number of cells is estimated on the basis of the cell mass, spectroscopy, or by using the statistical method known as the most probable number.

1. The indirect and viable method involves a statistical estimation based on growth patterns. Enumeration is done on the basis of MPN.

2. The indirect and total method involves measuring the transmittance of sample. Spectroscopy is used for enumeration.