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Sunday, October 17, 2004

Quantification of Microorganisms

The counting of bacteria is important if you want to know the number of bacteria in a sample. The common methods are: Plate count, Direct count, and Turbidometric

Plate count- is used on the premise that each viable bacterium will produce a colony when growing on a agar plate. A sample of the material to be counted is suspended in liquid and placed in a empty petri plate. Next, melted agar is poured into the plate. After incubation, each organism produces a colony in the agar that can be counted.
There are two main advantages of the plate count over other methods. Only viable organisms are counted, which are the ones considered to be important. Samples with small methods can also be counted.
The disadvantages are related to size and frequency. Bacteria are usually present in large numbers. E.coli could easily contain over one billion cells/ml. Some bacteria will stick together, gving rise that two different organisms may produce the appearance of only one colony.
Direct count - Organisms in a suspension of bacteria are placed on a slide that has been ruled into squares and can hold a specific amount of volume. By counting the bacteria that appear on the grid areas, the number of organisms in a sample can be calculated.
The direct count is much faster than the plate count but has its disadvantages. There must be a certain multiple number of organisms before there are enough to be seen, and both viable and nonviable organisms appear the same under the microscope.
Turbidometric - turbid simply means cloudy. In this method, a spectrophotometer measures the turbidity on bacteria in a broth. The more bacteria present, the cloudier the broth.

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