Instruments For Estimation Of Microbial Populations And Biomass

Many methods have been developed in recent years to estimate the total number of microorganisms by parameters other than the viable colony count as described in the previous section. For a new method to be acceptable, it should have some direct correlation with the total viable cell count. Thus, standard curves correlating parameters such as adenosine triphosphate (ATP) level, detection time of electrical impedance or conductance, generation of heat, radioactive C02, and so on, with viable cell counts of the same sample series must be made. In general, the larger the number of viable cells in the sample, the shorter the detection time of these systems. A scattergram is then plotted and used for further comparison of unknown samples. The assumption is that as the number of microorganisms increase in the sample, these physical, biophysical, and biochemical events will also increase accordingly. When a sample has 105 to 106 organisms/mL, detection can be achieved in about 4 to 6 h.

All living things utilize ATP. In the presence of a firefly enzyme system (luciferase and luciferin system), oxygen, and magnesium ions, ATP will facilitate the reaction to generate light. The amount of light generated by this reaction is proportional to the amount of ATP in the sample; thus, the light units can be used to estimate the biomass of cells in a sample. The light emitted by this process can be monitored by a variety of fluorimeters. These procedures can be automated for handling large numbers of samples. Some of the instruments can detect as little as 102 to 103 fg. The amount of ATP in one colony-forming unit has been reported as 0.47 fg with a range of 0.22 to 1.03 fg. Using this principle, many researchers have tested the efficacy of using ATP to estimate microbial cells in foods and beverages.

Lumac (Landgraaf, the Netherlands) markets several models of ATP instruments and provides customers with test kits with all necessary reagents, such as a fruit juice kit, hygiene monitoring kit, and so on. The reagents are

Table 2. Comparison of the Standard Plate Count Method and the Petrifilm Method for Viable Cell Counts of Shrimp, Perch, Cod, and Whiting

Colony-forming units/gm

Table 2. Comparison of the Standard Plate Count Method and the Petrifilm Method for Viable Cell Counts of Shrimp, Perch, Cod, and Whiting

Colony-forming units/gm

Sample

Standard plate count

Petrifilm SM

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