Explain the Direct Microscopic Counts (DMC)

The process includes making a smear of food sample on a microscope slide followed by staining with an appropriate dye and viewing and counting under the microscope.  It is widely used for assessing the microbial quality of the milk and dairy products (Breed Count).

The method involves spreading 0.01 ml of the milk sample on 1 cm area of a slide.  After drying and staining the slide, average number of bacteria per microscopic field is counted. The diameter and area of the microscopic field is calculated by micrometry.  Number of bacteria per ml of the milk sample is calculated as:

Number of bacteria/ml = average number of bacteria per microscopic field x number of field x dilution factor.

Number of Microscopic fields = area over which milk is spread ÷ area of one microscopic field  = 100 mm2  ÷ 0.02 mm2 = 5000 field

Dilution factor = 100 (0.01 ml of the sample is plated)

It is rapid and simple method by which cell morphology can be detected. Efficiency can be increased by using fluorescent dye. The method, however, gives the count of both viable and nonviable cells. Sometimes food particles may not be distinguishable from microorganisms. Some microorganisms may not take stain and not be counted. In spite of these drawbacks, it is used for quick assessment of microbial quality of foods. The discussion above focused on the methods we can use to assess the microbiological quality of food samples.