Microbial analysis is used in the food industry to determine the microbial content of foods and as a verification, measurement to determine the adequacy of cleaning and sanitation. Common tests that you will encounter include the following.

  1. Aerobic Plate Count (APC): This test is also called aerobic colony count, standard plate count (SPC) and total plate count (TPC). It determines the total bacteria present that are able to grow aerobically at mesophilic temperatures (25-40oC). It does not differentiate the type of bacteria present
  2. Total Coliform: This test tells you the total amount of coliform present. Coliform is a group of harmless bacteria naturally present in the environment. These include bacteria of the genus Escherichia, Citrobacter, Enterobacter and Klebsiella. Because they are more persistent than pathogens, higher numbers of coliform is used as an indicator for the amount of pathogens that could be present
  3. Fecal Coliform: Fecal coliforms is a subgroup of coliform bacteria. They are normally found in the gastrointestinal tract and feces of man and animals. Therefore their presence is a good indication of fecal contamination and presence of dangerous pathogens. Usually when samples are tested positive for coliform, the follow up test would be fecal coliform and if present, the sample is tested for E. coli which is a subgroup of fecal coliform. While most species of E. coli are harmless, E. coli O157 is found to be pathogenic
  4. Yeast and Mold Count: This test is a type of aerobic plate count that determines the total amount of viable yeast and mold present in the sample

Sometimes the bacteria population that you are testing is too high and therefore will be too numerous to count after they are developed on the Petrifilm or Petri dish. One way to avoid this is to dilute the sample. This is typically done following the serial dilution method. In this method, the sample is diluted several times by the same factor. Consider Figure 1 below. One ml of the original sample is first added to 9 ml of a diluent (e.g. water). This makes a 1:10 dilution. In the next step, 1 ml is taken from the 1:10 tube and placed in the next tube containing 9 parts of the diluent. Now we have a 1:100 dilution. The same procedure continues, each time taking 1 ml of sample from the previous tube and adding it to 9 ml of the following tube. This creates a series of dilutions from 1:10 (10-1) to 1: 100,000 (10-5). Notice that each time you are simply multiplying the previous dilution by a factor of 1/10 to get the next dilution. Therefore, if you were doing a serial dilution of ½ instead of 1/10 you would multiply each dilution by ½ to get the next dilution.

Practice Question

Following the example shown in the figure above, draw a diagram to show how you would prepare:

  • A 1:2 serial dilution beginning with 100 µl of the inoculum
  • A 1:4 serial dilution beginning with 100 µl of the inoculum
  • A 1:5 serial dilution beginning with 100 µl of the inoculum

Courtney Simons
Administrator
Courtney Simons is a food science professor. He holds a BS degree in food science and a Ph.D. in cereal science from North Dakota State University.
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