Laboratory Investigations in Microbiology

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Chapter 27: Bacterial counts in food

Outbreaks of food-borne illnesses occur quite frequently in the United States, despite technologies that have made food preparation faster and safer. The CDC estimates 5,000 deaths from food-related illnesses annually. Of these, roughly 500 deaths annually result from Salmonella poisoning, and 500 deaths from Listeria. With such large numbers of illnesses and fatalities resulting from a source that is generally preventable, it is no surprise that a lot of resources are committed to testing the safety of food.

Similar to water quality testing, much of food safety testing relies on the detection of indicator organisms. Since many food-borne illnesses utilize the oral-fecal route, monitoring for coliform contamination is a good indicator for the general sanitary status of most foods. While viruses and protozoan contaminants cannot be detected this way, the most common bacterial pathogens can be avoided. 

So what number of coliforms is acceptable on food? Most states have their own regulations. Typically, coliform counts on foods are permitted to be ~ 50 - 100 per gram on uncooked chicken and hamburger. To determine the coliform count on food products, a selective/differential medium will be used. VRBA is selective for Gram-negative enterobacteria and differential for lactose fermentation.

Another index of food safety is the total bacterial count. This count includes many of the Gram-positive spoilage bacteria that are restricted by the VRBA medium. 

The usefulness of these food counts is twofold: to indicate how close the food is to the point of spoilage. There are no hard and fast rules, but food counts should be < 100,000 colonies per gram (ref). Another indicator of food pertains to potential pathogens. Whereas the limit for known pathogens (E. coli, Salmonella, Listeria) is zero, other organisms can indicate the sanitary condition of food as well. A coliform count of < 100 per gram is desirable. An E. coli count should be 0 - 50 /gram (depending on food type).

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Materials per lab group
Procedure   
  1. Weigh out 20 g of your food
  2. Add it to 180 ml of sterile water in the blender. This makes it a 10-1 dilution
  3. Blend on high setting for 3 minutes, then pour into a flask
  4. Add 1 ml of the blended mix to 9 ml of sterile water. This is a 10-2 dilution
  5. Add 1 ml of the 10-2 dilution to a 9 ml tube. This is a 10-3 dilution
  6. Add 1 ml of each dilution (10-1, 10-2, 10-3) to a sterile Petri plate. Label plates
  7. Pour 1 tube of melted VRBA into each plate. Mix in a Figure-8 motion for 30 seconds, then allow it to harden.
  8. Repeat steps 6 & 7 for the TSA agar plates.
  9. Incubate plates 48 hrs at 35°C.

Thursday

  1. Count the number of purple colonies surrounded by a halo of precipitated bile on each plate (see image to the right). 
  2. Calculate the number of coliforms per food sample using the formula used for the viable plate count method.

Data sheet & Review Questions

© 2003 - 2015 José de Ondarza, Ph.D.