Microbiology Food Handout 

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Microbiology of Foods

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The presence of microorganisms in food does not necessarily indicate that the food is spoiled or has the potential to cause disease. Some foods can have high counts because microorganisms are used for production. Yogurt, sauerkraut and summer sausage are examples of foods prepared by microbial fermentation and, therefore, have high bacterial counts due to production. However, post-production treatments such as pasteurization or smoking will significantly reduce the numbers of bacteria present. During processing and preparation, food can become contaminated with bacteria, which naturally occur in the environment. These bacteria may not be necessarily harmful or pathogenic. Some bacteria are naturally associated with foods during the growing and harvesting period. For example, green beans, potatoes and beets contain attached soil bacteria when harvested. Even after washing some bacteria can remain and will be preserved with the food when frozen.

The chalky appearance of grapes is due to yeasts that are naturally associated with grapes and many other fruits. Milk in the udders of healthy cows is sterile, but bacterial flora normally associated with the outside of the udder, such as Streptococcus and Lactobacillus, are introduced during milking and processing. Pasteurization kills many of the bacteria introduced during processing and any pathogens that may be present, but does not kill all the bacteria present in milk. Some bacteria in milk can survive pasteurization temperatures and eventually cause spoilage and souring of milk. These are called thermoduric bacteria. Hamburger can also have high counts of bacteria introduced during processing and grinding of the meat. Many bacteria in hamburger are harmless saprophytes (organisms that live on decaying plant and animal material) that come from the environment where processing occurs. For example, endospore-forming bacteria and others can be introduced into ground beef during preparation (Brown and Smith, 2015).

Food can also be an important means for the transmission of disease. The Centers for Disease Control estimates that 76 million people per year in the United States become ill, 300,000 are hospitalized and 5,000 people die from foodborne illnesses. Foodborne illnesses usually result because pathogenic bacteria or bacterial toxins are introduced into food products during processing, handling or preparation. Food handlers can transmit opportunistic pathogens associated with the human body, such as, Staphylococcus aureus or intestinal bacteria, because of unsanitary practices such as failure to wash hands before preparing or handling food. Botulism food poisoning results from ingesting a toxin produced by Clostridium botulinum when endospores grow in improperly canned foods. The endospores occur in the soil and the environment and contaminate the prepared vegetables. Salmonella and Campylobacter are associated with poultry and eggs and can cause illness if these foods are not properly prepared. Escherichia coli O157:H7 is found in the intestines of cattle and can become associated with meat if fecal material from the animal’s intestines contaminates meat during the butchering process. This pathogen is then incorporated into hamburger during grinding and processing. Serious illness results from eating improperly cooked hamburger because cooking temperatures are insufficient to kill the organism. Transmission of this pathogen has also occurred when fecal material of cattle contaminated fruits and vegetables such as lettuce and spinach (Brown and Smith, 2015).


Although high bacterial counts in food do not necessarily mean that the food is spoiled or harbors disease-causing organisms, it can suggest the potential for more rapid spoilage of the food. Coliforms are organisms such as Escherichia coli that occur in the intestines of humans and warm-blooded animals. When present in food or water, fecal contamination has occurred and that there is the high potential for the spread of serious disease such as typhoid fever, bacillary dysentery, cholera and intestinal viral diseases (Brown and Smith, 2015).

In this exercise, the quantity and identification of bacteria in food will be examined.

Material and Method

Tube containing a sample of lunch

Label plate with initials
Break food into small pieces and place back into the test tube. Add sterile water to test tube containing food and mix vigorously.
Remove one mL of slurry and add to TSA plate and spread evenly. Keep plate upright until slurry has absorbed into the agar.


Presence of Microorganisms in Food

__1. Coliform bacteria are common contaminants of meat.

a. a. Why might one expect to find coliforms in samples of meat?

b. b. High coliform counts in food indicate the potential for finding which intestinal pathogens?

c. In terms of food safety, why is it suggested to cook hamburgers medium-well to well-done whereas steaks can be cooked rare?

2. What considerations should be made to safely thaw frozen foods for later consumption?

3. Why is refrigeration not always an effective means for preventing food spoilage?

4. Why are dried fruits somewhat resistant to spoilage?

5. Give two examples of foods that have high bacterial counts but are not spoiled.

(Brown and Smith, 2015)

Literature Cited

Modified from Brown, Alfred and Smith, Heidi. Benson’s Microbiological Applications. Laboratory Manual in General Microbiology. New York: McGraw Hill Education. 2015.

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