25: BACTERIAL COUNTS ON FOOD
- Page ID
- 157093
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Differentiate between food infection and food intoxication, and explain how each causes illness.
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Perform serial dilutions and plate food samples accurately to obtain countable colony-forming units (CFUs).
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Observe, count, and calculate colony-forming units (CFUs) to estimate microbial load in different foods.
BACKGROUND
Microorganisms in food can cause illness in two main ways. In some cases, food carries live, disease-causing microbes that begin to grow and multiply inside the body after the food is eaten. This is known as a food infection. In other cases, microorganisms produce toxins as they grow in food, and these toxins can remain behind even if the microbes themselves are dead or removed. When illness is caused by these pre-formed toxins, it is called a food intoxication. Although both are often described as food poisoning, food intoxications and food infections differ in their underlying causes and the way they impact the body.
Most foods are not sterile. Canned and bottled items are designed to be free of microorganisms until they are opened, but fresh meats, vegetables, dairy products, seafood, and grains commonly contain some level of microbial life. Normally, the low pH of the stomach and digestive enzymes are enough to destroy these microbes. Illness becomes more likely when the number of organisms is unusually high, when the microbe is especially virulent, or when toxins have accumulated in the food before ingestion.
Proper handling and storage are critical to preventing microbial growth. A small number of bacteria can multiply into millions in just a few hours under the right conditions. Refrigeration, acidic environments, and foods with high salt or sugar concentrations help limit microbial growth. Contamination can also occur when food is handled by someone who is ill or asymptomatically carrying an infectious agent. Because of these risks, public health agencies monitor food workers and routinely inspect food products for microbial contamination.
A well-known case that illustrates the risks of microbial contamination occurred during a community picnic in a rural town. Several attendees fell ill with symptoms including nausea, vomiting, and abdominal cramps just hours after eating. Public health officials traced the outbreak to a large bowl of potato salad that had been prepared the night before and left at room temperature for several hours before serving. Testing revealed the presence of Staphylococcus aureus, a bacterium commonly found on the skin and in the nasal passages. While the initial contamination likely occurred during preparation, the real danger came from the bacteria multiplying and producing toxins while the salad sat unrefrigerated. Even though reheating could have killed the bacteria, the heat-stable toxins remained in the food and caused the illness. Cases like this highlight the importance of proper preparation, handling and storage of food.
To reduce the risk of foodborne illness and food intoxication, the food industry is required to follow strict regulations that ensure safe food handling, storage, and preparation. These standards include guidelines for cleanliness, temperature control, and limits on the number of microorganisms allowed in food products. Microbial testing plays a vital role in determining whether food has been safely prepared, handled, and stored.
In this lab, you will investigate the total number of microorganisms present in various foods to explore how different items may support microbial growth. The focus will be on overall microbial levels rather than on identifying specific types of organisms. Gaining a better understanding of how and why microbial contamination occurs is essential for preventing future outbreaks and promoting public health.
MATERIALS (Per Group of 2)
1 Gram food sample (Brought by Student)
2 Nutrient agar plates
3 empty dilution tubes
4 Sterile 1.0 ml pipets
1 Graduated cylinder
1 2 x 2 piece of Parafilm
METHODS/PROCEDURES
1. Label two dilution tubes 1:100 and 1:1000 then add 9.0 ml of DI water to each.
2. Use a digital scale to measure 1.0 gram of your food sample.
3. Transfer the 1.0 gram of food into a clean 10 ml graduated cylinder.
4. Carefully add DI water to the cylinder until the total volume reaches exactly 10 ml.
5. Cover the top of the cylinder with Parafilm.
6. Mix the contents thoroughly by inverting or gently shaking the cylinder for one full minute. This is your first dilution (1:10).
7. Let the sample sit for a few minutes undisturbed to allow solid particles to settle.
8. Using a sterile transfer pipet, remove 1.0 ml of liquid from the top of the graduated cylinder (without disturbing the settled solids) and transfer it into
the 1:100 dilution tube containing 9.0 ml of DI water.
9. Use a new pipet to transfer 1.0 ml from the 1:100 dilution tube to the 1:1000 tube with 9.0 ml of DI water. Mix thoroughly.
10. Label one nutrient agar plate with the following information: 1:100, name of the food tested, the date, your first initial, last name.
11. From the 1:100 dilution tube transfer 0.1 ml a nutrient agar plate using a sterile pipet.
12. Create a bacterial lawn on the agar plate by spreading the liquid evenly across the surface of the agar using a sterile spreader and turn plate. Make sure
to spread the liquid evenly over the entire plate.
13. Label the second nutrient agar plate with the following information: 1:1000, name of the food tested, the date, your first initial, last name.
14. Using a sterile pipet from the 1:1000 dilution tube transfer 0.1 ml onto the second nutrient agar plate.
15. Create a bacterial lawn on the agar plate by spreading the liquid evenly across the surface of the agar using a sterile spreader and turn plate. Make sure
to spread the liquid over the entire plate.
16. Incubate the plates agar side down at 30°C. until the next class meeting.
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You are also welcome to print the PDF and turn in a physical copy of the following.
Exercise #30
NAME ______________________
EXPECTATIONS
Among the food items being tested today, which do you expect to have the highest microbial counts, and which do you expect to have the lowest? Explain your reasoning
RESULTS
After incubation, count the Colony Forming Units (CFU) on each of your plates, If the colonies are merging together there may be too many CFUs to count accurately. Ask your instructor how to estimate the CFUs or if the plate should be discarded.
Record the CFUs on each of your plates
CFU plate 1__________ Plate 1 dilution ___________
CFU plate 2__________ Plate 2 dilution ___________
Calculate the concentration of CFU/gram of food. Show your calculations.
CFU/Gram of food________
Add your data and the class data from the board to the table below.
Class Data
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Food Source |
CFU/gram of food |
Food Source |
CFU/gram of food |
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CONCLUSIONS
1. What characteristics of the food samples such as moisture, pH, nutrient content etc, might explain the different levels of microbial growth? Explain how.
2. If two students tested the same type of food but obtained different CFU counts, what factors could explain the difference?
3. Explain the difference between a food infection and a food intoxication.