Measuring Bacteria from Chicken Wings
Introduction
Bacteria are all around us – they can be found on nearly every inner and outer body surface. This creates a problem for meat suppliers, because the bacterial contamination from animal manure and digestive tracts can cause serious illnesses in humans.
In this lab, you will perform a technique called serial dilution to estimate the amount of bacteria present in a sample of chicken meat. These tests are similar to the ones conducted by federal inspectors.
Procedure
1. Obtain three test tubes. Fill each one with exactly 9mL of salt solution. Label the test tubes:
Test Tube 1: 1/10 Dilution
Test Tube 2: 1/100 Dilution
Test Tube 3: 1/1000 Dilution
2. Put on gloves. Raw meat must be handled carefully. Assume that it may contain bacteria that can cause food poisoning.
- Do not let the raw meat come into contact with lab tables.
- Sterilize your work area with disinfectant when you are finished.
3. Obtain a plastic sandwich bag. Place one chicken wing into the bag. Add 10mL of the salt solution the bag. Press as much of the air out as possible, then seal the bag.
4. Wash the wings in the saline solution by gently massaging the bag for about 5 minutes.
5. Take an unused plastic pipet and remove 1mL of the salt solution from the sandwich bag and place it into the first test tube. Swirl the contents of the test tube to mix.
6. Using the same pipet, take 1mL from the 1/10 test tube and add it to the 1/100 test tube. Mix.
7. Finally, take 1mL from the 1/100 test tube and add it to the 1/1000 test tube. Mix.
Source: Hoefnagels, M.H. and M.E. Walvoord. 2006. Chicken wing microbiology: a lesson in food safety and microbiological technique. Pages 185-200, in Tested Studies for Laboratory Teaching, Volume 27 (M.A. O'Donnell, Editor). Proceedings of the 27th Workshop/Conference of the Association for Biology Laboratory Education (ABLE)
8. Take the pipet in the 1/10 dilution test tube and place 1mL of solution in one of the sterile petri dishes. Place the cover back on. Gently move the dish around to all the solution to move across the entire surface of the agar.
9. Tape around the edge of the petri dish to prevent it from being opened again. Label the dish with the date, initials of your group members, and the dilution (1/10).
10. Repeat steps 8 and 9 for the 1/100 and 1/1000 dilution.
11. Place each of the petri dishes in a incubator, set for 98-100°F.
12. Clean your lab area with disinfectant.
13. Retrieve the petri dishes after 48 hours.
Analysis
Examine each of the three petri dishes for areas of bacterial growth. Count the number of colonies in each petri dish. If there are more than 300 colonies, or they grow and merge together record them as “TNTC” (Too Numerous To Count).
These are called Colony-Forming Units (CFUs), and are a good indirect way of measuring the amount of bacteria in a sample.
Finally, calculate the CFUs per milliliter by using this formula:
Dilution / CFUs / CFUs/mL1/10
1/100
1/1000
Discussion Questions
1. Why would CFUs used as a measurement of bacterial contamination instead of counting each individual bacterial cell in the sample?
2. Why are there bacteria on the chicken in the first place? Is this an indication that something is wrong at the processing plant where the wings were prepared?
3. If you wanted to do a comparison of CFUs/mL of different chicken wings, what would you test? Brands? Organic vs. conventional? Predict what you think the outcome of that experiment would be, and explain why.
4. Why is it safer to store meat for long periods of time in a freezer? What do you predict would happen if you measured the CFUs/mL of chicken that was instead stored at room temperature?
5. The recommended cooking temperature for poultry is 165°F. How do you think this was established? How would you expect the CFUs/mL of cooked chicken wings to compare to the one you tested?
6. If you were planning to do this test again on ground beef, which is more likely to have bacterial contamination, which dilution (1/10, 1/100, or 1/1000) would you use and why?