1. You are testing ground beef to determine how many bacteria are present per gram of meat. You place 1 gram of meat in sterile water, mix vigorously, and then do serial dilutions as shown below. You inoculate a plate with 0.1 ml of the sample from the last tube. The next day you count 122 colonies. How many bacteria/ml were in the undiluted sample?
Figure 9.4.1: Serial Dilutions for Question 1
2. When doing serial dilutions, why is it necessary to plate more than one dilution?
3. If you observe Escherichia coli in a food sample, what could this indicate?
4. You left a carton of orange juice on your counter for 3 days. When you taste it, it is very bubbly (as though it was carbonated), and it tastes more bitter than usual. (Note: acids taste bitter). What do you think could have happened? (Note: you can’t just say that bacteria grew—you must explain how the growth of the bacteria resulted in the changes in the orange juice.)
5. Based on what you now know about the presence of bacteria on chicken and beef, why is it a good idea to use separate chopping boards for meat and for vegetables?
6. You are testing unpasteurized milk for the presence of bacterial contamination. Starting from the undiluted milk, you do serial dilutions as shown below, and plate 1.0 ml of each dilution on agar. If the undiluted milk contains 5 x 106 bacteria/ml, how many colonies would you expect to see on each plate?
Figure 9.4.2: Serial Dilutions for Question 6
7. Do you think standard plate counts are very accurate? Why or why not?