Use this graph showing the standard death curves of three microbial species to answer the following two questions. The three microbial species shown in the graph where all treated at 70° C. If you had to sterilize a food product that contained 107 bacteria, how long would it take for each bacterial species? Calculate the D70 values for these three bacterial populations.
Use this graph showing the standard death curves of three microbial species to answer the following two questions.
- The three microbial species shown in the graph where all treated at 70° C. If you had to sterilize a food product that contained 107 bacteria, how long would it take for each bacterial species?
- Calculate the D70 values for these three bacterial populations.
For the next three questions, assume D90 = 22 minutes.
- At what temperature will you treat a food product?
- How long does it take you to kill 90% of bacteria?
- How long will it take you to sterilize a food product if it had 109 bacteria on it?
Serial Dilutions
6a. A sample is given to you that contains 6 million cells per ml. You must make a series of serial dilutions in order to get a countable number on a plate to prove that there are indeed 6 million cells per ml. The plate that you eventually put a sample of bacteria on must have between 10 and 100 colonies for ease of counting. What dilutions do you make to get this many colonies on a plate? How many colonies do you have on that plate? Show what dilutions you make (Write exactly how much liquid you move into each test tube and how much liquid was in the test tube, ex. Move 1 ml of culture into 9 ml of water – 10x dilution). Don’t forget to tell me how much liquid you eventually put on your plate. (If you want to draw something out, you could draw then take a picture of it with your phone and submit the image along with the worksheet).
6b. Do the same problem as in 6a, but now you have to do it in 3 test tubes or less. What dilutions do you do now?
7a. A petri plate is given to you with 80 colonies on it. I tell you that I did five 10x dilutions and then plated 0.1 ml from my last tube to get 80 colonies on a plate. How many living bacteria per ml were in that original sample? Draw out (or describe) all of the dilutions I would have done to get this number.
7b. Again, I have a plate with 80 colonies on it, but I tell you that I did one 100x dilution and two 10x dilutions before I plated that 0.1ml from my last tube. How many living bacteria per ml were in that original sample? Draw out (or describe) all of the dilutions I would have done to get this number.
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