1. Assume you are looking for a particular pathogen in a sample and you are using a highly selective medium, and so you make five 1:10 dilutions of the sample and you plate 0.2 mLs of each dilution. The following day you find that you have 80, 50, 25, and 1 colonies on plates 2, 3, 4, and 5. How many pathogens per mL do you have?

2. You have 1mL containing 10^8 cells and you need 2.5×10^7 cells. What do you do? (how many cells in each 0.1mL)

3. You have 10 mL of 10^6 cells/mL prep and you need 1mL of 10^7 cells. What do you do? Hint: you can se cetrifugation, but then you have to perform another step)

4. You are generating a vaccine prep that contains 10^8 microbes per mL and you need to know the ID50 of your prep. So you run a challenge study with 50 mice where you inoculate each mouse with 50 uL of the prep. Within 3 days you record the number of sick mice and you find that 30 of the mice are sick. How can you estimate the ID50, and what is that estimate? (use a ration)