1. In the image above, the final dilution is 1: 2. You make six 2-fold serial di
ID: 64443 • Letter: 1
Question
1. In the image above, the final dilution is 1: 2. You make six 2-fold serial dilutions of a solution. Your final dilution will be 1: 3. You dilute 10 ul of an unknown sample of bacteria to 1 x 10^-6 and plate out 10 mu l onto a Petri dish. The next day you count 64 colonies. The number of viable bacteria in the original sample was a. 6.4 x 10^6 cells/ul b. 6.4 x 10^7 cells/ul c. 6.4 x 10^8 cells/ul d. 6.4 x 10^9 cells/ul 4. A 10 ul sample of bacteria was diluted to 10^-6 and 10 mu l was plated on the Petri dish at right. What was the approximate concentration of the original sample? 5. What are two reasons serial dilution is a useful technique for counting populations of bacteria?Explanation / Answer
1. 1:100000000
3.
Final concentration (cells/mL)
= 1000*64/(10-6*100)
= 64*107
= 6.4*108
Hence, the correct option is (c) 6.4*108 cells/ul
4. The approximate concentration of the original sample is 108
5. Serial dilution reduces the concentration of bacteria and makes the counting possible.
Calculation of the final concentration of bacteria is simple and easy to perform.
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