Question

Hello, just wanted to check my answers. Answer as many as you like/can. Thankyou.

1) Four grams of soil are added to 36 ml sterile water and mixed well. 0.1 ml of this mix was added to 9.9 ml sterile water. This was diluted by 4 successive 1/10 dilutions. 1.0 ml from the last dilution was used to prepare a pour plate. After incubation 289 colonies were counted. Calculate the # CFU/g soil.

2) A 0.1 ml aliquot of a 1X10-3 dilution tube was placed into a plate and covered with molten agar. After 48 hours 121 colonies were counted. Calculate the number of cells per ml of the original culture.

3) How would you record your observation of a plate containing 27 colonies? How would you record your observation of a plate containing 345 colonies?

4) You have 4 mls of a bacterial culture. One ml of the bacteria is added to 99 ml of water and mixed well to make tube A. One ml of tube A is added to 9 ml of water to make tube B. One ml of tube B produced 45 colonies on a plate after incubation. Calculate the total CFU of the original sample.

Show all work. Include units of measure. Be sure you can do this without a calculator.

5) One thousand μl (1000 ul) of sample is added to 9 ml of water. This is labeled as Mix A. Then, 10 μl of Mix A is added to 990 μl of water and labeled as Mix B. One thousand μl of Mix B is added to an agar plate. After incubation this plate contained 85 colonies. Calculate the CFU/ml. Show all work

Answer 1

1. Final coloniy count = 289
4 successive dilutions of 1:10; Cumulative dilution factor = 1:10000 = 10^-4
0.1 mL to 9.9 mL = 100 fold dilution = 1:100 = 10^-2
4 g in 36 mL = 1 g in 9 mL = 1:10 dilution = 10^-1

Final dilution factor = 10^-7
Original CFU count = 2.89 X 10^9 CFU/g soil