BIOL 230 Lab Manual: Direct Count Practice Problem 2 Answer

Formula to be used:

the number of bacter per cc =

the average number of bacteria per large, double-lined square X

the dilution factor of the large square (1,250,000 or 1.25 X 10⁶) X

the dilution factor of any dilutions made prior to placing the sample in
the counting chamber, e.g., mixing the bacteria with dye

First find the average number of bacteria per square by adding the 5 numbers and dividing by 5.
- 61+74+78+63+64 = 340
- 340 divided by 5 = 68. There is an average of 68 bacteria per square. (6.8 X 10¹ in scientific notation.)
Multiply 68 times the dilution factor of the square which is always 1,250,000. (The square holds 1/1,250,000 of a cc. The dilution factor or inverse of 1/1,250,000 is 1,250,000/1.)
- 68 X 1,250,000 = 85,000,000. (6.8 X 10¹ X 1.25 X 10⁶ = 8.5 X 10⁷ in scientific notation).
Now multiply that number by the dilution factor of the dilution made when the bacteria was mixed with dye.
- One ml of bacter was mixed with 1ml of dye. This is a 1/2 dilution. The dilution factor or inverse of 1/2 is 2/1.
- 85,000,000 X 2 = 170,000,000. (8.5 X 10⁷ X 2 = 1.7 X 10⁸ in scientific notation.)
  There are 170,000,000 E. coli per cc in the original sample. (In scientific notation, the answer would be 1.7 X 10⁸.)

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