Answer to Direct Microscopic Count
Practice Problem #2
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 10 6) X
the dilution factor of any dilutions
made prior to placing the sample in
the counting chamber, e.g., mixing the bacteria with dye
to Lab 4
- 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.
- 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
- 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 = . There
are 170,000,000 E. coli per cc in the original sample. (In scientific
notation, the answer would be 1.7 X 10 8