Colony Count Calculations
Q1. One ml of a sample was mixed with 99 ml of buffer. One ml of this was plated (using the pour
plate method) in nutrient agar. After incubation, 241 colonies were present on the plate. How
many colony-forming units were present per ml of the original sample ? State your answer in
CFU/ml.
1ml E.coli was put into 99ml buffer
Dilution information (any of the following can be said):



1 in 100 dilution (99 + 1 = 100) or
Dilution factor of 100 or
10-2 dilution ( 1  100 = 0.01 = 10-2)
Colony information :
1ml of the dilution was plated = 241 colonies grew
To calculate the CFU per ml in the original sample:

Multiply the no. of colonies by the dilution factor or
241 x 100 = 24 100 colonies/ ml

Divide by the dilution
241  10-2 = 24 100 colonies/ ml
Q2 One ml of a sample was mixed with 99 ml buffer. One-tenth of a ml of this was plated on nutrient
agar. After incubation, 142 colonies were present on the plate. How many colony-forming units
were present per ml of the original sample?
1ml E.coli was put into 99ml buffer
Dilution information (any of the following can be said):



1 in 100 dilution (99 + 1 = 100) or
Dilution factor of 100 or
10-2 dilution ( 1  100 = 0.01 = 10-2)
Colony information :
1/10 ml of the dilution was plated = 142 colonies grew
To calculate the CFU per 1/10 ml in the original sample:

Multiply the no. of colonies by the dilution factor or
142 x 100 = 14 200 colonies

Divide by the dilution
241  10-2 = 14 200 colonies
To calculate the CFU per 1 ml in the original sample:
Multiply the no. of colonies in 1/10 ml by 10:
14 200 x 10 = 142 000 colonies/ml
Q3. An overnight culture of Escherichia coli is used as a sample. One ml of this culture is added to a
bottle containing 99ml of buffer. This dilution is mixed well (as all dilutions are!), and one ml of
this is mixed in 9 ml of buffer. This second dilution is diluted by three successive 1/10 dilutions.
The last (fifth) dilution is plated, i.e. 0.1 ml is plated on nutrient agar. After incubating the plate, 56
colonies are counted. How many colony-forming units were present per ml of E. coli culture?
1ml E.coli was put into 99ml buffer
Dilution information (any of the following can be said):



1 in 100 dilution (99 + 1 = 100) or
Dilution factor of 100 or
10-2 dilution ( 1  100 = 0.01 = 10-2)
1 ml of the previous dilution was put into 9ml buffer
(1 in 10 dilution is made of a sample that has already been diluted 1 in 100)


Dilution factor of 1000. (100 x 10 = 1000) or
10-3 dilution (1/10 x 1/100 = 1/1000 = 10-3)
This sample is then diluted a further 3 times, each 1/10 (10-1) dilutions


Dilution factor (1000 x 10 x 10 x 10 = 1 000 000 = 106) or
10-6 dilution (1/1000 x 1/10 x 1/10 x 1/10 = 1/ 1 000 000 = 10 -6)
Colony information :
0.1ml of the dilution was plated = 56 colonies grew
To calculate the CFU per 0.1ml in the original sample:

Multiply the no. of colonies by the dilution factor or
56 x 106 = 5.6 x 107 colonies/ 0.1ml

Divide by the dilution
56  10-6 = 5.6 x 107 colonies/ 0.1ml
To calculate the CFU per 1ml in the original sample:
Multiply the no. of colonies in 0.1ml by 10:
(5.6 x 107) x 10 = 5.6 x 108 colonies/ml