3. You perform a series of serial dilutions. You decide to plate each of your dilutions using the
spot plate method because your lab is broke and you are trying to conserve plates. You set up 8
serial 10-fold dilutions from an unknown sample. After plating three 25 ul aliquots of each
dilution and allowing for proper incubation, you count 12, 15, and 17 colonies on your 104 plate,
and 5, 3, and 7 colonies on your 10s plate. On the 103 plate, the growth is very dense, but you
think you count 37, 39, and 42 colonies. (a) What is the cfu/ml of this culture? (b) Which plate
did you choose for your calculation and why? o plate Avence is [4.6 109 plate Aes 5 S 39,33 4.
You perform a series of serial dilutions. You decide to plate each of your dilutions using the spot
plate method because your lab is broke and you are trying to conserve plates. You set up 8 serial
10-fold dilutions from an unknown sample. After the plating your samples, you gasp and realize
your P200 was set to 50 ul instead of 25 ul. On the 10 plate you count 10, 12, and 16 colonies,
while on the 10 plate you count 1, 4, and 6 colonies. (a) What is the cfu/ml of this culture? (b)
Which plate did you choose for your calculation and why?
Solution
3.
(a) CFU/ml of Culture = (No. of colonies X Dilution factor) / Vol. of culture plated
= (39.33 x 103) / 0.03
= 1311000 CFU/ml
= 1.3 x 106 CFU/ml
(b) I have chosen plate 10-3 dilution where the viable count came around 37, 39 and 42 colonies.
For calculating the colony forming unit per ml a Petri plate in which the viable cell is between 30
to 300 is considered for calculation. Hence, the 10-3 dilution plate was chosen.
4.
(a) CFU/ml of Culture = (No. of colonies X Dilution factor) / Vol. of culture plated
= (3.6 x 108) / 0.05
= 720000000
= 7.2 x 109 CFU/ml
(b) The 10-8 dilution plate must be used for calculating the CFU/ml. Since the P200 was set at
50 micro litre if we calculate the CFU/ml from 10-7 plate we would have overestimated the
colony forming unit growing in the Petri plate.
spot plate method because your lab is broke and you are trying to conserve plates. You set up 8
serial 10-fold dilutions from an unknown sample. After plating three 25 ul aliquots of each
dilution and allowing for proper incubation, you count 12, 15, and 17 colonies on your 104 plate,
and 5, 3, and 7 colonies on your 10s plate. On the 103 plate, the growth is very dense, but you
think you count 37, 39, and 42 colonies. (a) What is the cfu/ml of this culture? (b) Which plate
did you choose for your calculation and why? o plate Avence is [4.6 109 plate Aes 5 S 39,33 4.
You perform a series of serial dilutions. You decide to plate each of your dilutions using the spot
plate method because your lab is broke and you are trying to conserve plates. You set up 8 serial
10-fold dilutions from an unknown sample. After the plating your samples, you gasp and realize
your P200 was set to 50 ul instead of 25 ul. On the 10 plate you count 10, 12, and 16 colonies,
while on the 10 plate you count 1, 4, and 6 colonies. (a) What is the cfu/ml of this culture? (b)
Which plate did you choose for your calculation and why?
Solution
3.
(a) CFU/ml of Culture = (No. of colonies X Dilution factor) / Vol. of culture plated
= (39.33 x 103) / 0.03
= 1311000 CFU/ml
= 1.3 x 106 CFU/ml
(b) I have chosen plate 10-3 dilution where the viable count came around 37, 39 and 42 colonies.
For calculating the colony forming unit per ml a Petri plate in which the viable cell is between 30
to 300 is considered for calculation. Hence, the 10-3 dilution plate was chosen.
4.
(a) CFU/ml of Culture = (No. of colonies X Dilution factor) / Vol. of culture plated
= (3.6 x 108) / 0.05
= 720000000
= 7.2 x 109 CFU/ml
(b) The 10-8 dilution plate must be used for calculating the CFU/ml. Since the P200 was set at
50 micro litre if we calculate the CFU/ml from 10-7 plate we would have overestimated the
colony forming unit growing in the Petri plate.
