Bio2 LabName
Culturing Bacteria & CFU Determination Worksheet
(Total points = 10)
1. How do colonies form on agar?
Section #
2. Will the bacteria we culture from our river sediment samples reflect the entire population of
bacterial species present in the river? Why or why not?
3. When determining CFUs for a dilution of the same sample at 10-2 and 10-3 which of the
following results would seem the most accurate to you? Explain why in the space below.
A. 110,000 CFU/mL for 10-2; and 140,000 CFU/mL 10-³
B. 110,000 CFU/mL for 10-2; and 14,000 CFU/mL 10-³
C. 10,000 CFU/mL for 10-2; and 140,000 CFU/mL 10-³
D. none of the above4. You used 1mL of a river water sample at a 10-³ dilution and added this to an agar plate. After
incubation, you count 60 colonies. How many colony forming units (CFUs) are present? Show
your work.
5. River water samples are diluted to 10-2, 10-³, and 10-4. You plate 500µl of each dilution,
incubate overnight, and find that only the 10-4 plate had isolated, countable colonies; however,
colonies on the other plates are “Too Numerous To Count” (TNTC).
(a) Calculate the CFU/mL if you counted 100 colonies on the 10-4 plate. Show your work.
(b) State a modification you could make to the protocol in order to obtain isolated,
quantifiable colonies on all three agar plates.
6. What is the goal of calculating CFU’s?
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