You collected 1000 mL of river water from the stream running through campus. You prepared dilutions
of the water as follows:
Tube A. 0.5 mL of river water + 99.5 mL of sterile water
Tube B. 1 mL from Tube A + 9 mL of sterile water
Tube C. 1 mL from Tube B + 9 mL of sterile water
Tube D. 1 mL from Tube C + 9 mL of sterile water
You then plated 0.1 mL of each dilution onto each of three MacConkey agar plates. (MacConkey agar is type of selective,
differential medium. It inhibits the growth of Gram-positive microorganisms and non-enteric Gram-negative bacteria.
Among the bacteria not inhibited by the medium, lactose-fermenting bacteria form pink colonies and bacteria that do not
ferment lactose form colorless colonies.) After incubation, the colonies were too numerous to count on the plates
prepared from dilutions A, B, and C. For the plates prepared from dilution D you counted the following numbers of
colonies:
Plate 1: 155 pink colonies and 50 colorless colonies
Plate 2: 165 pink colonies and 55 colorless colonies
Plate 3: 175 pink colonies and 60 colorless colonies
a. Determine the total dilution factors for each of the tubes.
b. Calculate the concentration of lactose-fermenting enteric Gram-negative bacteria (CFUs) per mL that were present in
the original water sample.
c. Calculate the TOTAL number of lactose-fermenting enteric Gram-negative bacteria (CFUs) that were present in the
original water sample.
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