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: 157 pink colonies and 62 colorless colonies Plate 2: 167 pink colonies and 67 colorless colonies Plate 3: 177 pink colonies and 66 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.
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: 157 pink colonies and 62 colorless colonies
Plate 2: 167 pink colonies and 67 colorless colonies
Plate 3: 177 pink colonies and 66 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.
Trending now
This is a popular solution!
Step by step
Solved in 2 steps
