Pseudomonas aeruginosa is able to colonize the respiratory mucosa and cause recu
ID: 254273 • Letter: P
Question
Pseudomonas aeruginosa is able to colonize the respiratory mucosa and cause recurring pneumonia in cystic fibrosis patients. The following experiments were performed to determine how P. aeruginosa can colonize mucous membranes. 0.1 mL broth culture was added to 0.4 mL serum and incubated for 1 hour; 0.1 mL of the same broth culture was added to 0.4 mL isotonic saline and incubated for 1 hour. Plate counts were performed on 1-mL samples to determine the number of bacteria. Immunoelectrophoresis was then used to compare normal serum to the serum incubated with Serratia and Pseudomonas.
Dilution
Number of Colonies
Serratia in serum
Serratia in saline
1:102
92
TMTC
1:104
10
TMTC
1:106
0
293
Pseudomonas in serum
Pseudomonas in saline
1:102
TMTC
TMTC
1:104
600
150
1:106
65
12
What was the purpose of the saline? The purpose of the saline is to provide a medium to survive the growth of both the bacteria.
Dilution
Number of Colonies
Serratia in serum
Serratia in saline
1:102
92
TMTC
1:104
10
TMTC
1:106
0
293
Pseudomonas in serum
Pseudomonas in saline
1:102
TMTC
TMTC
1:104
600
150
1:106
65
12
Explanation / Answer
CFU/ml = Number of colonies * dilution factor
Let's consider the dilution factor 1 : 104.
Pseudomonas in serum:
Number of colonies = 600
CFU/ml = 600*104=6*106
Serratia in serum:
Number of colonies = 600
CFU/ml = 10*104=105
In serum, CFU/ml of Pseudomonas > CFU/ml of Serratia.
Similarly for saline,
Let's consider the dilution factor 1 : 106.
Pseudomonas in saline:
Number of colonies = 12
CFU/ml = 12*106=12*106
Serratia in saline:
Number of colonies = 293
CFU/ml = 293*106=293*106
In saline, CFU/ml of Pseudomonas < CFU/ml of Serratia.
Thus Pseudomonas produce more colonies than Serratia in serum. Because the serum provided the growth factor which enhanced the growth of Pseudomonas.
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