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.

Answer 1

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.