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

Calculate the number of bacteria (CFU/ml) in each serum and saline sample.

Answer 1

Serratia in serum:

In 10² dilution, Serratia in serum = 92

So, in 0.1 ml broth culture, it will be 92 X 10²

In 0.5 ml serum solution, the number of bacteria is 92 X 10²

So, the CFU/ml = 18.4 X 10³

Serratia in saline:

In 10⁶ dilution, Serratia in saline = 293

So, in 0.1 ml broth culture, it will be 293 X 10⁶

In 0.5 ml saline solution, the number of bacteria is 293 X 10⁶

So, the CFU/ml = 5.86 X 10⁸

Pseudomonas in serum:

In 10⁴ dilution, Pseudomonas in serum = 600

So, in 0.1 ml broth culture, it will be 600 X 10⁴

In 0.5 ml serum solution, the number of bacteria is 600 X 10⁴

So, the CFU/ml = 12 X 10⁶

Pseudomonas in saline:

In 10⁴ dilution, Pseudomonas in saline = 150

So, in 0.1 ml broth culture, it will be 150 X 10⁴

In 0.5 ml saline solution, the number of bacteria is 150 X 10⁴

So, the CFU/ml = 3 X 10⁶