Question

One g of fecal matter was suspended in 99 ml saline solution (tube 1). After mixing gently but thoroughly, one ml from tube 1 was removed and added to tube with 99 ml saline solution (tube 2). One ml suspension from tube 2 was played in MacConkey agar. After 24 h incubation, 30 red colonies and 50 colorless colonies were counted. Determine cfu/g Gram-negative bacteria in the fecal sample. Your answer must be in scientific notation such as 1.0 x 10^2. Round your answer to the nearest 10th. For example, your answer is 1.675 x 10^5 cfu/g, type 1.7 x 10^5 cfu/g. Put a space before and after the x. The x is lower case.

Answer 1

Answer: the cfu/g Gram-negative bacteria in the fecal sample is C = 3.0 × 10^3

Explanation:

We know that; Gram negative bacteria looks  pale reddish in color under a light microscope from Gram staining.

therefore

There are 30 red bacterial colonies counted.

1 mL of from tube 1 was removed and added to tube with 99 mL saline (tube 2) dilution is 1/100.

transferred volume  into the plate is 1 mL.

Now, we have to determine the cfu/g Gram-negative bacteria in the fecal sample

Formula to calculate CFU/g bacteria in fecal sample is expressed as;

C = n/(s×d )

where C is concentration (CFU/g) , n is number of colonies , s is volume transferred to plate , d is dilution factor.

so we substitute

C = 30 / ((1/100) × 1)

C = 30 / 0.01

C = 3000

C = 3.0 × 10^3

THERFERE, the cfu/g Gram-negative bacteria in the fecal sample is C = 3.0 × 10^3