Question

2. A series of 1 ml samples from tube #4 of the dilution s eries below are spread on agar plates, resulting in the growth of an average of 68 colonies per plate. What is the concentration of colony-forming units (cfu/mi) present in the original culture? 0.1 ml 0.1 ml 1 ml 1 ml 100 ml 9.9 ml 9.9 ml 9 ml 9 ml 2 4 3. The culture flask at left contains a total of 3.2 x 10s cfu. How man total cfu's are present in dilution tube #52 If you were to plate a 0.2 ml (200 μ) sample from tube #4, how many colonies would you expect to grow? 1 ml 0.1 ml I ml 1 ml 1 ml 200 ml 9 ml 9.9 ml 1 ml 1 ml 1 ml 4

Answer 1

2- CFU/ml = number of colonies/ dilution factor * volume plated in ml.

Dilution factor = 10^-2*10^-2*10^-1*10^-1 = 10^-6.

Volume Plated - 1ml.

The number of colonies = 68.

So CFU/ml in original culture = 68 / 10^-6

= 68*10⁶

6.8 *10⁷ CFU/ml

3- -

CFU/ml = number of colonies/ dilution factor * volume plated in ml.

Dilution factor = 10^-1*10^-2*10^-1*10^-1 *10^-1 = 10^-6.

Volume Plated - 1ml.

So CFU/ml in original culture = 3.2* 10⁸

3.2* 10⁸ = Number of colonies / 10^-6 * 1

Number of colonies = 3.2* 10^{8 *} 10^-6

⁼ 3.2* 100 or 320 colonies.

So we will get 320 colonies from the fifth tube when we plate 1ml of culture.

Dilution for the fourth tube is 10-1*10-2*10-1*10-1= 10-5.

Volume Plated = 0.2ml.

3.2* 108 = Number of colonies / 10-5 * 0.2

Number of colonies = 3.2* 108  * 10-5 * 0.2

= 3.2*10³ * 0.2

= 3200 * 0.2 = 640 colonies.