Question

Imagine you plate .1ml of full strength milk on an NA plate and 250 colonies grew, how many CFU/ml are in the milk?

PLEASE TYPE YOUR ANSWER

Imagine you plate .1ml of full strength milk on an NA plate and 250 colonies grew, how many CFU/ml are in the milk?

NOW .2ml of a 1:100 dilution of milk on a EMB plate and 215 colonies grew. How many CFU/ml are in the original (full concentration) milk?

LASTLY, Imagine you plate .1 ml of a 1:10,000 dilution of milk on a plate 56 dark purple colonies and 144 clear colored colonies grew. How many CFU/ml of coliform bacteria are in the milk?

Answer 1

Answer:

1) Imagine you plate .1ml of full strength milk on an NA plate and 250 colonies grew, how many CFU/ml are in the milk?

• Number of colonies with 0.1ml of milk on Nutrient Agar plate = 250 colonies
• Number of colonies per 1ml of milk = 250 / 0.1 = 2500 CFU/ml = 2.5 x 10³ CFU/ml

2) NOW .2ml of a 1:100 dilution of milk on a EMB plate and 215 colonies grew. How many CFU/ml are in the original (full concentration) milk?

• Number of colonies with 0.2ml of 1:100 dilution of milk on EMB plate = 215 colonies
• Dilution factor of plate (fold dilution) = 100x
• Number of colonies per 1ml of original undiluted sample of milk = (215 *fold dilution)/0.2
• = (215*100) / 0.2 = 107500 CFU/ml = 1.075 x 10⁵ CFU/ml

3) LASTLY, Imagine you plate .1 ml of a 1:10,000 dilution of milk on a plate 56 dark purple colonies and 144 clear colored colonies grew. How many CFU/ml of coliform bacteria are in the milk?

• Number of dark purple colonies with 0.1ml of 1:10,000 dilution of milk on plate = 56 colonies
• Dilution factor of plate (fold dilution) = 10000x
• Number of colonies per 1ml of original undiluted sample of milk = (215 *fold dilution)/0.1
• = (56*10000) / 0.1 = 5600000 CFU/ml = 5.6 x 10⁶ CFU/ml