Question

(Microbiology Dilution Scheme) You are working with a water source that has approximately 1 billion CFU/mL. The actual sample you are given is 0.5mL of the original water diluted to a total of 10mL. Based upon this information, diagram a dilution series that should result in not only countable plates, but confirm the initial assessment of 1 billion CFU. There are several solutions, but full credit goes to the most efficient scheme.

Answer 1

Total count is 1000000000 CFU/ml.

0.5mL of the original water diluted to a total of 10mL. That means 20 times dilution. The count is 50000000 CFU/ml

Take 1 ml solution in 999 ml of water (total volume 1 L), that means 1000 times dilution. The count is 50000 CFU/ml

Again, Take 1 ml solution in 999 ml of water (total volume 1 L), that means 1000 times dilution. The count is 50 CFU/ml.

Therefore, 50 CFU/ml in plates is countable, and it confirms the initial assesment of 1 billion CFU/ml.