Plate Count/ Dilutions

Question 1

Define what plate counts purpose is.

Answer 1

A method to count bacteria in a given sample.

Question 2

What principle does plate counts come from?

Answer 2

That one cell placed on/embedded in agar will multiply to form one colony. The number of colonies in the agar after inoculation reflect the number of viable cells in the original sample.

Question 3

Define confluent growth. (Lawn on cells)

Answer 3

If too many cells are inoculated into the agar.

Question 4

Why do we often refer to our plate count results as colony-forming units? (cfu)

Answer 4

Because only about 1% of bacteria can even be grown in a lab, so our sample may have other bacteria in it, but it’s unable to thrive in the agar nutrients.

Question 5

What is the formula for calculating a dilution?

Answer 5

Sample volume / (sample volume + diluent volume)

Question 6

What is the formula for calculating the total dilution.

Answer 6

By multiplying each individual dilution.
Ex. (1/100)(1/10)(1/10)= 1/10,000

Question 7

What is the formula for calculating cell concentrations? (Cells/mL)

Answer 7

of colonies * (1/volume plated) * (1/total dilution)

Ex. If you obtained 80 colonies by plating 0.5 mL of 10^-6 dilution.

80 * (1/0.5) * (1/10^-6) = 80 * 2 * 10^6 = 1.6 * 10^8 cfu/mL

Question 8

What must you always remember about dilutions? (4)

Answer 8

1. Always record your answer in terms of cells/mL or CFU/mL.
2. Use only the plates showing 30 to 300 colonies per plate to calculate cell numbers.
3. Average the colony numbers of replicate plates from the same dilution, but not of plates from different dilutions.
4. A negative exponent always means the number is less than one. Ex. 1x10^-5 = 0.00001 cells/mL