Q: Which equation is used to calculate the original cell density (OCD) in a standard plate count?

$$OCD = \frac{CFU}{D \times V}$$

Q: If you plate 0.1 mL of a $$10^{-3}$$ dilution and count 150 colonies, what is the original cell density (OCD) in CFU/mL?

$$OCD = \frac{150}{0.001 \times 0.1} = 1,500,000$$ CFU/mL

Q: A student performs a serial dilution and plates 0.1 mL from a $$10^{-2}$$ dilution, counting 76 colonies. What is the OCD?

$$OCD = \frac{76}{0.01 \times 0.1} = 76,000$$ CFU/mL

Q: If a plate is inoculated with 1 mL of a $$10^{-4}$$ dilution and 250 colonies are counted, what is the OCD?

$$OCD = \frac{250}{0.0001 \times 1} = 2,500,000$$ CFU/mL

Question 1

Which of the following best describes the purpose of performing a standard plate count with serial dilutions in a clinical or food safety laboratory?

Accepted Answer

To estimate the number of live microorganisms in a sample by counting colonies formed after incubation.

Question 2

A urine sample is tested for a urinary tract infection (UTI) using the standard plate count method. Which result would indicate a likely infection?

Accepted Answer

Greater than 100,000 CFU/mL

Question 3

Which equation is used to calculate the original cell density (OCD) in a standard plate count?

Accepted Answer

$$OCD = \frac{CFU}{D 	imes V}$$

Question 4

If you plate 0.1 mL of a $$10^{-3}$$ dilution and count 150 colonies, what is the original cell density (OCD) in CFU/mL?

Accepted Answer

$$OCD = \frac{150}{0.001 	imes 0.1} = 1,500,000$$ CFU/mL

Question 5

Why are only plates with between 30 and 300 colonies counted in the standard plate count method?

Accepted Answer

Plates with fewer than 30 colonies are statistically unreliable, and plates with more than 300 colonies may have overlapping colonies, making accurate counting difficult.

Question 6

A milk sample is tested for safety using the standard plate count. Which result would indicate the milk is safe for consumption according to the guidelines provided?

Accepted Answer

Less than 20,000 CFU/mL

Question 7

In the serial dilution process, what is the main reason for mixing each dilution thoroughly before transferring to the next tube?

Accepted Answer

To ensure an even distribution of microorganisms, resulting in accurate and reproducible dilutions.

Question 8

A student performs a serial dilution and plates 0.1 mL from a $$10^{-2}$$ dilution, counting 76 colonies. What is the OCD?

Accepted Answer

$$OCD = \frac{76}{0.01 	imes 0.1} = 76,000$$ CFU/mL

Question 9

Which of the following is a common error that could lead to an inaccurate standard plate count result?

Accepted Answer

Not mixing the dilution tubes thoroughly before plating.

Question 10

A laboratory technician counts 25 colonies on a plate inoculated with 0.1 mL of a $$10^{-3}$$ dilution. What should they do according to standard plate count guidelines?

Accepted Answer

Disregard the plate because it has fewer than 30 colonies and is statistically unreliable.

Question 11

Which of the following best explains why serial dilutions are necessary before plating a bacterial sample?

Accepted Answer

To reduce the concentration of bacteria so that individual colonies can be counted accurately.

Question 12

If a plate is inoculated with 1 mL of a $$10^{-4}$$ dilution and 250 colonies are counted, what is the OCD?

Accepted Answer

$$OCD = \frac{250}{0.0001 	imes 1} = 2,500,000$$ CFU/mL

Question 13

A food safety inspector finds that a pasteurized milk sample has 50,000 CFU/mL. What is the most appropriate action based on the guidelines?

Accepted Answer

Reject the milk as unsafe because it exceeds the 20,000 CFU/mL limit.

Question 14

Which of the following best describes a colony-forming unit (CFU)?

Accepted Answer

A single viable microorganism or a group of microorganisms that forms a visible colony on an agar plate.

Question 15

A student accidentally plates 1 mL instead of 0.1 mL from a $$10^{-2}$$ dilution and counts 200 colonies. What is the calculated OCD?

Accepted Answer

$$OCD = \frac{200}{0.01 	imes 1} = 20,000$$ CFU/mL

Standard Plate Count and Serial Dilution in Microbiology

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