Question 1

What is the primary goal of performing a serial dilution when working with bacterial samples?

Accepted Answer

To reduce the concentration of bacteria to a countable range for accurate quantification

Question 2

If you transfer 1 mL of a bacterial stock solution into 9 mL of diluent, what is the dilution factor for this step?

Accepted Answer

$$1:10$$

Question 3

A student performs a serial dilution by transferring 1 mL from each tube to the next, each containing 9 mL of diluent, for a total of 6 tubes. What is the final dilution in the sixth tube?

Accepted Answer

$$1:1,000,000$$

Question 4

Why is it important to use filter tips when pipetting bacterial samples?

Accepted Answer

To prevent contamination of the pipet and sample

Question 5

Which pipet would you use to accurately measure and transfer 150 µL of a solution?

Accepted Answer

P200

Question 6

You plate 0.1 mL of a $$10^{-5}$$ dilution and count 120 colonies. What is the concentration of bacteria in the original sample (in cells/mL)?

Accepted Answer

$$1.2 	imes 10$$^{7}$$

Question 7

If you observe 15 colonies on a plate after plating 0.1 mL of a $$10^{-4}$$ dilution, how would you report this result?

Accepted Answer

Too few to count (TFTC)

Question 8

Which of the following best describes the correct sequence of steps when using a micropipette?

Accepted Answer

Depress, hold, aspirate, dispense and blowout, eject

Question 9

A student adds 1 mL of a milk sample to a 99 mL bottle of diluent. What is the dilution factor after this step?

Accepted Answer

$$1:100$$

Question 10

You plate 0.1 mL from a $$10^{-6}$$ dilution and count 55 colonies. What is the cell concentration in the original sample in scientific notation?

Accepted Answer

$$5.5 	imes 10$$^{7}$$

Question 11

Why is it important to mix dilution tubes by rolling them between your hands rather than shaking them vigorously?

Accepted Answer

To prevent the formation of bubbles and ensure even mixing

Question 12

If you need to plate 100 µL of a dilution, which pipet would be most appropriate to use?

Accepted Answer

P200

Question 13

A student performs a serial dilution and obtains the following colony counts: $$10^{-3}$$ plate: 350 colonies, $$10^{-4}$$ plate: 120 colonies, $$10^{-5}$$ plate: 8 colonies. Which plate should be used to calculate the original cell concentration?

Accepted Answer

$$10^{-4}$$ plate

Question 14

Which of the following is NOT a reason for performing serial dilutions in microbiology?

Accepted Answer

To reduce the risk of contamination

Question 15

If you start with a $$1:100$$ dilution and then perform three consecutive $$1:10$$ dilutions, what is the final dilution factor?

Accepted Answer

$$1:100,000$$

Question 16

Which of the following best describes the term 'diluent'?

Accepted Answer

A substance used to dilute something

Question 17

A student accidentally aspirates liquid into the pipet body. What is the most likely consequence?

Accepted Answer

The pipet may become contaminated and inaccurate

Question 18

Which of the following is the correct scientific notation for a dilution factor of $$1:1,000$$?

Accepted Answer

$$1 	imes 10$$^{-3}$$

Question 19

If you plate 0.1 mL of a $$10^{-4}$$ dilution and count 250 colonies, what is the concentration of bacteria in the original sample?

Accepted Answer

$$2.5 	imes 10$$^{6}$$

Question 20

Which of the following best explains why plates with fewer than 30 colonies are considered 'too few to count' (TFTC)?

Accepted Answer

Statistical accuracy is low due to random variation

Serial Dilution and Standard Plate Count in Microbiology

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