Question 1

What is the primary purpose of performing a quadrant streak plate as shown in the image?

Accepted Answer

To isolate individual bacterial colonies from a mixed sample

Question 2

Which step in the quadrant streak plate technique is essential for sequentially diluting the bacterial sample across the plate?

Accepted Answer

Sterilizing the inoculating loop between each quadrant

Question 3

If you observe isolated colonies only in the fourth quadrant of your streak plate, what does this indicate about your technique?

Accepted Answer

You successfully diluted the sample to achieve isolation

Question 4

Why is it important to use aseptic technique when performing streak or spread plate methods?

Accepted Answer

To prevent contamination of the culture and ensure accurate results

Question 5

A student performs a spread plate using 100 $$\muL of a$$ $$10^{-6}$$ dilution and counts 120 colonies. What is the original cell density (OCD) in CFU/mL? Use the formula $$OCD = \frac{CFU 	imes TDF}{V}$$, where $$TDF = 10^6$ and V = 0.1$ mL.

Accepted Answer

$$1.2 	imes 10^8$ CFU/mL

Question 6

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

Accepted Answer

A group of genetically identical cells originating from a single cell or small cluster

Question 7

During a serial dilution, you transfer 100 $$\muL $$from a $$10^{-2}$$ tube to a $$10^{-3}$$ tube. What is the dilution factor of the $$10^{-3}$$ tube relative to the original sample?

Accepted Answer

$$10^{-3}$$

Question 8

If a spread plate has 400 colonies, how should the result be reported according to standard plate count guidelines?

Accepted Answer

TNTC (too numerous to count)

Question 9

Which of the following errors would most likely result in no growth in quadrants 2, 3, and 4 of a streak plate?

Accepted Answer

Failing to transfer bacteria from quadrant 1 to quadrant 2

Question 10

A student counts 85 colonies on a plate inoculated with 150 $$\muL $$from a $$10^{-4}$$ dilution. What is the original cell density (OCD) in CFU/mL? Use $$OCD = \frac{CFU 	imes TDF}{V}$$, $$TDF = 10^4$, V = 0.15$ mL.

Accepted Answer

$$5.7 	imes 10^5$ CFU/mL

Question 11

What is the main advantage of using the spread plate method for bacterial enumeration?

Accepted Answer

It provides a quantitative estimate of viable cell numbers in a sample

Question 12

Which of the following best explains why plates with fewer than 25 colonies are considered TFTC (too few to count)?

Accepted Answer

Statistical accuracy is low due to sampling error

Question 13

Why is it necessary to convert $$\muL to mL $$when calculating OCD in CFU/mL?

Accepted Answer

Because OCD is always reported per mL, and the plated volume is often in $$\muL$$

Question 14

Which of the following is NOT a reason for using a pure culture in microbiological testing?

Accepted Answer

To study interactions between multiple species

Question 15

A student accidentally uses the same pipette tip for all serial dilutions. What is the most likely consequence?

Accepted Answer

Cross-contamination will occur, invalidating the dilution series

Question 16

Which of the following colony characteristics can help differentiate between Serratia marcescens and Staphylococcus epidermidis?

Accepted Answer

Colony pigment and morphology

Isolation Techniques in Microbiology: Streak Plate, Spread Plate, and Standard Plate Count

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