Apps Week 5 Review Quiz

A scientist wanted to produce 40ml of solution with a dilution factor of 15. What is required to achieve this?

• 2.67ml stock solution & 37.33ml sterile water

• 15ml stock solution & 25ml sterile water

• 10ml stock solution and 30ml distilled water

• 6ml stock solution and 34ml distilled water

Answer explanation

Divide the total volume by the dilution factor e.g. 40/15 = 2.67.

What is the log the number 16?

• 160

• 1.20

• 15.8

• 2.56

What is the antilog of 1.20?

• 0.08

• 1.44

• 15.8

• 120

Which of the following statements are correct?

• A viable count will count cells capable of dividing

• Haemocytometry can be used to obtain a total count

• Turbidimetry is used to obtain a total count

• Dilution plating can be used to obtain a viable count

• A total count only includes living cells

When making a 1:10 serial dilution with a final volume of 20ml, which of the following statements are correct?

• Put 1ml of stock solution into test tube 1

• Put 2ml of stock solution into test tube 1

• Put 19ml of distilled water into test tube 1 then all subsequent test tubes

• Put 18ml of distilled water into test tube 1 then all subsequent test tubes

• Discard 2 ml of the final test tube at the lowest concentration

Four agar plates contained the following numbers of bacterial colonies:

A - 7

B - 44

C - 84

D - 389

Which agar plate would you use to estimate the number of bacteria in the original stock solution?

• A

• B

• C

• D

Answer explanation

If the numbers are too high they are difficult to count, but if they are too low they could be inaccurate as the margin for error is larger.

If there were 29 bacterial colonies in 0.1ml of stock solution on an agar plate with a dilution factor of 1:10,000, what is the number of bacteria in the stock solution per ml?

• 29,000

• 29,000,000

• 2,900,000

• 2,900,000,000

Answer explanation

29/0.1 to calculate how many in 1ml. Then multiply by the dilution factor of 10,000 to give 2,900,000.

Put the statements below regarding haemocytometry in the correct order:

1. 1. Using the dilution factor the number in the original sample is estimated.

2. 2. A coverslip is added to the haemocytometer.

3. 3. A drop of sample is placed on the slide.

4. 4. The slide is viewed under a microscope.

5. 5. The number of microorganisms in several squares (usually 5) are counted and an average found.

• 3, 2, 4, 1, 5

• 2, 3, 4, 1, 5

• 3, 2, 4, 5, 1

• 2, 3, 4, 5, 1

What is the mean number of cells per square in the haemocytometer?

• 5

• 6

• 6.2

• 4.4

Answer explanation

Count 5 squares - the four corners and centre. Count only cells touching the top and left (North/West rule).

Which of the following statements regarding turbidimetry is incorrect?

• If there are more cells present in the solution, the measurement of absorbance will increase

• If there are more cells present in the solution, the measurement of absorbance will decrease

• A serial dilution is required (or known concentrations of microorganisms)

• It cannot distinguish living cells from dead cells

Answer explanation

If there are no cells transmission would be 100% and absorbance zero. The more cells you introduce to the solution, the more transmission will decrease (less light will pass through) as more will be absorbed.