Bacterial enumeration - Dilutions and Plate counts

Question 1

Blank Describes individual colonies on a plate’s surface

Answer 1

Colony forming unit

Question 2

The number of individuals doubles each generation

Answer 2

Exponential growth

Question 3

Cloudiness of a fluid caused by large numbers of particles/organisms

Answer 3

Turbidity

Question 4

The measure of the light intensity absorbed by a sample

Answer 4

absorbance

Question 5

Asexual reproduction resulting in two daughter cells

Answer 5

Binary Fission

Question 6

Order the steps for performing a dilution series

Answer 6

Determine the dilutions of the dilution tubes.
Determine the dilutions of the agar plates.
Determine which plate is the countable plate.
Calculate the concentration in the original tube.

Question 7

True/False: Binary fission is a method of sexual reproduction where a single cell (parent cell) will split into four cells (daughter cells).

Answer 7

False –> 2 daughter cells

Question 8

Define viable plate count

Answer 8

A common method used to determine the number of organisms in a culture

Question 9

Define colony forming unit (CFU).

Answer 9

Each colony on a plate

Question 10

How many CFUs for the plate to be considered viable.

Answer 10

30 - 300 CFUs

Question 11

What is the dilution formula?

Answer 11

Dilution = Volume of sample added mL/ Total volume (mL of Sample + mL of Diluent)

Question 12

If 1 mL of sample is added to 9mL of diluent, what is the dilution?

Answer 12

Dilution = 1/ 1+9 = 1/10

Question 13

What are successive dilutions?

Answer 13

Dilutions that are multiplied together to determine the final total dilution.

Question 14

Dilution of 3rd tube

Answer 14

= 1/10 x 1/10 x 1/10 = 1/1000

Question 15

How to determine the number of CFUs in the original plate.

Answer 15

The number of colonies on the viable plate is multiplied by the total dilution factor. For example, if it is determined that there are 57 CFUs on a plate that was diluted 1000 fold, then the original culture would have contained 57,000 CFU/mL.

Question 16

True/False: If there is more than one plate for a dilution, the CFUs of all the plates that have that dilution are averaged together prior to multiplying by the dilution factor.

Answer 16

True

Question 17

True/False: If more than one dilution that falls within the 30-300 CFU range, only the least diluted plate is used in the calculations.

Answer 17

True

Question 18

To determine the number of CFUs in the original culture, the number of colonies on the viable plate is _____ the total dilution factor of that plate.
Responses

Answer 18

Multiplied by

Question 19

Step 1: Determine the dilutions of the dilution tubes (first series of dilutions). In the example, 0.5 mL is removed from the original tube and is added to 4.5 mL of sterile diluent to create a 1:10 dilution. In this example, each of the subsequent tubes created are also 1:10 dilutions.

Answer 19

Dilution of 1st tube = 0.5mL/5ml = 1/10

Question 20

Step 1: Determine the dilutions of the dilution tubes (first series of dilutions). In the example, 0.5 mL is removed from the original tube and is added to 4.5 mL of sterile diluent to create a 1:10 dilution. In this example, each of the subsequent tubes created are also 1:10 dilutions.

Find dilution of 3rd tube

Answer 20

0.5mL/5mL x 1/10 x 1/10 = 1/1000

Question 21

Step 2: Determine which plate is the countable plate.

What is a countable plate?

Answer 21

30 - 300

Question 22

Step 3: Calculate the concentration in the original tube.

How do you do this?

Answer 22

Number of colonies counted should be divided by final dilution of plate.

Concentration of original tube = Colonies counted/ dilution X volume

Question 23

Which plate should you count?

Plate A = 500 colonies
Plate B = 50 colonies
Plate C = 100 colonies

Answer 23

Plate C bc it is least diluted.

Question 24

Binary fission is a method of sexual reproduction where a single cell (parent cell) will split into four cells (daughter cells).

True/False

Answer 24

False

Question 25

Order the steps for performing a dilution series.

Answer 25

1.) Determine the dilution of the dilution tubes
2.) Determine which plate is the countable plate
3.) Calculate the concentration in the original tube

Question 26

What is a colony forming unit and how is it used to infer microbial numbers in a sample?

Answer 26

A colony is group of microorganisms grown on a single agar plate. Colonies are used to estimate the microbial numbers in a sample by multiplying the visible number of colonies by the dilution factor to provide a CFU/mL result.

Question 27

A viable plate has a count of 95 colonies produced from 0.125 mL of a 10-4 dilution of the original sample. The original contains _____ CFU/mL.

Answer 27

7,600,000

Question 28

Determine the Concentration of the original sample.

Dilution plate CFU = 120

Dilution factor of viable plate = 1/1000

Volume of diluent added to plate = 0.125mL

Answer 28

CFU/mL original = 0.96 = 120/(0.125) x 1000

Question 29

In binary fission, bacterial cells grow _____

Answer 29

Exponentially

Question 30

When one bacterial cell undergoes binary fission, there will be _____ cells present on the 5th generation.

Answer 30

16

Question 31

A viable plate count is a method of calculating the number of cells in a dilution by the counting the number of _____ on agar plates.

Answer 31

colonies

Question 32

Performing a 1:8 dilution from the original sample 3 times would produce a dilution of _____.

Answer 32

1: 512

Question 33

True or False

To create a series of dilutions, an identical volume of stock (undiluted) solution is added to each of the dilution test tubes.

Answer 33

False