Lab Three

Question 1

What is a viable count?

Answer 1

counting of growth of individual cells present in the initial sample

Question 2

what is the countable range of a viable plate count?

Answer 2

25-250 colonies

Question 3

what is the major advantage to a viable count?

Answer 3

it only counts living cells

Question 4

what is a 10-fold dilution?

Answer 4

dilute 1 in 10

is the most common dilution series

Question 5

what is the spread plate procedure?

Answer 5

take a sample of known dilution and spread it onto the agar plate, using a spreader. Individual colonies can be counted after incubation.

Question 6

what could cause an error in the viable count on a pour plate?

Answer 6

• not sterilizing the equipment
• not working near a bunsen
• not having the agar too hot

Question 7

what could cause an error in the viable count on a spread plate?

Answer 7

• not cooling the spreader after sterilization
• not having the agar plate at room temperature
• having pooling of the inoculum on the plate surface

Question 8

what is the advantage to pour plates?

Answer 8

they can incorporate volumes greater than 100 microlitres into a place if needed and give a similar result to the spread plate procedure

Question 9

What is the pour plate procedure?

Answer 9

1. add the bacterial culture into molten agar (not too hot)
2. allow the agar to set and then invert
3. Prepare positive (undiluted) and negative (saline) controls.

Question 10

what are the two kinds of direct count?

Answer 10

turbidity measurements and the direct microscopic cell count

Question 11

what is a turbidity measurement?

Answer 11

the greater the number of cells, the greater the turbidity, the greater the amount of light scattered and absorbed by the solution.

Question 12

what is the measurement unit for turbidity?

Answer 12

Optical density (OD)

Question 13

what is a direct microscopic count?

Answer 13

Involves the estimation of the total number of microbial cells in a sample using a counting chamber and a microscope

Question 14

what are the advantages of direct count?

Answer 14

• an easy method that requires minimal equipment
• you can observe cell morphology when counting
• you can count any solution as long as it is diluted correctly

Question 15

What is a 2-fold serial dilution?

Answer 15

The volume of culture is the same as the volume of the diluent. The number of cells is halved in each dilution

Question 16

How is optical density measured?

Answer 16

measured using a spectrophotometer

Question 17

what is the relationship between optical density and the number of cells?

Answer 17

the greater the optical density, the more cells present in the culture

Question 18

How do you calibrate a spectrophotometer?

Answer 18

1. load a cuvette with 1ml of diluent (blank)

2. Press CAL button and wait until the display is zero

Question 19

How do you measure the absorbance on the spectrophotometer?

Answer 19

1. load 1ml of a sample into the same cuvette in the zeroed machine
2. read the absorbance

Question 20

How do you plot absorbance results?

Answer 20

Plot the dilutions (x axis) against optical density (y axis)

Question 21

what is that direct microscopic cell count?

Answer 21

commonly used to estimate the total number of cells

Question 22

what is the structure of a counting chamber?

Answer 22

The counting chamber slide has two chambers, each has a microscopic grid etched on the glass surface. A glass slip is put on the top, exactly 0.1mm above the chamber floor.
The volume in each chamber is known

Question 23

how do you calculate the number of cells from a direct microscopic cell count?

Answer 23

The volume in each chamber is known.
Count the number of colonies in 5 cells and add
Then multiply by the number of squares in the section
Multiply the total cells in the section by the inverse of the dilution
Multiply by the chamber volume

Question 24

when is a direct count more useful than a viable count?

Answer 24

It is quicker

Question 25

What does a viable count, count?

Answer 25

Estimates the number of living or viable cells

Question 26

What does a direct count, count?

Answer 26

Estimates the total number of cells, both living and dead