4.3: Obtaining a Pure Culture

Question 1

What is a clinical specimen?

Answer 1

Is a sample that is collected from a patient in order to identify the pathogen that may be causing their infection or disease.

Question 2

What is a pure culture?

Answer 2

A Pure Culture is a culture only growing one species of cells.

Question 3

A culture only growing one species of cells.

Answer 3

Pure Culture

Question 4

The process of spreading a bacterial culture onto a petri dish filled with agar is called ______.

Answer 4

Plating

Question 5

What is Plating?

Answer 5

The process of spreading a bacterial culture onto a petri dish filled with agar is called plating

Question 6

Plating can be done using a ___1___, ___2___, and ___3___.

Answer 6

1. sterile loop
2. sterile swab
3. sterilized wire loop.

Question 7

The primary advantage of plating a bacterial sample onto agar is……

Answer 7

……that cells are held in place.

Question 8

Bacteria plated onto agar are fixed in such a way as to support the ______ of colonies.

Answer 8

formation and visualization

Question 9

True or False: Colonies are visible to the naked eye but only after the bacterial cell has multiplied, often a million times over.

Answer 9

True

Question 10

Each colony is derived from a single ______.

Answer 10

Cell

Question 11

To isolate a microbe on an agar plate, microbiologists often utilize a ______ approach.

Answer 11

quadrant streak (or phase-dilution)

Question 12

A pure culture is free of ___1___ and can be traced back to a single origin.

Answer 12

outside contaminants

Question 13

Once isolated and expanded, a pure culture can be further examined for its……

Answer 13

…… size and shape, motility, Gram status, biochemical properties, etc.

Question 14

The sample is spread across the plate in such a way as to establish a ______.

Answer 14

dilution gradient

Question 15

For a ______, the sample is spread into four regions where each new region is perpendicular to the previous region, such that each region will become more diluted than the previous.

Answer 15

four-phase dilution gradient

Question 16

True or False: As the four-phase dilution gradient process is carried out, the same loop must be used each time a new phase is started.

Answer 16

False. As the four-phase dilution gradient process is carried out, a new (or sterilized) loop must be used each time a new phase is started.

Question 17

Once the samples are appropriately streaked onto the media (and the lids placed back onto the plate), they are ___1___ to prevent any potential contaminants from settling onto the surface of the agar and placed in an incubator set at 37°C for ___2___.

Answer 17

1. inverted
2. ~12-24 hours

Question 18

What temperature is most commonly used for incubation and why?

Answer 18

Maintaining a temperature of 37°C is optimal for promoting growth and is the temperature most commonly used by researchers.

Question 19

Individual colonies are then isolated, picked, and ___1___ in nutrient ___2___ for the expansion of the now pure culture to be used for laboratory testing.

Answer 19

1. re-inoculated
2. broth (or onto nutrient agar)

Question 20

True or False: Different strains of bacteria vary in their generation time (time it takes them to divide or double).

Answer 20

True

Question 21

Unlike bacteria, other organisms such as yeast preferentially grow at ______.

Answer 21

30°C

Question 22

True or False: Pathogenic strains of bacteria tend to grow faster than non-pathogenic strains at 37°C, researchers may set incubators at 25°C to restrict pathogenic growth.

Answer 22

True

Question 23

Describe what is being shown in the picture.

Answer 23

Agar Streak. A 4-phase dilution streak is shown beginning with the first phase (P1) and ending with the fourth phase (P4). A sterile loop must be used when switching between phases in order to accurately generate a dilution series. Single colonies should be evident within P3 or P4.

Question 24

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Question 25

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Question 26

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Question 27

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Question 28

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Question 29

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Question 30

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Question 31

Answer 31

Question 32

The purpose of the ______ is to generate an individual colony so that a single (pure) bacterial sample can be picked from the plate.

Answer 32

quadrant streak (or phase-dilution) method