Experiment #13: Use of Selective, Differential, and Enriched Media

question

Selective Media:

answer

Selects for certain microbes while suppressing others

question

Selective media is usually used towards

answer

separating gram neg. & pos. bacteria.

question

Differential Media:

answer

Allow the growth of more than one microorganism, all of which have distinguishable colonies based on the biochemical properties of the organisms

question

Often times, differential media contain

answer

a pH indicator.

question

Enriched Media:

answer

Contains growth factors that support bacterial growth

question

Mannitol Salt Agar (MSA):

answer

Selective & Differential

question

MSA selective component:

answer

7.5% NaCl, which selects for halotolerant organisms such as members of the Staphylococcus genus.

question

MSA selects for

answer

halotolerant organisms.

question

MSA differential component:

answer

-Mannitol (carbohydrate substrate)
-Phenol red (pH indicator)

question

MSA is differential for

answer

mannitol fermentation.

question

Organisms which are able to ferment the mannitol will produce

answer

acid fermentation products which lower the pH, causing the phenol red to turn yellow.

question

Blood Agar (5% sheep's blood):

answer

Differential

question

Blood agar differential component:

answer

The degree of hemolysis

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Hemolysis:

answer

Break down of RBC's

question

Three types of hemolysis:

answer

-Alpha
-Beta
-Gamma

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Alpha hemolysis:

answer

-Partial lysis
-Green/Brown/Yellow discoloration of the blood around the colonies

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Beta hemolysis:

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-Complete lysis
-Complete clearing of the blood around colonies

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Gamma hemolysis:

answer

-No lysis
-No change in the medium

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MacConkey Agar:

answer

Selective & Differential

question

MacConkey selective component:

answer

-Crystal violet
-Bile salts
*Inhibit most gram-positive organisms

question

MacConkey Agar selects for

answer

gram-negative organisms.

question

MacConkey differential component:

answer

-Lactose (substrate)
-Neutral red (pH indicator)

question

MacConkey agar differentiates

answer

gram-negative bacteria based on their ability to ferment lactose (strong vs. weak).

question

Lactose fermenters:

answer

Coliforms

question

MacConkey lactose fermentation results in

answer

acid by-products in the media, resulting in colonial growth turning pinkish-red.

question

If an organism is unable to ferment lactose,

answer

the colonies will be colorless, taking on the color of the medium.

question

E. coli & MacConkey lactose fermentation:

answer

-Turns bacteria & media bright pink
-Strong fermenter

question

E. aerogenes & MacConkey lactose fermentation:

answer

-Only turns bacteria pink
-Weak fermenter

question

Eosin Methylene Blue Agar (EMB):

answer

Selective & Differential

question

EMB selective components:

answer

-Eosin
-Methylene blue
*Inhibit gram-positive growth

question

EMB selects for

answer

gram-negative organisms

question

EMB differentiation component:

answer

-Lactose

question

EMB differentiates between organisms

answer

which ferment lactose and produce acid end-products, and organisms that do not ferment lactose

question

In EMB, small amounts of acid production result in

answer

pink colored growth (Enterobacter aerogenes).

question

In EMB, large amounts of acid cause

answer

the acid to precipitate on the colony, resulting in a greenish, metallic sheen (Escherichia coli).

question

Columbia C-NA Agar (CCNA):

answer

Selective & Differential

question

CCNA selective components:

answer

Two antibiotics:
-Colistin
-Nalidixic
*Prohibit growth in gram-negative organisms

question

CCNA is selective for

answer

gram-positive organisms.

question

CCNA allows differentiation by

answer

hemolytic patterns with the addition of blood agar.

question

Cultures:

answer

-Enterobacter aerogenes
-Escherichia coli
-Streptococcus zooepidemicus
-Streptococcus mutans
-Enterococcus faecalis
-Morganella morganii
-Staphylococcus xylosus
-Staphylococcus epidermidis

question

What properties of gram-negative organisms would likely make them more resistant to crystal violet and bile salts than gram-positive organisms?

answer

The external lipopolysaccharide membrane makes gram-negative organisms resistant to crystal violet and bile salts.

question

An unknown bacterium produces colorless colonies when inoculated onto an EMB plate. Predict what you would see if you inoculated that same unknown onto the following media, and why:

MacConkey Agar:

answer

The unknown bacteria will grow and produce colorless colonies because both EMB and MacConkey agar are selective for gram-negative organisms and differential for lactose fermenters. Clear colonies indicate that the unknown bacterium is not a lactose fermenter.

question

An unknown bacterium produces colorless colonies when inoculated onto an EMB plate. Predict what you would see if you inoculated that same unknown onto the following media, and why:

C-CNA Agar:

answer

The unknown bacteria will have no growth because it is gram-negative while C-CNA agar is selective for gram-positive organisms.

1 of 43

question

Selective Media:

answer

Selects for certain microbes while suppressing others

question

Selective media is usually used towards

answer

separating gram neg. & pos. bacteria.

question

Differential Media:

answer

Allow the growth of more than one microorganism, all of which have distinguishable colonies based on the biochemical properties of the organisms

question

Often times, differential media contain

answer

a pH indicator.

question

Enriched Media:

answer

Contains growth factors that support bacterial growth

question

Mannitol Salt Agar (MSA):

answer

Selective & Differential

question

MSA selective component:

answer

7.5% NaCl, which selects for halotolerant organisms such as members of the Staphylococcus genus.

question

MSA selects for

answer

halotolerant organisms.

question

MSA differential component:

answer

-Mannitol (carbohydrate substrate)
-Phenol red (pH indicator)

question

MSA is differential for

answer

mannitol fermentation.

question

Organisms which are able to ferment the mannitol will produce

answer

acid fermentation products which lower the pH, causing the phenol red to turn yellow.

question

Blood Agar (5% sheep's blood):

answer

Differential

question

Blood agar differential component:

answer

The degree of hemolysis

question

Hemolysis:

answer

Break down of RBC's

question

Three types of hemolysis:

answer

-Alpha
-Beta
-Gamma

question

Alpha hemolysis:

answer

-Partial lysis
-Green/Brown/Yellow discoloration of the blood around the colonies

question

Beta hemolysis:

answer

-Complete lysis
-Complete clearing of the blood around colonies

question

Gamma hemolysis:

answer

-No lysis
-No change in the medium

question

MacConkey Agar:

answer

Selective & Differential

question

MacConkey selective component:

answer

-Crystal violet
-Bile salts
*Inhibit most gram-positive organisms

question

MacConkey Agar selects for

answer

gram-negative organisms.

question

MacConkey differential component:

answer

-Lactose (substrate)
-Neutral red (pH indicator)

question

MacConkey agar differentiates

answer

gram-negative bacteria based on their ability to ferment lactose (strong vs. weak).

question

Lactose fermenters:

answer

Coliforms

question

MacConkey lactose fermentation results in

answer

acid by-products in the media, resulting in colonial growth turning pinkish-red.

question

If an organism is unable to ferment lactose,

answer

the colonies will be colorless, taking on the color of the medium.

question

E. coli & MacConkey lactose fermentation:

answer

-Turns bacteria & media bright pink
-Strong fermenter

question

E. aerogenes & MacConkey lactose fermentation:

answer

-Only turns bacteria pink
-Weak fermenter

question

Eosin Methylene Blue Agar (EMB):

answer

Selective & Differential

question

EMB selective components:

answer

-Eosin
-Methylene blue
*Inhibit gram-positive growth

question

EMB selects for

answer

gram-negative organisms

question

EMB differentiation component:

answer

-Lactose

question

EMB differentiates between organisms

answer

which ferment lactose and produce acid end-products, and organisms that do not ferment lactose

question

In EMB, small amounts of acid production result in

answer

pink colored growth (Enterobacter aerogenes).

question

In EMB, large amounts of acid cause

answer

the acid to precipitate on the colony, resulting in a greenish, metallic sheen (Escherichia coli).

question

Columbia C-NA Agar (CCNA):

answer

Selective & Differential

question

CCNA selective components:

answer

Two antibiotics:
-Colistin
-Nalidixic
*Prohibit growth in gram-negative organisms

question

CCNA is selective for

answer

gram-positive organisms.

question

CCNA allows differentiation by

answer

hemolytic patterns with the addition of blood agar.

question

Cultures:

answer

-Enterobacter aerogenes
-Escherichia coli
-Streptococcus zooepidemicus
-Streptococcus mutans
-Enterococcus faecalis
-Morganella morganii
-Staphylococcus xylosus
-Staphylococcus epidermidis

question

What properties of gram-negative organisms would likely make them more resistant to crystal violet and bile salts than gram-positive organisms?

answer

The external lipopolysaccharide membrane makes gram-negative organisms resistant to crystal violet and bile salts.

question

An unknown bacterium produces colorless colonies when inoculated onto an EMB plate. Predict what you would see if you inoculated that same unknown onto the following media, and why:

MacConkey Agar:

answer

question

An unknown bacterium produces colorless colonies when inoculated onto an EMB plate. Predict what you would see if you inoculated that same unknown onto the following media, and why:

C-CNA Agar:

answer

The unknown bacteria will have no growth because it is gram-negative while C-CNA agar is selective for gram-positive organisms.

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