Lab #18: Selective, Differential, and Enriched Media

Define:

Selective Media

Contains ingredients that inhibit the growth of some bacteria, while allowing growth of other bacteria

How is selective media useful for microbiologists?

Useful for isolating bacteria from highly mixed contaminated specimens

Define:

Differential Media

Contains ingredients to distinguish one organism from another

How is differential media useful for microbiologists?

Useful to characterize and identify media

Define:

Enriched media

Contains extra nutrients, growth stimulants or growth factors

How is enriched media useful for microbiologists?

Useful to enable fastidious organisms to grow

What type of media/s is Mannitol Salt Agar? Why?

• Selective- Sodium chloride (NaCl) makes it so that its concentration is high enough to dehydrate and kill most bacteria

• Differential- Mannitol provides the substance for fermentation + phenol red

What bacterial species tolerate the high salt?

Staphylococci

Fill in the blank:

Organisms fermenting mannitol produce _____ and pH indicator turns ____

• Acid

• Yellow

What color is mannitol not fermented?

pink/red (no color change)

T/F: Staphylococcus aureus ferments mannitol

True

What is the observation and Conclusion of this agar plate?

1. Growth- Tolerates salt

2. Yellow- Mannitol fermented

What type of media/s is MacConkey Agar? Why?

• Selective- Contains bile salts and crystal violet: these inhibit growth of gram positive bacteria

• Differential- Contains lactose and neural red: lactose fermentation bacteria produce acid

The ph indicator turns red at ____

ph less than 6.8

What is the observation and Conclusion of this agar plate? (left vs right)

• Growth: Gram negative, pink: fermented lactose

• Growth: Gram negative, colorless: no fermentation

McConkey plate is useful in identifying ______

enterics

What type of media/s is Blood Agar? Why

• Differential- Useful for identifying which bacteria hemolyze RBC’s

• Enriched- Serum + RBC’s provide additional nutrients & growth factors

Fill in the blanks:

1. Alpha hemolysis is a _____ lysis and is ____ around the colony

2. Beta hemolysis is a _______ lysis and is ____ around the colony

3. Gamma (No hemolysis) has no/yes change in medium

1. incomplete, green

2. complete, clear

3. no

Review:

1. What Media has 7.5% NaCl and has Mannitol and phenol red

2. What media has Bile slats + Crystal violet and lactose + neutral red

3. What media has TSA and sheep blood

1. Mannitol Agar

2. MacConkey agar

3. Blood Agar