Bacterial Growth Media and Agar

Why is a colony considered a pure culture?

An isolated colony contains a single type of organism that originated from the same bacterium or fungus.

What types of media can be used to grow microorganisms?

Physical, chemical, and functional media.

What are the three physical forms of media?

Liquid, semisolid, and solid.

What is an agar plate used for?

Growing microorganisms and forming colonies.

What is the quadrant streak method used for?

Isolating individual colonies.

What is a pour plate?

A method used to grow and isolate microorganisms within and on agar.

What is a spread plate?

A method used to spread microorganisms across the surface of agar.

What are SIM tubes used for?

Testing certain bacterial characteristics, including motility.

What is liquid media?

A water-based solution called a broth.

What does turbidity in a broth indicate?

The density of bacterial growth in the culture.

What is semisolid media?

A clot-like medium containing a low percentage of agar.

What percentage of agar is usually found in semisolid media?

Approximately 0.4-0.7%.

What is semisolid media commonly used for?

Motility testing.

What does solid media allow bacteria to form?

Discrete colonies.

What percentage of agar is typically found in solid media?

Approximately 1.5%.

Is nutrient broth/nutrient agar complex or synthetic media?

Complex media.

What is complex media?

Nutrient-rich media containing at least one ingredient whose exact chemical composition is not fully defined.

What is enriched media?

Media with extra nutrients added to support fastidious organisms.

What is a fastidious organism?

An organism that requires special nutrients or conditions to grow.

What is blood agar used to grow?

Fastidious organisms such as Streptococcus.

What is chocolate agar used to grow?

Fastidious organisms such as Haemophilus.

What type of media is used to culture anaerobic organisms?

Reducing media.

What does reducing media contain?

A reducing agent.

What is an example of reducing media?

Fluid thioglycollate medium (FTM).

What is another example of media for anaerobic organisms?

Brewer's anaerobic agar.

What are two common products of bacterial sugar fermentation?

Acid and gas.

How is acid production detected during fermentation?

By a pH indicator that changes color.

How is gas production detected during fermentation?

By a Durham tube.

What does a Durham tube detect?

Gas produced during fermentation.

What does a pH indicator detect in carbohydrate fermentation?

Acid production through a color change.

What does a positive color change in fermentation media indicate?

The bacteria produced acid from fermentation.

What is mannitol salt agar (MSA) used for?

Isolating Staphylococcus species, especially Staphylococcus aureus.

What organism is commonly isolated using MSA?

Staphylococcus aureus.

What is the selective agent in MSA?

7.5% salt.

What organisms does MSA select for?

Salt-tolerant Staphylococcus species.

What property allows Staphylococcus to grow on MSA?

Its ability to tolerate high salt concentrations.

What is the differential agent in MSA?

Mannitol.

What does Staphylococcus aureus do to mannitol?

It ferments mannitol.

What happens when Staphylococcus aureus ferments mannitol on MSA?

The medium changes color due to acid production.

What are MacConkey agar and EMB agar commonly used to detect?

Gram-negative bacteria such as E. coli.

What type of bacteria do bile salts inhibit?

Most Gram-positive bacteria.

What type of bacteria are selected for by MacConkey agar?

Gram-negative bacteria.

Why can Gram-negative bacteria grow on MacConkey agar?

Bile salts inhibit most Gram-positive bacteria.

What dyes are found in EMB agar?

Eosin and methylene blue.

What type of bacteria does EMB agar generally select for?

Gram-negative bacteria.

What sugar is the differential agent in EMB agar?

Lactose.

What bacteria are differentiated on EMB agar?

Lactose-fermenting and non-lactose-fermenting bacteria.

What color are non-E. coli coliforms on EMB agar?

Red.

What appearance does E. coli have on EMB agar?

A metallic sheen.

What sugar is used to differentiate coliforms and noncoliforms?

Lactose.

What is a basic dye?

A positively charged (cationic) dye.

What charge does the chromophore of a basic dye have?

Positive.

Why do basic dyes stain bacteria well?

They bind to negatively charged molecules on bacterial cells.

What are examples of basic dye colors?

Blue and violet.

What is an acidic dye?

A negatively charged (anionic) dye.

What are examples of acidic dyes?

India ink and nigrosine.

Which type of dye is used in a positive stain?

Basic dye.

What does a positive stain color?

The microbial specimen.

Which type of dye is used in a negative stain?

Acidic dye.

What does a negative stain color?

The background.

Which dyes generally stain microbes directly?

Basic dyes.

What does incubated mean?

Allowed to grow under appropriate conditions.

What does inoculate mean?

To introduce microorganisms into a growth medium.

What are other names for the prokaryotic cell membrane?

Plasma membrane or cytoplasmic membrane.

What is the bacterial cytoplasmic membrane made of?

A phospholipid bilayer with proteins.

What is one function of the bacterial cell membrane?

Maintaining the cell boundary and integrity.

What is another function of the cell membrane?

Transporting materials.

Where does electron transport occur in bacteria?

On the cell membrane.

Why is electron transport important?

It is important for energy generation.

What type of barrier is the bacterial cell membrane?

A selectively permeable barrier.

Where is the bacterial cell wall located?

Outside the cell membrane.

What is a major function of the bacterial cell wall?

Maintaining cell shape.

How does the cell wall protect bacteria?

It helps prevent the cell from bursting when water enters.

What bacterial process is the cell wall involved in?

Cell division.

What is the major structural difference between Gram-positive and Gram-negative bacteria?

Gram-positive bacteria have thick peptidoglycan, while Gram-negative bacteria have thin peptidoglycan and an outer membrane.

What does Gram-positive bacteria have a thick layer of?

Peptidoglycan.

What acids are associated with Gram-positive cell walls?

Teichoic acid and lipoteichoic acid.

What color do Gram-positive bacteria appear after Gram staining?

Purple.

What is an example of a Gram-positive bacterium?

Staphylococcus aureus.

What additional structure do Gram-negative bacteria have?

An outer membrane.

What molecule is found in the outer membrane of Gram-negative bacteria?

Lipopolysaccharide (LPS).

What color do Gram-negative bacteria appear after Gram staining?

Pink.

Why do Gram-negative bacteria appear pink?

They lose the crystal violet during decolorization and take up the counterstain.

What is the main structural component of the bacterial cell wall?

Peptidoglycan.

What does NAG contribute to peptidoglycan?

The glycan chain.

What does NAM contribute to peptidoglycan?

The peptide crosslinks.

What does penicillin do to bacterial cell walls?

It prevents the formation of peptide crosslinks and weakens the cell wall.

What does lysozyme do to bacterial cell walls?

It hydrolyzes the glycan chains and weakens the cell wall.

Why are penicillin and lysozyme effective against bacteria?

They target structures in the bacterial cell wall.

What happens when a bacterial cell wall is destroyed?

The weakened cell can lose structural protection and may burst.

Why are Gram-negative bacteria often more resistant to disinfectants and drugs?

Their outer membrane provides additional chemical protection.

Which structure gives Gram-negative bacteria extra resistance?

The outer membrane.

What is LPS?

Lipopolysaccharide.

Which bacteria are associated with LPS?

Gram-negative bacteria.

Why is LPS called an endotoxin?

It is released when bacterial cells die and lyse.

What serious condition can endotoxin contribute to?

Sepsis.

Which genus is known for acid-fast bacteria?

Mycobacterium.

What other genus may be associated with acid-fast characteristics?

Nocardia.

What unique molecule is found in acid-fast bacterial cell walls?

Mycolic acid.

What is mycolic acid like?

A waxy, hydrophobic molecule.