Chapter 4: Microbial Growth - Culture Media

Question 1

What is a solid media? (2)

Answer 1

• Agar petri plates
• made by adding agar (solidifying agent) to liquid media

Question 2

Can agar be degraded by most bacteria?

Answer 2

• no
• only 2% of bacteria can grow in agar

Question 3

What does a solid media allow for? (3)

Answer 3

• growth of colonies
• a genetically identical population of cells
• allow the isolation of pure cultures

Question 4

Culture Media can be Chemically or unchemically _____.

Answer 4

• defined

Question 5

What is a chemically defined culture media? (3)

Answer 5

• the exact composition of the medium is known
• ex. media made from known qualities of salts and sugars
• also known as minimal media

Question 6

What is a chemically undefined culture media? (3)

Answer 6

• contains rich organic ingredients, so the chemical composition is not known
• ex. media containing yeast extract (all of the soluble components from crushed yeast cells)
• also known as complex media

Question 7

What is selective media? (3)

Answer 7

• prevents the growth of unwanted organisms, allowing only the desired microbes to grow
• ex. Bismuth sufite agar, used to culture Salmonella typhi
• inhibits the growth of all Gram positive and most Gram negative bacteria

Question 8

What is differential media?

Answer 8

• used to distinguish different bacteria
• all can grow, but colonies of certain bacteria look different on the plate
• ex. blood agar plates
• used to distinguish bacteria that can lyse (and eat) RBC
• ex. Streptococcus pyogenes

Question 9

What are two ways to count bacteria?

Answer 9

• direct count
• viable count

Question 10

What occurs in direct count? (3)

Answer 10

• cells are counted using a light microscope
• usually employs a special counting chamber
• Inaccurate because it counts both live and dead cells

Question 11

What occurs in viable count? (3)

Answer 11

• only live cells are counted
• A liquid culture is diluted and plated onto agar plates to grow colonies
• Each colony on a plate represents a single cell from the original culture
• colonies are counted and used to calc. number of bacteria in original culture

Question 12

What are the units for viable count?

Answer 12

• always expressed as cfu per ml
• colony forming units
• 1 cfu = 1 live bacterial cell