Lab 14 Selective and Differential Media

Question 1

What is the purpose of a general growth media?

list some examples

Answer 1

• supports the growth of many different microbes

- ex; tsa(tryptic soy agar), nutrient broth

Question 2

What is the purpose of selective media?

Answer 2

-allows some microbes to grow while deliberately suppressing other microbes

Question 3

What is the purpose of differential medium?

Answer 3

-allow several different groups of microbes to grow with different appearances for each group

Question 4

Can some media be both selective and differential? if so give examples

Answer 4

Yes

• Mannitol salt agar (MSA)
• Eosin Methylene blue (EMB)

Question 5

Explain the functions of the components of mannitol salt agar:
NaCl
Beef extract
Mannitol
Phenol Red

Answer 5

NaCl: inhibits the growth of bacteria

Beef extract: building blocks for biological molecules, can be digested for energy

Mannitol (carb): energy source for many microbes

phenol red: ph indicator

Question 6

In a MSA medium, once the mannitol fermenter creates acid, it lowers the ph of the medium, causing an acidic environment. What color appears?

Answer 6

yellow

Question 7

In a MSA medium, once the non- mannitol fermenter deaminates amino acids in peptone, it increases the ph of the environment. What color appears?

Answer 7

pink; if slight deamination

red; strong deamination

Question 8

Explain the functions of the ingredients in EMB
Peptone
lactose
sucrose
K2PO4
Eosin y
methylene blue
agar

Answer 8

• lactose and sucrose are carbs that are fermented by the bacteria that create acids to react with the Eosin y and methylene blu
• eosin y and methylene blue inhibit the growth of gram + cells and allow gram - cells to grow
• peptone is partially hydrolyzed protein

Question 9

Eosin Methylene Blue (EMB) is considered as what type of media?

Answer 9

selective and differential

Question 10

In an EMB medium, strong fermenters indicate what colors?

Answer 10

metallic green, black,purple

Question 11

In an EMB medium, slow fermenters indicate what kind of color?

Answer 11

pink precipitate

Question 12

In an EMB medium, non fermenters indicate what kind of colors?

Answer 12

clear/normal

Question 13

Define enteric bacteria

Answer 13

• bacteria of the small and large intestines (very diverse)

- also found in fecal matter

Question 14

Define enteric pathogen

Answer 14

-diseases that are contracted when exposed to an enteric bacteria

Question 15

What is a coliform?

Answer 15

• indicator of fecal matter
• function definition: non sporulating gram negative rod or cocobacillus

aerobe or facultative anaerobe

grows in the presence of bile salts

ferments lactose to acid and gas within 48 hours at 37 degrees celsius

Question 16

What is an indicator organism?

Answer 16

• not always a pathogen
• abundant in fecal matter (human)
• easy to detect
  ex. E. coli > vigorous fermenters