Agar plates

Question 0

what kind of agar is PEA?

Answer 0

selective

Question 1

what are the different agar types we are responsible for?

Answer 1

Phenylethly alcohol agar (PEA)
mannitol salt agar (MSA)
MacConkey Agar (MAC)
Eosin methylene blue agar (EMB)
Blood agar 
Tryptic soy agar (TSA)

Question 3

what does PEA select for?

Answer 3

inhibits the growth of gram -, encourages growth of gram +

Question 4

what does PEA agar contain?

Answer 4

• NaCl

- Phenylethyl alcohol

Question 5

how does the PEA agar select against gram - cells?

Answer 5

• by breaking down the membrane permeability barrier in the cells.
• this allows influx of substances usually blocked and leaking lare amounts of potassium
• this ultimately disrupts DNA synthesis

Question 6

what does MSA contain?

Answer 6

• Mannitol
• NaCl
• pH indicator (phenol red)

Question 7

what colors does the MSA pH indicator turn and what does it tell you?

Answer 7

• yellow (pH below 6.8)
• red (pH 7.4-8.4) –> no change
• pink color (pH 8.4 and above) –> no ferment.

tells you if fermentation occurred

Question 8

What color is the PEA agar?

Answer 8

Clear

Question 9

what kind of agar is MSA

Answer 9

differential, selective

Question 10

what organism does MSA select for?

Answer 10

staphylococcus aureus (bright yellow) 
- they also survive well in the salty conditions

Question 11

how does MSA select for fermenting organisms?

Answer 11

• mannitol is the substance for fermentation
• NaCl dehydrates and kills most bacteria
• staphylococci can ferment = produce acid = turns pH indicator yellow

Question 12

what kind of agar is MAC?

Answer 12

• differential

- selective

Question 13

what does MAC select for?

Answer 13

inhibits gram + cells , encourages growth of gram - cells

Question 14

what does MAC contain?

Answer 14

• lactose
• bile salt
• pH indicator (neutral red)
• crystal violet
• NaCl

Question 15

what are the pH indicator colors produced in MAC agar and what does it indicate?

Answer 15

colorless (pHabove 6.8)
red (pH lower than 6.8)

indicates fermentation

Question 16

what specific organisms does MAC select for?

Answer 16

• enterobacteria (ferment and turn pink)

Question 17

how does MAC select for gram - cells?

Answer 17

• bile saltsand crystal violet inhibits growth or gram + bacteria

Question 18

what kind or agar is eosin methylene blue agar?

Answer 18

differential, selective

Question 19

what does EMB select for?

Answer 19

inhibits gram+ cells

Question 20

what does EMB contain?

Answer 20

• methylene blue
• eosine y
• lactose
• sucrose

Question 21

what specific organism does EMB select for?

Answer 21

E. Coli (turns dark purble/black with metallic green sheen)

Question 22

what color is the EMB agar?

Answer 22

reddish-berry color

Question 23

how does the EMB agar select for gram - and E. coli?

Answer 23

the dyes are the main agent in the agar

• they inhibit growth of gram + organisms
• react with vigorous lactose fermenters and turn color

Question 24

what kind of agar is Blood agar?

Answer 24

enriched

differential

Question 25

how is blood agar differential?

Answer 25

based on hemolysis

• gama - no hemolysis
• beta - degrade RBC = clear agar
• alpha - degrade some components of RBC = greenish or brown agar

Question 26

why do we care about the results of a Blood agar plate?

Answer 26

if the organism is + for hemolysis it is probably pathogenic

Question 27

what does the blood agar contain?

Answer 27

• sheep blood in TSA agar

Question 28

what can grow on TSA agar?

Answer 28

pretty much anything

Question 29

What does the pink color change on EMB plate mean?

Answer 29

Ferments lactose with little acid production (possible coliform)