Lab Practical 1 #2

Question 1

Is B. Cereus gram positive or gram negative?

Answer 1

G-pos

Question 2

Is E. Coli gram positive or gram negative?

Answer 2

G-neg

Question 3

What is selective media?

Answer 3

• Selective media suppresses the growth of some organisms and allows for the growth of others.
• Interpreted with regard to the quality or amount of growth
• Must always be used with a positive control for growth, like TSA

Question 4

What is differential media?

Answer 4

• Contains reagents that change color or undergo a reaction secondary to growth or metabolism of the bacteria
• Not interpreted based on growth, but based on changes due to growth

Question 5

Do you evaluate selective or differential first?

Answer 5

Selective takes precedence over differential. If an organism does not grow we cannot make conclusions about if it ferments etc. Without growth there is no metabolism.

Question 6

MacConkey Agar

Answer 6

• Selective AND differential

- 1.5% bile salt, 0.1% crystal violet, lactose, pH indicator neutral red

Question 7

What does MAC select for?

Answer 7

Gram-negative bacteria. Bile salts and crystal violet inhibit the growth of Gram-positive bacteria.

Question 8

What does MAC differentiate for?

Answer 8

• If a Gram-negative organism is able to ferment lactose the colonies will produce acid, lowering the pH, causing a brighter pink, or pink haze.
• Gram-negative that cannot ferment lactose will have growth but no color

Question 9

Mannitol Salt Agar

Answer 9

• Selective AND differential

- 7.5% NaCl, mannitol, pH indicator phenol red

Question 10

What does MSA select for?

Answer 10

Halotolerant/halophylic (bacteria that can survive and thrive in a relatively high salt environment)

Question 11

In the clinical environment what is MSA used for?

Answer 11

• To select between Staphylococcus aureus and other species of staphylococcus
• It mimics the salt levels in human skin

Question 12

What does MSA differentiate for?

Answer 12

The fermenting of mannitol. If a bacteria is halotolerant/halophylic (salt loving) and can ferment mannitol it will be YELLOW. Fermenting mannitol lowers the pH, causing the phenol red to turn yellow.

Question 13

Phenylethyl Alcohol Agar

Answer 13

• Selective ONLY.

- 2.5% phenyl ethyl alcohol

Question 14

What does PEA select for?

Answer 14

The growth of gram-positive bacteria. It does not allow gram-negative bacteria to grow.

Question 15

What happens to KOH with

• Gram-positive cells
• Gram-negative cells

Answer 15

• G-pos: nothing

- G-neg: thick and gooey

Question 16

Why do Gram-negative cells get gooey with the KOH test?

Answer 16

KOH causes lysis of Gram-negative cells, so their proteins and other macromolecules denature and form a sticky liquid.

Question 17

What happens to Gram-positive cells with the KOH test?

Answer 17

Nothing, the liquid remains watery.

Question 18

What is denaturation?

Answer 18

A process in which proteins or nucleic acids lose the quaternary structure, tertiary structure and secondary structure which is present in their native state.

Question 19

What is Thioglycolate Medium?

Answer 19

• Differential

- Resazurin, sodium thioglycolate, 0.05% agar

Question 20

What does the resazurin in thioglycolate indicate?

Answer 20

Resazurin acts as an oxygen indicator. As oxygen diffuses into the top portion the resazurin turns lavender/pink, indicating that the upper portion of the north is aerobic. Bottom part of the tube is not pink aka anaerobic. It is often colorless after incubation (why you draw the wax pencil line).

Question 21

What does the agar in thioglycolate broth do?

Answer 21

Keeps the top and bottom layers from mixing, maintains the oxygen gradient.

Question 22

Obligate aerobe

Answer 22

Growth only in the aerobic zone. Require oxygen for their metabolism, cannot do fermentation or anaerobic respiration.

Question 23

Facultative anaerobe

Answer 23

Whole tube. Growth in both aerobic and anaerobic zones. Use oxygen when available and fermentation when not available. (Both B. and E.)

Question 24

What is the function of sodium thioglycolate?

Answer 24

It keeps the bottom of the tube anaerobic.

Question 25

Obligate aerobe

Answer 25

Only in top of tube (aerobic zone)

Question 26

Obligate anaerobe

Answer 26

Growth only in bottom of tube (anaerobic zone).

Question 27

Aerotolerant anaerobes

Answer 27

Whole tube. Use fermentation and can grow in the presence or absence of oxygen.

Question 28

What metabolic pathway do facultative anaerobes use?

Answer 28

Aerobic respiration and/or fermentation

Question 29

What metabolic pathway do aerotolerant anaerobes use?

Answer 29

Fermentation

Question 30

What metabolic pathway do obligate anaerobes use?

Answer 30

Fermentation

Question 31

What metabolic pathway do obligate aerobes use?

Answer 31

Aerobic respiration