Lab Exam 2: Ex. 4-6

Question 1

What does HE agar stand for?

Answer 1

Hektoen Enteric Agar

Question 2

What type of medium is HE Agar?

Answer 2

complex (undefined), moderately selective, and differential medium.

Question 3

What is HE Agar selective for?

Answer 3

Gram-negative

Question 4

What is HE Agar differential for?

Answer 4

It differentiates between coliforms, which produce acid from lactose fermentation, and noncoliforms, and also identifies sulfur reducers.

Question 5

What is a coliform?

Answer 5

a type of bacteria that lives in the intestines.

Question 6

What is Hektoen Enteric agar designed to do? (typically used for?)

Answer 6

It is designed to isolate Salmonella and Shigella species from other Gram-negative enterics organisms in a patients stool sample (typically used to isolate members of the Enterobacteriaceae)

Question 7

What is the HE Agar test based on?

Answer 7

the ability to ferment lactose, sucrose, or salicin to acid end-products, and to reduce sulfur to hydrogen sulfide gas.

Question 8

What is the source of sulfur in HE Agar?

Answer 8

sodium thiosulfate

Question 9

What is the selective agent in HE Agar?

Answer 9

Bile salts inhibit most Gram-positive cocci

Question 10

What is the differential agent in HE Agar? What does it do?

Answer 10

ferric ammonium citrate - reacts with H2S (hydrogen sulfide gas) to form a black precipitate

Question 11

What are the dyes included in HE Agar and what do they do?

Answer 11

Bromothymol blue and acid fuchsin dyes are added to indicate pH changes.

Question 12

When using Hektoen Enteric Agar what does a poor growth or no growth result indicate?

Answer 12

The organism is inhibited by bile and/or one of the dyes included and is Gram-positive

Question 13

When using Hektoen Enteric Agar what does a good growth result indicate?

Answer 13

The organism is not inhibited by bile or any of the dyes included and is Gram-negative

Question 14

When using Hektoen Enteric Agar what does a pink to orange growth result indicate?

Answer 14

The organism produces acid from lactose, sucrose, and/or salicin fermentation and is NOT Shigella or Salmonella.

Question 15

When using Hektoen Enteric Agar what does a blue-green growth with black preicpitate result indicate?

Answer 15

The organism does not ferment lactose, sucrose, or salicin, but reduces sulfur to hydrogen sulfide (H2S) and is most likely Salmonella.

Question 16

When using Hektoen Enteric Agar what does a blue-green growth without black precipitate result indicate?

Answer 16

The organism does not ferment lactose, sucrose, or salicin, or reduce sulfur and is most likely Shigella (very rarely Salmonella)

Question 17

What microbes did we look at using HE Agar and what was the results?

Answer 17

Enterococcus faecalis - no growth
Escherichia coli - orange/yellow growth
Providencia stuartii - no growth