MIDTERM 2 Hydrogen Sulfide Test Flashcards Preview

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Flashcards in MIDTERM 2 Hydrogen Sulfide Test Deck (22):

What can be said about an organism with the ability to produce hydrogen sulfide?

It helps identify facultative anaerobic gram-negative rods


True or false: aerobes can produce hydrogen sulfide too



Why does the H2S production test favor fermenters?

because it requires a low pH


Microbes can produce Hydrogen sulfide in 2 ways. what are they?

1. As end product of cystein or methionine metabolism. The -SH group is removed from the carbon backbone and is reduced to produce hydrogen sulfide
2. Through process of anaerobic respiration. sulfate or thiosulfate is used as an alternate electron acceptor


Why isnt odor used as a diagnostic feature of Hydrogen Sulfide?

Because inhaling the gas can be very toxic


Why cant you use the durham tube to test for Hydrogen sulfide Gas?

because the gas is VERY SOLUBLE in water


What is used for the detection of hydrogen sulfide?

Medium containing Fe2+ ions that will allow the Hydrogen sulfide to produce a black ferrous sulfide precipitate.


What media is used to test Hydrogen sulfide?

Kligler iron Agar


What does Kligler iron agar contain?

1. It contains peptone
2. and Thiosulfate
3. Glucose
4. Lactose
5. Phenol red


What is the significance of peptone in the KIA media?

it is rich in amino acids such as cysteine and methionine


What is the significance of Thiosulfate in the KIA media?

its an additional source of sulfur and an alternative terminal electron acceptor


Which sugar ( glucose or Lactose ) is found in higher quantity in the KIA media ?

Lactose is found in much higher quantity


explain the priority of bioorganic molecules a microorganism uses as a source of energy?

Weither they are Aerobic or Anaerobic, they use
1. Glucose first
2. peptone second ( if they can use lactose, theyll use it next)


If organism uses Glucose Aerobically, What happens to the KIA medium and why?

Theres little to no color change in the first 24 hours because the by-products of glucose catabolism do not significantly alter the pH


If organism uses Glucose via fermentation, What happens to the KIA medium and why?

The acid produced drops the pH and causes the phenol red to turn yellow. a yellow butt and a yellow slantwould appear after 24 hours.


How is there a yellow slant in the KIA medium if an organism uses glucose via fermentation

Upward diffusion of acid from the anaerobic butt


Explain what can happen once glucose is exhausted from the KIA medium

the organism will ferment lactose. Since lactose is in such a great amount, the tube will remain yellow at the 48 hour observation period.


explain what can happen once glucose is exhausted from the kia medium but the organism cant use lactose

Breakdown of the amino acids in peptone occurs
Ammonia is released as a by product
Ammonia shifts pH to a more alkaline value
This changes color of the phenol red to a reddish pink


What can be determined if the slant of the KIA is pinkish red?

organism is capable of aerobic peptone utilization only


What can be determined if both the slant and the butt of the KIA medium is red

Organism is capable of anaerobic peptone utilization


What can be said about the black precipitate of ferrous sulfide in the medium?

1. forms under acidic conditions
2. indicates fermentation is occurring even if it obscures butt and slant


How dos KIA medium turn black?

When hydrogen sulfide is produced because ferrous iron is present in the medium