Question

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1. Why do we use sugar as a component when performing triple Sugar Iron Agar tests?
2. How do we know that the bacteria produced gasses as a result of metabolizing sugar?
3. What is meant by a non-fermenter?
4. Why can some organisms live with or without oxygen
5. Why is a non-metallic instrument used in the Oxidase and O-F Glucose Tests experiment?

Answer 1

1. Triple sugar iron agar (TSI) is a differential medium that is made up of lactose, sucrose, a small amount of dextrose, ferrous sulfate, and the pH indicator phenol red. It is used to differentiate enterics based on the ability to reduce sulfur and ferment carbohydrates. Fermentation of available sugars is an important part of the TSI test.Sugars are readily used up by bacteria for energy.If the organism can ferment all sugars in the media,it will turn yellow. If an organism can only ferment lactose(i.e;lactose becomes lactic acid), the small amount of lactose in the medium will be used up by  the organism within say,the first ten hours of incubation. After that time, the reaction that produced acid stops in the aerobic areas of the culture, and the medium in those areas turns red, indicating alkaline conditions.Therefore depending on the ability of the bacteria to breakdown complex sugars alone,they can be easily identified or classified.This is why sugars are used in TSI medium.

2.The presence of bubbles in the medium indicate that the bacteria has produced gasses as a result of metabolization of sugars.

3.Non-fermenters are bacteria that cannot break-down or catabolize glucose.This implies that they cannot glucose in a media.

4.Some organisms can use both aerobic and anaerobic respiration for survival.In the presence of oxygen,they use aerobic respiration to create ATP. When oxygen is not present,they use anaerobic mechanism of respiration in which other electron acceptors like sulfur replace the function of oxygen.

5.Oxidase tests use a reagent that changes color when it is oxidized by enzymes like cytochrome C oxidase.Only aerobic and facultative anaerobic microbes have this enzyme and hence this test can be used to identify the presence of these organisms.If a metallic instrument is used in these tests,it may oxidize the reagent giving false positive results.This is why only metallic instruments are used.