Question

What are the Selective Agents in the followings Micro Biology Tests

* Oxidase Test

*Catalase Test

*Methyl Red Test

*Voges-Proskaurer Test

*Sulfur Reduction Test

*Citrate Utilization Test

*Nitrate Reduction Test

*Motility Test

*Indole Production Test

Answer 1

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Name of test	Selective agent	Results
Oxidase test	Is used to identify the organism which produces enzyme cytochrome oxidase. Substrate tetramethyl-p-phenylenediamine is being oxidized into indophenol in the presence of cytochrome c oxidase.	Development of dark purple color on the disk containing the substrate shows the presence of an enzyme
Catalase test	Is used to determine whether the microbial cells produce Catalase enzyme or not. This enzyme (catalase) protects cells from the oxidative damage of hydrogen peroxide.	If the microorganism is Catalase positive then on the addition of hydrogen peroxide on cell there is the formation of air bubbles took place which indicates the breakdown of hydrogen peroxide into water and oxygen.
Methyl red test	Is used to determine whether the microorganism is able to perform mixed acid fermentation of glucose.MR broth media is used to grow the culture and methyl red is used as an indicator.	If organism shows the positive MR test results then glucose is being converted into acids which lower down the pH of media and on the addition of methyl red the color of media converted into the red as due to acidic pH. And if it is negative then the color of media remains yellow on the addition of methyl red.
Vogas-Proskauer test	The medium used: VP broth Reagents: 1. Alpha-naphthol, 5% color intensifier Alpha α-5g Absolute ethyl alcohol- 100 mL 2.Potassium Hydroxide, 40%, oxidizing agent Potassium hydroxide 40g Distilled water: 100 mL	Positive test results show the development of red color in the media after addition of reagents.
Sulfur Reduction Test	SIM agar media is used. Composition: Pancreatic digest of casein 20.0g, Peptic digest of animal tissue 6.1g, Agar 3.5g, Fe(NH4)2(SO4)2·6H2O 0.2g, Na2S2O3·5H2O 0.2g, pH 7.3 ± 0.2 at 25°C	If sulfur reduction took place then the appearance of dark precipitates took place in yellow media. In a negative test result, there is no appearance of dark precipitates in the media.
Citrate Utilization Test	This test is used to determine the capability of an organism to use the citrate as an energy source. Simmon’s citrate agar is used.per litre composition: Sodium Chloride       5.0 gm,Sodium Citrate (dehydrate)    2.0 gm,Ammonium Dihydrogen Phosphate     1.0 gm, Dipotassium Phosphate     1.0 gm, Magnesium Sulfate (heptahydrate)      0.2 gm Bromothymol Blue    0.08 gm, Agar 15.0 gm.Final pH 6.9 +/- 0.2 at 25 degrees C.	If the test result is positive then the color of media changes from the green to blue.
Nitrate Reduction Test	Used to detect the capability of an organism to reduce nitrate to nitrite. Nitrate broth is used having composition: Peptone 5 g/l, Meat extract 3 g/l, Potassium nitrate 1 g/l.Final pH 7.0 ± 0.2 at 25°C.Reagents: Nitrate Reagents A and B and zinc powder	If the test result is positive then there is a development of cherry red color on the addition of reagent A and B took place. In negative test result development of red color on the addition of Zn powder.
Motility Test	Used to detect the mobility of an organism. semisolid agar medium i.e. Sulfide Indole Motility (SIM) Medium. composition Pancreatic digest of casein 20.0g, Peptic digest of animal tissue 6.1g, Agar 3.5g, Fe(NH4)2(SO4)2·6H2O 0.2g, Na2S2O3·5H2O 0.2g, pH 7.3 ± 0.2 at 25°C A stab line is generated with inoculum loop and microorganism is allowed to grow along this line.	If growth is along with the stab line then it represents the non-motile nature of the microorganism. And if growth is diffusible, hazy and spread in a large area it confirms the motile nature of the organism.
Indole Production Test	Use to determine the ability of some bacteria to decompose the tryptophane to indole. Tryptophan broth is used. Reagents used are Indole Spot Reagent: p-Dimethylaminocinnamaldehyde (DMACA)   10.0 gm Hydrochloric Acid, 37%   100.0 ml Deionized Water 900.0 ml Indole Kovacs Reagent: p-Dimethylaminobenzaldehyde       50.0 gm Hydrochloric Acid, 37%   250.0 ml Amyl Alcohol     750.0 ml	If results are positive then there is the formation of a pink to red color in the reagent layers on top of the medium took place. In a negative test result, there is no color change.