عدد المساهمات : 99
تاريخ التسجيل : 26/03/2008
العمر : 33
|موضوع: BIOCHEMICAL TESTS الأحد مايو 04, 2008 12:12 am|| |
Some bacteria and macrophages can reduce diatomic oxygen to hydrogen peroxide or superoxide. Both of these molecules are toxic to bacteria. Some bacteria, however, possess a defense mechanism which can minimize the harm done by the two compounds. These resistant bacteria use two enzymes to catalyze the conversion of hydrogen peroxide and superoxide back into diatomic oxygen and water. One of these enzymes is catalase and its presence can be detected by a simple test. The catalase test involves adding hydrogen peroxide to a culture sample or agar slant. If the bacteria in question produce catalase, they will convert the hydrogen peroxide and oxygen gas will be evolved. The evolution of gas causes bubbles to form and is indicative of a positive test.
The citrate test is used to determine the ability of a bacterium to utilize citrate as its only source of carbon. Bacteria can break the conjugate base salt of citrate into organic acids and carbon dioxide. The carbon dioxide can combine with the sodium from the conjugate base salt to form a basic compound, sodium carbonate. A pH indicator in the medium detects the presence of this compound by turning blue (a positive test).
Like the mannitol salts agar, the coagulase test is another method for differienting between pathogenic and non-pathogenic strains of Staphylococcus. Bacteria that produce coagulase use it as a defense mechanism by clotting the areas of plasma around them, thereby enabling themselves to resist phagocytosis by the host's immune system. The sample in question is usually inoculated onto 0.5 ml of rabbit plasma and incubated at 37 degrees celsius for one to four hours. A positive test is denoted by a clot formation in the test tube after the allotted time.
Indole is a component of the amino acid tryptophan. Some bacteria have the ability to break down tryptophan for nutritional needs using the enzyme tryptophanase. When tryptophan is broken down, the presence of indole can be detected through the use of Kovacs' reagent. Kovac's reagent, which is yellow, reacts with indole and produces a red color on the surface of the test tube.
The optochin test is a presumptive test that is used to identify strains of Streptococcus pneumoniae. Optochin (ethyl hydrocupreine) disks are placed on inoculated blood agar plates. Because S. pneumoniae is not optochin resistant, a zone of inhibition will develop around the disk where the bacteria have been lysed. This zone is typically 14mm from the disk or greater.
Cytochrome oxidase is an enzyme found in some bacteria that transfers electrons to oxygen, the final electron acceptor in some electron transport chains. Thus, the enzyme oxidizes reduced cytochrome c to make this transfer of energy. Presence of cytochrome oxidase can be detected through the use of an Oxidase Disk which acts as an electron donator to cytochrome oxidase. If the bacteria oxidize the disk (remove electrons) the disk will turn purple, indicating a positive test. No color change indicates a negative test.
Urease is an enzyme that breaks the carbon-nitrogen bond of amides to form carbon dioxide, ammonia, and water. Members of genus Proteus are known to produce urease. Urease can be detected by plating bacteria onto an amide containing medium, specifically urea. When urea is broken down, ammonia is released and the pH of the medium increases (becomes more basic). This pH change is detected by a pH indicator that turns pink in a basic environment. A pink medium indicates a positive test for urease
|د. عبدالرؤوف المناعمة|
عدد المساهمات : 146
تاريخ التسجيل : 21/03/2008
|موضوع: رد: BIOCHEMICAL TESTS الأحد مايو 04, 2008 10:54 am|| |
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|bassem abu alrub|
عدد المساهمات : 103
تاريخ التسجيل : 24/03/2008
|موضوع: رد: BIOCHEMICAL TESTS الإثنين مايو 05, 2008 3:27 pm|| |