Wednesday, 18 February 2015

Voges Proskauer and Citrate Utilization Test For Bacteria


About Test For Bacteria
Voges Proskauer Test:
This test determines the ability of microorganism to ferment glucose. The end products
of glucose metabolism,pyruvic acid, is further metabolized by using Butylene glucol pathway to produce neutral end such as acetoin and 2,3 butanediol. When Barrit's reagent A ( 40% KOH) and Barrit's reagent B (5% solution of alpha naphthol) is added it will detect the presence of acetoin, the precursor in the 2,3- butanediol synthesis. Acetoin in the presence of Oxygen and Barrit's reagent is oxidized to diacetyl, where alpha naphthol act as a catalyst. Diacetyl then reacts with guanidine components of peptone to produce a cherry red colour
Citrate Utilization Test:
About Test For Bacteria

This test determines the ability of microorganism to utilize Citrate. Some bacteria have the capability to convert the salts of organic acids, for example, Sodium citrate to alkaline carbonates. Sodium citrate is one of the important metabolite of Kreb's cycle. Certain bacteria use citrate as the sole carbon source. Citrate utilization requires a specific membrane transporter and citrate lyase activity. Citrate is converted to Oxalo acetic acid by citrate lyase and oxaloacetate decarboxylase activity will convert oxaloacetate to pyruvate with the release of carbondioxide. The other products of the reaction are acetate, Lactic acid, formic acid etc. The carbondioxide reacts with sodium and water to form sodium carbonat.
TThe triple sugar- iron agar test is designed to differentiate among the different groups or genera of the Enterobacteriaceae, which are all gram negative bacilli capable of fermenting glucose with the production of acid, and to distinguish them from other gram negative intestinal bacilli. This differentiation is based on the differences in carbohydrate fermentation patterns and hydrogen sulfide production by the various groups of intestinal organisms. Carbohydrate fermentation is detected by the presence of gas and a visible colour change (from red to yellow) of the pH indicator, phenol red. The production of hydrogen sulfide is indicated by the presence of a precipitate that blackens the medium in the butt of the tube. TSI Agar contains three fermentative sugars, lactose and sucrose in 1% concentrations and glucose in a concentration of 0.1%. Due to the building of acid during fermentation, the pH falls. The acid base indicator Phenol red is incorporated for detecting carbohydrate fermentation that is indicated by the change in colour of the medium from orange red to yellow in the presence of acids. In case of oxidative decarboxylation of peptone, alkaline products are built and the pH rises. This is indicated by the change in colour of the medium from orange red to deep red. Sodium thiosulfate and ferrous ammonium sulfate present in the medium detects the production of hydrogen sulfide (indicated by blackening in the butt of the tube). To facilitate the detection of organisms that only ferment glucose, the glucose concentration is one-tenth the concentration of lactose or sucrose. The small amount of acid produced in the slant of the tube during glucose fermentation oxidizes rapidly, causing the medium to remain orange red or revert to an alkaline pH. In contrast, the acid reaction (yellow) is maintained in the butt of the tube since it is under lower oxygen tension. After depletion of the limited glucose, organisms able to do so will begin to utilize the lactose or sucrose. To enhance the alkaline condition of the slant, free exchange of air must be permitted by closing the tube cap loosely. If the tube is tightly closed, an acid reaction (caused solely by glucose fermentation) will also involve the slant.

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