Answer to Question #174879 in Microbiology for qiiq

Question #174879

What is the principle of the dextrose broth test (a biochemical test)? is it to test the ability to utilize dextrose as carbon source? but can I have an in-depth elaboration/explanation about this test?

Expert's answer

The principle of dextrose broth test states that the action of organism on a carbohydrate substrate results in acidification of the medium, detected by a pH indicator dye. Carbohydrate fermentation is the process microorganisms use to produce energy.

the test used to test carbon sources is refered to as malonate test.. as follows;

An organism that simultaneously can utilize sodium malonate as its carbon source and ammonium sulfate as its nitrogen source produces an alkalinity due to the formation of sodium hydroxide. This results in an alkaline reaction which in a medium containing malonate, changes the indicator (bromothymol blue) from its original green color to light blue or Prussian blue. Organisms which cannot utilize malonate and ammonium sulfate and do not ferment dextrose produce no color change. Organisms which are malonate-negative but do ferment dextrose result in the development of a yellow color due to increased acidity in the medium.

In the lab, an inoculum from a pure culture is transferred aseptically to a sterile tube of malonate broth. The inoculated tube is incubated at 35-37 C for 24 hours and the results are determined. Abundant growth and a change from green to blue in the medium indicates a positive test for growth using malonate. Hence, if a microbe can use malonate for carbon and energy, it will grow on malonate broth. The use of malonate leads to a rise in pH of the medium, and a pH indicator changes color.

process of thedextrose broth test(a biochemichal test)

This is a differential medium. It tests an organism's ability to ferment the sugar glucose as well as its ability to convert the end product of glycolysis, pyruvic acid into gaseous byproducts. This is a test commonly used when trying to identify Gram-negative enteric bacteria, all of which are glucose fermenters but only some of which produce gas.

Like MSA, this medium also contains the pH indicator, phenol red. If an organism is capable of fermenting the sugar glucose, then acidic byproducts are formed and the pH indicator turns yellow. Escherichia coli is capable of fermenting glucose as are Proteus mirabilis (far right) and Shigella dysenteriae (far left).  Pseudomonas aeruginosa (center) is a nonfermenter.

The end product of glycolysis is pyruvate. Organisms that are capable of converting pyruvate to formic acid and formic acid to H2(g) and CO2 (g), via the action of the enzyme formic hydrogen lyase, emit gas. This gas is trapped in the Durham tube and appears as a bubble at the top of the tube. Escherichia coli and Proteus mirabilis (far right) are both gas producers. Notice that Shigella dysenteriae (far left) ferments glucose but does not produce gas.

Note - broth tubes can be made containing sugars other than glucose (e.g. lactose and mannitol). Because the same pH indicator (phenol red) is also used in these fermentation tubes, the same results are considered positive (e.g. a lactose broth tube that turns yellow after incubation has been inoculated with an organism that can ferment lactose).

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24.03.21, 15:13

but i have been told that a positive result is when the media turning turbid

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