Question

stion 24

A positive test result on Simmons Citrate Agar occurs when bacteria ____.

		produce base, causing the pH to increase.
		produce acid, causing the pH to increase.
		produce base, causing the pH to decrease.
		produce acid, causing the pH to decrease.

Answer 1

Ans.

Simmon Citrate agar ;

• It is an agar medium which differentiate enterobacteriacea based upon their utilization of citrate as a sole carbon source.
• It identify organisms capable of using citrate as their energy source.
• Sole source of nitrogen in this test is ammonium dihydrogen phosphate and diptassium phosphate is used as a buffer.
• To maintain the osmotic balance of the medium, sodium chloride is used and magnesium sulfate is used as a cofactor.
• Only microbes which can use ammonium dihydrogen phosphate and citrate, will grow without any restriction, on this medium.
• Utilization of citrate will result in accumulation of alkaline byproducts in microbes.
• An enzyme, citrate-permiase is produced by those bacterias which are capable of growing on this medium.
• This enzyme coverts citrate to pyruvate which can enter in the metabolic cycle of organism and produce energy.
• Utilization of citrate results in break down of ammoinum salts into ammonia resulting in an increase in alkalinity.
• This shift in pH is associated with a color change of bromophenol blue indicator from green to blue color above 7.6 pH.

Interpretation of result ;

• Positive result is indicated by the production of an alkaline reaction and a color change of medium from green to blue and shows that microbe used citrate for energy production.
• This positive result and production of base is associated with an increase in pH of the medium.
• Negative result is shown by no color change indicating that bacteria did not used citrate.