The basic principle of this test is to detect the ability of an organism which can utilize citrate as sole source of carbon for their metabolism with resulting alkalinity. All coliforms metabolize citrate when molecule is generated inside the bacterial cell. But not all coliforms produce transport enzymes that bring citrate from the environment across the cytoplasmic membrane & into the cell.Normally, the citrate metabolism involves the condensation of acetyl with Coenzyme A resulting in the formation of oxaloacetate which enters the TCA cycle. 
    In the bacteria that utilize citrate, the cleavage of citrate involves an enzyme system without the intervention  of  Coenzyme A. This enzyme system is known as citrase or citrate permease. This enzyme system requires a divalent cation for its activity which supplied by the Magnesium+ ion or Mn+ ion. The  product obtained

 from the citrate metabolism depends on the pH of the medium. In alkaline medium, mostly, acetic acid & formic acid are produced. In acidic medium, acetoin & lactic acid are mostly produced. 
In the both cases carbon dioxide is produced which then combines with the sodium present in medium to sodium carbonate. This carbonate rises the pH of the medium thereby causing alkalinity.The organism capable of utilizing citrate as a sole carbon source can also utilize ammonium salts such as sole source of Nitrogen. When ammonium salts are used by microorganisms it is broken down to ammonia which also increases the pH. The medium used for this test is Simmons Citrate agar which has a pH indicator- bromothymol blue. The indicator is green at neutral pH & changes to deep Prussian blue at alkaline pH above 7.6.

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Cited By Kamal Singh Khadka 
Assistant Professor Of PU, RE-COST, PNC, LA, NA




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