CHRISTENSEN CITRATE SULPHITE AGAR

Description

Christensen Citrate Sulphite Agar is a modification of the Christensen Iron Agar. This modification was described by Edwards and Ewing. Christensen reported that all members of genera Escherichia, Enterobacter, Citrobacter and Salmonella as well as Alkalescens-Dispar were capable of utilizing citrate as a source of energy while Shigella species failed to utilize citrate. Organisms that metabolize citrate as a sole source of carbon cleave citrate to oxaloacetate and acetate via the citritase enzyme. Another enzyme, oxaloacetate decarboxylase, then converts oxaloacetate to pyruvate and CO2. Further, this CO2combines with sodium and water to form sodium carbonate, an alkaline compound. As a result, the pH of medium rises and the indicator, phenol red changes from orange red to cerise. Presence of the cerise colour indicates a positive finding for citrate utilization. Care should be taken while inoculating, as, a too heavy inoculum may give a false positive result. The reduction of ferric ammonium citrate to iron sulphide by H2S producing organisms is indicated by blackening of the medium. Sodium thiosulphate enhances H2S production. Strong positive cultures upon prolonged incubation turn the entire butt black. Some members of Salmonella like Salmonella Typhi are weakly positive and require 2-5 days for hydrogen sulphite production.