Koser’s Citrate Medium

Principle And Interpretation

Coliform bacteria serve as bacterial indicators of sanitary quality of food and water. These bacteria are normally found in the intestinal tract of humans and many warm-blooded animals (1). Coliforms encompasses mostly of Enterobacteriaceae from the genera Enterobacter, Klebsiella, Escherichia and Citrobacter. The characteristics of the members of Enterobacteriaceae are that they are gram-negative rods and ferment glucose to form acid along with gas production (2). Two important members of the Enterobacteriaceae family are Escherichia coli and Enterobacter aerogenes. Both can be differentiated on the basis of IMViC test. Enterobacter species are able to utilize sodium citrate as the sole carbon source while E.coli fail to do so. This property is used to differentiate the coli-aerogenes group (3). Koser Citrate Medium is used as a base for studying citrate utilization tests. This medium is recommended by APHA, and others, to presumptively identify coliforms encountered in the food and dairy industry (3-7). The various salts used serve as source of carbon and nitrogen to the organisms. Citric acid or its sodium salt is utilized as a sole source of carbon and ammonium salt as the sole source of nitrogen by E.aerogenes while E.coli does not utilize these salts and hence fail to grow. Koser Citrate Medium may be used in place of Simmon Citrate Agar (M099). Inoculating into Koser Citrate Medium further identifies coli-like colonies from Endo or EMB Agar plates. After 24-48 hours incubation, tubes showing marked turbidity may be assumed to contain organisms from aerogenes group and if the medium remains clear it may be considered as coli. Presumptive positive organisms identified on this medium should be further confirmed by performing the biochemical tests.