Overview
This test enables us to distinguish between a group of bacteria that can grow with citrate as the only source of carbon and inorganic ammonium salts as the only source of nitrogen, which alkalizes the medium.

The medium to use is Simmons’ Citrate agar, which contains sodium citrate, ammonium phosphate and bromothymol blue as a pH indicator.

Only the bacteria that are capable of metabolizing the citrate will be able to multiply in this medium and they will release ammonium ions which, together with the citrate metabolism, will produce a strong basification of the culture medium, which will become apparent with the change in color from green to blue.

Carrying out the practical
The slant agar culture is inoculated on the surface by means of an inoculation loop.

Incubate at 37ºC for 24 hours.

After the incubation period the results are read. If the medium turns blue it will be positive citrate, otherwise it will be negative citrate.

Recently inoculated medium	citrate +         citrate –

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