Saturated ammonium sulfate is a reagent used to determine the production of the enzyme gelatinase. The gelatinase test is performed by streaking the microbe being tested on an agar medium containing 3% gelatin and allowing the culture to grow overnight. About 5 ml of ammonium sulfate is then used to flood the plate, where it reacts with gelatin in the medium to produce a cloudy precipitate. When gelatin has been digested by the enzyme gelatinase, the region around a colony will be gelatin-free and will lack the cloudiness found elsewhere in the agar. This will result in a clear halo surrounding the colony for a positive test for gelatinase.
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