Nutrient Gelatin

For the plate count of organisms in water, this medium is replaced by solid media with agar.

Nutrient Gelatin was one of the first solidifying agents used at the beginning of bacteriology and was originally used in the standard method for water and wastewater as a direct plate count technique, replacing the dilution method. As this method required incubation at approximately 20 °C, it was not ideal for most organisms. Gelatin peptone and beef extract provide nitrogen, vitamins, minerals and amino acids essential for growth.

Preparation

Suspend 128 grams of the medium in one liter of distilled water. Mix well and dissolve by heating with frequent agitation. Boil for one minute until complete dissolution. Dispense into appropriate containers and sterilize in autoclave at 121 °C for 15 minutes.

Instructions for use

1. Inoculate the tubes by stabbing with a needle (straight wire) and incubate at 35 ± 2 °C for 7 days, or up to 15 days if required.
2. Refrigerate the test cultures together with an uninoculated Nutrient Gelatin control tube and read the reactions as soon as the control tube has hardened, by inverting the tube.
3. Detection of proteolysis: strong positive remains liquid.
4. If plates of Nutrient Gelatin are utilized, they can be streaked. Check for the hydrolysis of gelatin on the streaked plate by adding a drop of saturated ammonium sulfate or 20% sulfosalicylic acid to an isolated colony. Look for a zone of clearing around the colony (Stone reaction) after 10 minutes.
5. The Stone reaction is also used on Staphylococcus Medium No. 110 (LC-1032).

• Ewing Enterobacteriaceae USPHS Publication 734 Washington, 1960.
• Edwards and Ewing. Identification of Enterobacteriaceae, Burgess Publ. Co. Minneapolis, Minn., 1962.
• Standard Methods for the Examination of Water and Sewage, Ninth Edition APHA Inc. New York, 1960.