Chapter II: Materials

1. _Water._—Distilled water shall be used in the preparation of all culture media and reagents.

2. _Meat Extract._—Liebig’s meat extract shall be used in place of meat infusion. Other brands may be substituted for Liebig’s when comparative tests have shown that they give equivalent results.

3. _Peptone._—Armour’s, Digestive Ferments Company’s, Fairchild’s, or any other peptone which gives equivalent results may be used.

4. _Sugars._—All sugars used shall be of the highest purity obtainable.

5. _Agar._—The agar used shall be of the best quality and shall be dried for one-half hour at 105° C. before weighing. Much of the agar on the market contains considerable amounts of sea salts.[221][225][228] These may be removed by soaking in water and draining before use.

6. _Gelatin._—The gelatin used shall be of light color, shall contain not more than a trace of arsenic, copper, sulfides, and shall be free from preservatives, and of such a melting point that a 10 per cent. standard nutrient gelatin shall have a melting point of 25° C. or over. Gelatin shall be dried for one-half hour at 105° C. before weighing.

7. _Litmus._—Reagent litmus of highest purity (not litmus cubes) or azolitmin (Kahlbaum) shall be used for all media requiring a litmus indicator.

8. _General Chemicals._—Special effort shall be made to have all the other ingredients used for culture media chemically pure.