Broth may be prepared from Liebig’s or Armour’s meat extract by adding 5 grams of either, 10 grams peptone and 5 grams NaCl to 1000 cc of water, boiling to dissolve, then titrating and filtering as above.
The author after much experience finds meat juice preferable to meat extract for broth and other media for pathogenic bacteria, and has abandoned the use of meat extracts for these organisms.
Glycerin Broth.—Glycerin broth is made by adding 4 to 6 per cent. of glycerin to the broth just previous to the sterilization. The glycerin serves as a source of carbon to certain bacteria which will not grow on the ordinary broth—as Mycobacterium tuberculosis.
Sugar Broths.—Sugar broths are used for determining the action of bacteria on these carbohydrates, since this is a valuable means of differentiating certain forms, especially those from the intestinal tract. Broth free from sugar must first be made. This is done by adding to broth prepared as already described, just previous to final filtering and sterilization, a culture of some sugar-destroying organism (Bacterium coli is ordinarily used), and then allowing the organism to grow in the raw broth at body temperature for twenty-four hours. Any carbohydrate in the broth is destroyed by the Bacterium coli. This mixture is then boiled to kill the Bacterium coli, restandardized and then 1 per cent. by weight of required sugar is added. Dextrose, saccharose and lactose are the most used, though many others are used for special purposes. After the sugar is added the medium must be sterilized by discontinuous heating at 100° for three or four successive days, because long boiling or heating in the autoclave splits up the di- and polysaccharids into simpler sugars and may even convert the simple sugars (dextrose) into acid.
Various other modified broths are frequently used for special purposes but need not be discussed here.
Dunham’s peptone solution, frequently used to determine indol production, is a solution of 1 per cent. of peptone and 0.5 per cent. of salt in tap water. It does not need to be titrated, but should be boiled and filtered into tubes or flasks and sterilized.
Nitrate Broth.—Nitrate broth for determining nitrate reduction is 1 per cent. of peptone, 0.2 per cent. of C. P. potassium nitrate dissolved in distilled water and sterilized.
Milk.—Milk is a natural culture medium much used. It should be fresh and thoroughly skimmed, best by a separator or centrifuge to get rid of the fat. If the milk is not fresh, it should be titrated as for broth and the reaction adjusted. The milk should be sterilized discontinuously to avoid splitting up the lactose as well as action on the casein and calcium phosphate.
Litmus Milk.—Litmus milk is milk as above to which litmus has been added as an acid production indicator. The milk should show blue when the litmus is added or be made to by the addition of normal NaOH solution. It should be sterilized discontinuously. Frequently on heating litmus milk the blue color disappears due to a reduction of the litmus. This blue color will reappear on shaking with air or on standing several days, due to absorption of O and oxidation of the reduced litmus, provided the heating has produced no other change in the milk, as proper heating will not.
Gelatin Culture Medium.—Gelatin to the extent of 10 to 15 per cent. is frequently added to broth and gives a culture medium of many advantages. It is solid at temperatures up to about 25° and fluid above this temperature, a property which is of great advantage in the isolation of bacteria. (See [Chapter XVIII].) Further gelatin is liquefied (that is digested, converted into gelatin proteose and gelatin peptone, which are soluble in water and do not gelatinize) by many bacteria and not by others, a valuable diagnostic feature. The gelatin colonies of many bacteria are very characteristic in appearance, as is the growth of many on gelatin in culture tubes.