190 MICROSCOPIC AND CULTURAL STUDY OF BACTERIA 



through several layers of cheese-cloth, the volume recorded, then 

 heated to boiling. The coagulum which forms is removed by filtration 

 through filter paper and the clear, amber-colored fluid, after restoring 

 the loss due to evaporation, is run into flasks and sterilized in an 

 autoclave at 15 pounds' pressure for fifteen minutes. This plain meat 

 infusion contains but little protein; it is relatively rich, however, in 

 soluble meat extractives, soluble salts and muscle-sugar dextrose. 

 It is not suitable in itself as a complete nutritive medium for most 

 bacteria, but it forms the basis of many of the commonly used nutri- 

 tive media. Meat extract (Liebig's or other kinds) is frequently 

 substituted for meat infusion. Three grams of meat extract are dis- 

 solved in a small volume of water, filtered through a cold wet filter 

 paper to remove fat, arid made up to a volume of a liter. The solution 

 contains some meat extractives, including a relatively large propor- 

 tion of xanthin bases and a very considerable amount of salts, par- 

 ticularly sodium chloride. Little or no muscle-sugar is present. It 

 is distinctly inferior to meat infusion, however, as a basis for cultural 

 media, especially for the more delicate pathogenic organisms. 



The Reaction of Media. Bacteria are relatively sensitive to com- 

 paratively slight changes in the reaction of their nutritional environ- 

 ment, and it is essential to create a suitable initial degree of acidity 

 or alkalinity in media to favor their growth. A reaction neutral to 

 phenolphthalein slightly alkaline to litmus is suitable for most of 

 the bacteria pathogenic for man human tissues and blood are slightly 

 alkaline to litmus. A reaction of 1 per cent, acid (+1.0), using phenol- 

 phthalein as an indicator, has been recommended by the Laboratory 

 Section of the American Public Health Association for the routine 

 bacterial examination of water, ice, sewage, milk, cream, and ice-cream. 

 A reaction of 1 per cent, signifies that 1 c.c. of normal NaOH would 

 be required to neutralize the acid in 100 c.c. of the medium. Ten 

 c.c. of Y NaOH would be required to exactly neutralize one liter of 

 medium having an acidity of 1 per cent. 



The reaction may be determined accurately in the following manner : 

 to 45 c.c. of distilled water, contained in a porcelain evaporating dish 

 of 100 c.c. capacity, are delivered exactly 5 c.c. of the medium from a 

 graduated pipette. The solution is brought to the boiling-point over 

 the free flame to expel CO 2 and 1 c.c. of a solution of phenolphthalein 1 



1 Made by dissolving 0.5 gram phenolphthalein in 100 c.c. 50 per cent, alcohol. This 

 indicator is colorless in acid solution pink in an alkaline solution. CO2 interferes with 

 its accuracy as an indicator. It is especially sensitive to organic acids which occur in 

 ordinary media, hence its value in media titrations. 



