40 MEDIA AND THEIR PREPARATION 
pared which embody the essential food substance. Some of the patho- 
genic bacteria grow better if a little blood serum or hemoglobin is 
added to our common media. The following substances are in common 
use by bacteriologists for preparing media. 
Peptone. This is added to common culture media to satisfy the 
organic nitrogen requirement of the bacteria. The peptones which 
are used in media are mixtures of the various cleavage products of 
the protein molecule. Important among these are the amino acids, 
which are precursors for many of the characteristic products of bac- 
terial activity. These must be hydrolyzed from the protein molecule. 
Many analyses of peptones are available in the literature, but as 
Banzhaf and Hirshleifer (1914) have pointed out, they may mean 
very little because in many cases the methods of analysis are not 
known. Generally, peptones consist of salts, among which are the 
phosphates, amino acids and proteoses. 
Witte’s peptone has formerly been specified for all bacteriological 
media. Berry (1914-1915) and Conn (1915) have found that equally 
good results are secured with the domestic peptones as with Witte’s. 
Berry secured her data by comparing colonies of B. typhi, B. colt and 
streptococcus on agar containing 2 per cent of the different peptones. 
Conn compared two samples of American-made peptone to Witte’s 
when he studied the methods of milk analysis in New York City. The 
three peptones showed such slight difference in average that Conn 
regarded one as good as the other. 
Meat Extract. This is an important constituent of laboratory 
media. For bacteriological purposes it is usually secured in jars. Meat 
infusion formerly was used in place of meat extract. 
This is made by soaking 500 gms. of lean chopped beef in 500 e.c. 
of distilled water for twenty-four hours. By keeping this in the refrig- 
erator all bacterial changes are inhibited. At the end of this time it is 
filtered through cheese cloth or flannel and brought back to the original 
volume by adding distilled water. This meat infusion is used as the 
base to which the other constituents are added. Its chemical composi- 
tion is not far different from that of the commercial meat extract. 
Gage and Adams (1904), however, have shown that the chemical com- 
position is variable. The reaction and the organic solids are variable. 
This variation is probably due to the different grades of meat which are 
used. High amounts of fat and connective tissue will tend to give a 
product weaker in strength. The kind of beef might also explain the 
variation in chemical content. The variation is illustrated by the fol- 
lowing data which are taken from Bigelow and Cook (1908). 
