PURE CULTURE METHODS 115 
TaBLE VII 
SHOWING DEGREES OF ACIDITY EASILY RECOGNIZED IN CLEAR 
MEDIA 
(After Conn ct al., 1918) 
| ork 
Acidity. Indicator Reactions. a oximate 
| | ory Value. 
Neutral. ...... Blue or green to brom thymol blue. . ........... Over 6.2 
Weak......... Yellow to brom thymol blue. .. 0 ........... . ..| 3,.2-6.0 
Purple to brom cresol purple.* 
Moderate...... Yellow to brom eresol purple... ... 0.0.0.0. .0.000 4.6-5.0 
Orange to methyl red.t 
Maximum red to methyl red. 
Strong........ Blue or green to brom phenol blue.*.... 0... 0... 3.24.4 
Very strong....| Yellow to bromphenol blue ..................0.. Under 3.0 
' Use a 004 per cent aleoholic solution. 
t Use a 0.02 per cent alcoholic solution, 
Litmus Milk. Growth in this medium is not much different from 
that in plain milk. If the tube turns to a white with a pink layer 
at the surface which is in contact with air, the presence of a reductase 
for litmus is indicated. Clark and Lubs (1917) have stated that brom 
eresol purple may be used as a substitute for litmus in milk. The 
committee regards this as not always to be recommended since this 
dye docs not show the reduction phenomena. 
Indol Production. Indo! is formed from tryptophane and is pro- 
duced in media which contains proteins or their split products con- 
taining this amino acid. Under routine conditions the production of 
indol may be determined in Dunham’s medium or in the plain broth 
culture. Zipfel (1912) has shown that trytophane in an inorganic 
medium will show indol formation in twenty-four hours. Since tryp- 
tophane is so expensive Cannon (1916) has prepared a hydrolyzed 
casein medium with which it is claimed that good indol formation is 
secured. This is prepared according: to directions which are given in 
the Chapter on Media. The following tests for indol may be used: 
Nitroso-Indol Nitrate Test: Add 0.5 c.c. of concentrated sulphuric 
acid and 0.5 c.c. of dilute sodium nitrite to the culture. If a red color 
is obtained the presence of indol is indicated. 
Ehrlich’s Test: Add 0.5 c.c. of paradimethylamidobenzaldehyde 
to the culture and if indol is present, a deep crimson color will be 
formed. 
