94 DETERMINATION OF SPECIES. 
(6) Add, in order, phenol-sulphonic acid,’ water, 
and sodium hydrate. A yellow. color shows the 
presence of nitrates. 
(j) Acid or Alkali Production. —For the deter- 
mination of acid or alkali production, cultures are 
made on solid media to which have been added. 1 
per cent. of dextrose, saccharose, or lactose, and a 
sufficient quantity of litmus solution to produce a 
blue tinge. A pink coloration of the colony or a 
reddening of the surrounding medium indicates 
acid production. 
(k) Test for Indol Production.—For the deter. 
mination of indol production, incubate the species: 
to be tested in the dextrose-free bouillon described 
on page 56. Add to the tube of culture 1 c.c. of a 
0.0I-0.02 per cent. solution of sodium or potassium 
nitrite. T’o each 10 c.c. of medium add one drop 
of concentrated sulphuric acid. A red coloration 
indicates indol. If no result is obtained at once, it 
is well to allow the tube to stand for one hour. 
Pathogenesis.—In Chapter VIII. are described 
the various methods of determining the pathogenic 
properties of bacteria. 
1. When a given form grows only at or below 
18° to 20° C. inoculation should be made into the 
dorsal lymph-sac of a frog, using about 1 per cent. 
of the body-weight of a bouillon culture seven. 
days old. _ 
2. When a given species grows at 35” C. or up- 
ward, inoculation should be. made into the peri-. 
1 See Appendix, page 174. 
