588 BACTERIOLOGY. 
10. Then try indol reacton with the same tubes. 
11. While these tubes are incubating use material 
from the suspected colonies on the plates (1 and 5) for 
hanging drop cultures. 
12. Meanwhile make stained cover-glass prepara- 
tions from other colonies of suspected cholera on the 
plates (1 and 5). 
13. Make gelatin tube cultures from colonies on 
plates (1 and 5). 
14. Make gelatin tube cultures daily for five or six 
days, to study shape of growth along the line of punc- 
ture to preserve the culture. 
B. Suspected water. 
Add to 500 e.c. or 1 litre of the water to be exam- 
ined in a flask half-full enough peptone-salt solution 
(20 per cent. peptone and 10 per cent. NaCl) to make 
a1 per cent. solution of peptone. Then proceed as in A. 
PFEIFFER’s SERUM Reaction. All authors now 
agree that the differentiation of the cholera vibrio from 
other similar vibrios cannot always be made by the cul- 
tural method, nor is the usual inoculation of animals 
sufficient. For this purpose serum is employed either 
by making intraperitoneal injections of a surely fatal 
dose of the suspected spirillum along with the serum 
of animals immunized to undoubted cholera cultures, 
or by watching the action of the spirillum in the hang- 
ing drop when added to a dilution of the above men- 
tioned serum, so as to note whether immobilization 
and clumping occurred. 
