182 
PRINCIPLES OF BACTERIOLOGY 
The bacilli are usually transmitted by rats and ground 
squirrels. One attack of plague usually confers im¬ 
munity. Both the vaccine and the serum treatments 
have been used for prevention with encouraging results. 
The Cholera Group 
Cholera, which at times becomes epidemic and invades 
large territories, is but little known in this country. It 
Fig. 39.—Spirillum cholera. Film preparation of a pure culture, x 1500. 
(Hewlett —Manual of Bacteriology.) 
is a very dangerous disease, characterized by persistent 
diarrhea and collapse. The “comma bacillus/’ so-called 
because of its shape, or the spirillum cholera was dis¬ 
covered by Koch in 1883. It is a small curved rod, ac¬ 
tively motile, has one flagellum, no spores or capsules, 
is Gram-negative and grows readily on meat extract 
media. It is an aerobic organism, and can be quickly 
killed by ordinary antiseptics. The disease is contracted 
by eating contaminated food or drinking impure water. 
It is essentially a “water-borne” disease, as has been 
