564 THE CHOLERA GROUP 
ture being in three planes of space in the Hving vibrios. Freshly 
isolated vibrios have slightly but distinctly pointed ends which are 
best observed in stained specimens made directly from cholera dejecta. 
Cultures grown for some time on artificial media lose their original 
uniformity of size and shape and tend to become less curved, many 
indi%'iduals even appearing as straight rods. The passage of these old 
cultures through animals is said to restore their original morphology. 
Cultures in artificial media several days old frequently exhibit involu- 
tion forms which are irregularly swollen or even coccoid in outline. 
Bacillary forms and even true spirillum forms also are not uncommonly 
seen. 
Cholera vibrios are actively motile and they possess a single polar 
flagellum — monotrichic flagellation.^ Xo capsule has been demon- 
strated and no spores are produced, although involution forms which 
stain somewhat irregularly may suggest spores. 
The cholera organism stains with ordinary anilin dyes, although less 
readily than the majority of pathogenic bacteria. This is particularly 
the case in freshly isolated cultures. Older cultures are more uniform 
in this respect. The organism is invariably Gram-negative. 
Isolation and Culture.— Cholera vibrios grow rapidly upon all ordinary 
artificial media, even at 20° C. Their nutritional requirements with 
respect to nitrogenous substances are less exacting than those of 
many pathogenic and non-pathogenic bacteria commonly found in 
the intestinal tract. Also the true cholera organisms are tolerant of 
a degree of alkalinity which is unsuited for the development of ordinary 
bacteria. Advantage is taken of these nutritional peculiarities in 
isolating cholera vibrios from the dejecta of cholera patients. A 
small portion of fecal mucus is emulsified in slightly alkaline Dunham's 
solution' and incubated for six to eight hours at 37° C. The cholera 
organisms increase in numbers with great rapidity and they will be 
found at the surface of the medium in considerable concentration, 
for they are strongly aerobic. The isolation of them in pure culture 
by plating is readily accomplished if the material for inoculation is 
taken from the surface of such a peptone culture.^ 
Growth in Artificial Media. — Colonies of cholera vibrios which appear 
on agar plates after twelve to eighteen hours' incubation at 37° C. 
are round, very thin and transparent, and when viewed by transmitted 
light they are nearly colorless. Colonies of colon and other intestinal 
bacteria are usually yellowish-brown under the same conditions. 
The colonies of freshly isolated cholera vibrios are even more trans- 
parent than colonies of typhoid, paratyphoid, or dysentery bacilli. 
Older cultures do not exhibit this transparency to such a degree. 
Colonies on gelatin ])lates present a somewhat characteristic appear- 
1 Loffler: Centralbl. f. Bakteriol., 1889, 6, 209. 
- Dunham solution: Peptone 1 gram, NaCl 0.5 gram, potassium nitrate 0.25 gram, 
sodium carbonate (cryst.) 0.5 gram, water 100 cc. 
3 See Bacteriological diagnosis for details. 
