[Illustration: BACILLUS TYPHOSUS (SHOWING FLAGELLA)
[Illustration: BACILLUS TYPHOSUS (WIDAL REACTION)
(Agglutination by serum from typhoid patient)
_By permission of the Scientific Press, Limited_]
Its cultivation characters are not always distinctive. Microscopically the young colonies in gelatine appear as cream-coloured, irregularly round, and granular. Liquefaction sets in on the second day, producing a somewhat marked "pitting" of the medium, which soon becomes reduced to fluid. In the depth of gelatine the growth is very characteristic. An abundant, white, thick growth exactly follows the track of the needle, here and there often showing a break in continuity. Liquefaction sets in on the second day, and produces a distinctive "bubble" at the surface. The liquefied gelatine does not fall from the sides of the tube, as in the Finkler-Prior comma of _cholera nostras_, but occurs inside the border where the gelatine joins the glass. In the course of a week or two all the gelatine may be reduced to fluid. On agar Koch's comma produces with rapidity a thick, greyish, irregular growth. On potato, especially if slightly alkaline, an abundant brownish layer is formed. Broth and peptone water are excellent media. In milk it rapidly multiplies, curdling the medium, with production of acid. Unlike _Bacillus coli_, it does not form gas, but, like _B. coli_, it produces large quantities of indol and a reduction of nitrates to nitrites. Hence the indol test may be applied by simply adding to the peptone culture several drops of strong sulphuric acid, when in the course of several hours, if not at once, there will be produced a pink colour, the "cholera red reaction." Although it readily loses virulence, and its resistance is little, the comma bacillus retains its vitality for considerable periods in moist cultures, upon moist linen, or in moist soil. In cholera stools kept at ordinary room temperature the cholera bacillus will soon be outgrown by the putrefactive bacteria. The same is true of sewage water.
The lower animals do not suffer from any disease at all similar to Asiatic cholera, and hence it is impossible to fulfil the postulate of Koch dealing with animal inoculation. In this respect it is like typhoid. It is, however, provisionally accepted that Koch's bacillus is the cause of the disease. The four or five other bacteria which have from time to time been put forward as the cause of cholera have comparatively little evidence in their support. It is less from these, and more from several spirilla occurring in natural waters, that difficulties of diagnosis arise.