386 THE ETIOLOGY OF TROPICAL DISEASES 



cultivation a large variety of involution forms. Unless the organism 

 is constantly being sub-cultured, it readily dies. Acid, even the '2 

 per cent, present in the gastric juice, readily kills it. Prolonged 

 drying, or heating to 55° C. for sixty minutes, or treatment with 

 weak chemicals has the same effect. The bacilH, however, have 

 comparatively high powers of resistance to cold. Unless examined 

 by the microscope in a fresh and young stage, it is difficult to 

 differentiate Koch's comma from many other curved bacilli. 



Its characters in culture are not always distinctive. Microscopic- 

 ally, the young colonies in gelatine appear as cream-coloured, irregu- 

 larly 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 process proceeds steadily, 

 at first a funnel-shaped liquefaction resulting, and then in the course 

 of a week or two all the gelatine may be reduced to fluid. On agar 

 Koch's comma bacillus produces within twenty-four hours a thick 

 greyish irregular growth. On potato, especially if slightly alkaline 

 and incubated at 37° C, an abundant brownish layer is formed. 

 Broth and peptone water are favourable media, and at 37° C. a general 

 turbidity occurs with the formation on the surface of a pellicle, 

 containing spirilla in active motility. In milk it rapidly multiplies, 

 curdling the medium, with production of acid. Unlike £. coli, it 

 does not form gas, but, like £. 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 a 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," due to the formation of nitroso-indol. Although the 

 bacillus readily loses virulence, and its resistance is little, it 

 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 out- 

 grown by the putrefactive bacteria. The same is true of sewage 

 water. 



The lower animals do not suffer from any disease exactly 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 

 the typhoid bacillus. 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 



