QUALITATIVE BACTERIOLOGICAL ANALYSIS. 237 



Agar-streak. A greyish-white growth, which is not char- 

 acteristic. 



Broth. A diffuse growth takes place and a thin pellicle 

 forms on the surface. 



Milk. The microbe grows well at 37 C., and produces coagu- 

 lation of the casein accompanied by an acid reaction of the medium. 



Potato. It does not grow on slightly acid potato at 22 C. ; 

 but at 37 C. it produces either a yellowish-brown or a non- 

 glistening white growth, which is hardly perceptible except 

 when the potato is held up to the light in a particular position. 

 On potato which has been rendered alkaline by soaking in a 2 

 per cent, solution of sodium carbonate it grows both at 22 C. 

 and at 37 C., producing a yellowish-brown growth. 



Peptone ( Witters) and Salt Solution. After twelve to twenty- 

 four hours incubation at 37 C., both indol and a nitrite are 

 formed, so that on the addition of a few drops of pure sulphuric 

 acid a red colour is produced. This is usually known as the 

 " cholera-red reaction." 



Microscopical Characters. The cholera spirillum is a small 

 curved organism resembling a comma ; usually only one curve 

 is seen, but sometimes two spirilla are attached together and 

 an S-shape is produced. It is actively motile and possesses a 

 single flagellum placed at one end. It does not form spores, 

 and involution forms are common, especially when the organism 

 has been kept in water ; it may then appear short and thick and 

 resemble a coccus. It is not stained by Gram's method. The 

 optimum temperature for its growth is between 35 and 38 C. ; 

 it grows, but more slowly, at a temperature as low as 17 C., 

 but under 16 C. no growth is visible. It is strongly aerobic. 



Reaction with Anti-cholera Serum. Pfeiffer suggested the 

 following test as a means of diagnosis : The blood serum of an 

 animal, which has been immunised to the highest degree possible 

 by several months treatment with cholera vibrios, is diluted 

 1-100 with ordinary nutrient bouillon. A loopful (2 mgm.) 

 of a twenty hours agar culture of the vibrio to be tested is 

 then added to 1 c.c. of the mixed serum and bouillon, and the 

 mixture injected into the peritoneal cavity of a guinea-pig 

 weighing 200 grammes ; every five minutes up to the end of 

 twenty minutes, when the experiment is ended, a portion of the 



