496 BACTERIOLOGY. 



A 1 per cent, solution of ammonium chloride and of gly- 

 cerin, according- to Fischer, likewise differentiates the colon 

 and typhoid bacilli. The former can live on nitrogen in the 

 form of ammonia, whereas the latter is unable to do so. 

 The typhoid bacillus may give a slight growth in solutions, 

 which contain nitrogen in the amido combination, but it is 

 especially dependent upon the nitrogen of protein matter 

 (pepton). 



Collodium Sacs. 



The usual method of increasing the virulence of an organ- 

 ism is by successive passage through animals as indicated on 

 p. 278. This, indeed, has been the only procedure until the 

 recent introduction of the collodium sac method by Metch- 

 nikoff, Roux and Salimbeni. In the hands of the French 

 investigators this method has yielded remarkable results, 

 notably in the study of cholera, pest, pleuro-pneumonia 

 and tuberculosis. 



The principle of the method consists in planting the 

 organism in a small collodium sac, which is then sealed 

 hermetically and placed within the peritoneal cavity of an 

 animal. Under these conditions diffusion takes place. The 

 bacterial products pass out, whereas the albuminous, highly 

 nutritive fluids of the living body pass into the sac. The 

 organism is thus enabled to grow in the fluids of the living 

 body unaffected, however, by phagocytes. As a result, it 

 grows luxuriantly and is markedly increased in virulence. 

 The growth is milky in appearance and the bacteria are 

 vastly more abundant than in ordinary liquid media. 



The method involves the most delicate bacteriological 

 technique, and, inasmuch as the details of the process have 

 not been heretofore published, the several steps will be 

 described as minutely as possible. 



The collodium on the market varies in composition according- to 

 the kind of nitro-celhilose employed. Hence every collodium cannot 



