154 BACTERIA IN RELATION TO DISEASE 



upon animals. The following species, on the other hand, are extremely 

 difficult, and in some individual cases impossible to determine with cer- 

 tainty. 



6. THE COLON BACILLUS (Bactridium \Bacillui\ coli commune] is found 

 constantly, and in large numbers, in the human intestine and in the excreta. 

 It is in general nothing but a harmless commensal organism, but it may 

 under certain circumstances become pathogenic both in animals and in 

 man. The fact that a number of metatrophic bacteria very similar to 

 B. coli are known makes its recognition very difficult, and it is moreover 

 almost indistinguishable from the parasite of typhoid fever, Bactridium typhi 

 (Figs. 28 and 5). Endless pains have been taken to find reliable and 

 constant points of difference between these two bacteria, and yet even 

 now the differential diagnosis of the two species is a work of great 

 delicacy (137). The very circumstance that the colon bacillus grows more 

 quickly than the typhoid organism only adds to the difficulty, because in 

 the attempts to isolate the latter from tissues or from water it is over- 

 powered and choked by the rapidly-multiplying colon bacillus. 



Common to both species is the rod-like shape (Fig. 28, h and i). 

 The dimensions are about the same : B. typhi, 1-4 p. long by 06-08 /* thick, 

 B. coli, 1-3 ju, long, 04-06 /x thick ; i. e. B. typhi is rather the larger of the 

 two. Both are actively motile and peritrichously ciliated. The cilia are 

 too delicate for their number to be of determinative value. They do not 

 liquefy gelatine, and spores are unknown in both species. In the absence 

 of morphological differences, physiological peculiarities have been utilized. 

 Such are, on the part of B. coli, the fermentative power, the production of 

 gas, the acidulation and coagulation of milk, and the indol reaction when 

 growing in peptone solutions. All these properties are absent in the case 

 of the typhoid bacillus. Furthermore must be remembered the quicker 

 growth of B, coli, and its more humble requirements as to food-stuffs. As 

 was remarked on pp. 55, 56, this last peculiarity is deserving of special atten- 

 tion in differential diagnosis. It shows also that the B. coli, an ' ammonia 

 bacterium,' is a very simple metatrophic organism, as is indicated by its 

 frequent presence in dirty water. The typhoid organism, on the other 

 hand, is paratrophic, and in all probability never occurs in potable water, 

 except where introduced in the dejecta of typhoid patients. Such dejecta 

 carry with them, of course, plenty of nourishment for the bacteria. 



These few remarks will suffice to indicate in outline what we know at 

 the present time concerning the biology of the typhoid parasite. In typhus 

 abdominalis the bacilli can be demonstrated in all the abdominal organs 

 (spleen, liver, kidney, lymphatic glands, &c.), generally in little heaps 

 between the cells, and also in the blood. Infection takes place undoubtedly 

 from the intestine. 



A species allied to the typhoid bacillus is B. typhi miirium, that was 



