INFECTIONS OF THE DIGESTIVE TRACT 111 



The agglutinative characters of many pathogenic bac- 

 teria are remarkably fixed properties. Dr. Theobald 

 Smith attempted to induce variations in the agglutina- 

 tive characters of hog-cholera bacilli and colon bacilli 

 by treating a series of sensitive experimental animals 

 with the bacteria to be tested. He met with no success. 

 Dr. Smith tells me, however, that he thinks the chances 

 for success in modifying the agglutinative characters of 

 the bacteria would have been greater had he tried more 

 resistent animals than were used, since this would have 

 afforded more opportunity to excite antagonism on the 

 part of the microorganisms, and with this, some altera- 

 tion in agglutinative properties. It is stated that the 

 agglutinative characters of some streptococci are more 

 subject to variation than those of the B. coli group. 



METHODS OF INVESTIGATION 



It is desirable to speak briefly here of the methods 

 that have been employed in some of the investigations 

 with which this volume deals. Some of these methods 

 are well known, others are new. They relate in part 

 to the study of the morphological and cultural charac- 

 ters of the bacterial organisms found in the digestive 

 tract under different conditions, but they have to do 

 mainly with the products of the life activities of these 

 bacteria upon different nutrient media. 



Character of the Microscopical Fields. By far the 

 most helpful method of studying the microscopical 

 fields is with the aid of the Gram stain. The routine 



