174 INFECTIONS OF THE DIGESTIVE TRACT 



shown, there are- pathogenic bacteria intermediate in 

 type between the two. This further distinction of a 

 third variety is justifiable on grounds of convenience. 

 Indeed, Shiga 1 has lately adopted the classification of 

 Hiss, which recognizes four groups based on fermenta- 

 tive characters and has added to this a fifth, interme- 

 diate between the acid bacilli and the non-acid bacilli. 



Like the typhoid bacilli, the bacilli of dysentery are 

 hemiparasites ; that is, their invasive and destructive 

 properties are developed only when the bacteria have 

 been introduced into the organism in considerable num- 

 bers or have had an opportunity to multiply owing to 

 feeble powers of resistance on the part of the infected 

 individual. Like typhoid bacilli, the bacilli of dysentery 

 form substances the so-called "aggressins" -which 

 are capable of inhibiting or even completely paralyzing 

 the natural protective action of the body fluids and thus 

 removing the most important obstacle to the invasion 

 of the bacteria which up to the moment of this oppor- 

 tunity had led a saprophytic existence. It is not clear 

 whether these "aggressins" are identical with the soluble 

 products of the dysentery bacilli which one obtains on 

 filtering an old bouillon culture or are bodies wholly 

 or in part distinct. It is well established that guinea- 

 pigs may be actively immunized against the severest 

 intraperitoneal infection by means of two or more 

 injections of sterile guinea-pig exudate containing 

 "aggressins" derived from the peritoneal cavity of a suit- 



1 "Observations on the Epidemiology of Dysentery in Japan," 

 The Philippine Journ. of Sci., i, p. 485, 1906. 



