CAN ORGANISM AL DIFFERENTIALS BE CHANGED? 585 



which apparently depended upon the individuality or species differentials of 

 two organisms. 



7. Somewhat comparable changes to those observed in protozoa have been 

 induced experimentally in various kinds of bacteria, and especially in 

 pneumococci. Through serial passages through animals belonging to a 

 susceptible species the virulence of bacteria can be raised. Similar changes 

 have been noticed in the case of viruses. Thus the effects of the virus of 

 poliomyelitis on mice and rats can be greatly increased by serial transfers to 

 mice ; but at the same time the virulence has become greater thereby also for 

 guinea pig and Rhesus monkey (Jungeblut and Sanders). In bacteria many 

 kinds of socalled "dissociation" have been observed and produced experi- 

 mentally; from apparently fixed bacterial forms, bacteria with different 

 characteristics have developed and these new types have remained constant. 

 Modifications of bacteria have also been produced under the influence of 

 bacteriophage or related substances. Increase in virulence for one host 

 species and decrease for another species may follow serial passage of 

 microorganisms or viruses through a certain species of animals. In the case 

 of viruses, this has followed cultivation on the chorio-allantois of chick 

 embryos. However, these modifications in the effects or reactions cannot 

 strictly be attributed to changes in the organismal differentials of bacteria 

 or viruses. 



Dawson observed, in 1919, that when bacterium coli was cultivated 

 through many generations in culture media, which differed from the usual ones 

 in their fat and protein content, definite peculiarities developed, which dis- 

 tinguished the strains thus produced from the original ones ; especially notice- 

 able was a specific change in the character of the antigens, which after injection 

 in rabbits called forth the production of immune agglutinins ; accordingly, the 

 character of the latter was also changed. More specific were the changes 

 which Burnet produced in a strain of B. melitensis. In this case, it was the 

 association with a heat-agglutinable paramelitensis strain which transmitted 

 to the melitensis strain characteristics similar to those of the paramelitensis 

 and modified the antigenic character of the B. melitensis. 



But the most striking results have been obtained with pneumococci. 

 Pneumococci were formerly classified into four types, which differed, above 

 all, in the character of the complex carbohydrates contained in their capsules. 

 More recently, Group IV has further been split into twenty-nine additional 

 types. In addition, it is possible to distinguish within at least some of the 

 different types between smooth (S) and rough (R) colonies. The bacteria 

 from smooth colonies possess their typical capsules and behave therefore in 

 a characteristic type-specific way. The pneumococci from rough colonies, on 

 the other hand, have lost their capsules, and with them their type specificity. 

 Furthermore, there exist within the cell-body proper of the pneumococci 

 proteins of a specific character. It can be shown that type specific S pneumo- 

 cocci can be transformed into R pneumococci by cultivating them in 

 homologous type-specific immune sera. According to Griffith, this transforma- 

 tion from the virulent S forms into the avirulent R forms may take place 



