AVERY, MACLEOD, MCCARTY 



mation m vitro. This they accom- 

 plished by growing R cells in a fluid 

 medium containing anti-R serum and 

 heat-killed encapsulated S cells. They 

 showed that in the test tube as in the 

 animal body transformation can be 

 selectively induced, depending on the 

 type specificity of the S cells used in 

 the reaction system. Later, Alloway 

 (6) was able to cause specific trans- 

 formation in vitro using sterile ex- 

 tracts of S cells from which all formed 

 elements and cellular debris had been 

 removed by Berkefeld filtration. He 

 thus showed that crude extracts con- 

 taining active transforming material in 

 soluble form are as effective in induc- 

 ing specific transformation as are the 

 intact cells from which the extracts 

 were prepared. 



Another example of transformation 

 which is analogous to the intercon- 

 vertibility of pneumococcal types lies 

 in the field of viruses. Berry and De- 

 drick (7) succeeded in changing the 

 virus of rabbit fibroma (Shope) into 

 that of infectious myxoma (Sanarelli). 

 These investigators inoculated rabbits 

 with a mixture of active fibroma virus 

 together with a suspension of heat-in- 

 activated myxoma virus and produced 

 in the animals the symptoms and path- 

 ological lesions characteristic of infec- 

 tious myxomatosis. On subsequent ani- 

 mal passage the transformed virus was 

 transmissible and induced myxomatous 

 infection typical of the naturally oc- 

 curring disease. Later Berry (8) was 

 successful in inducing the same trans- 

 formation using a heat-inactivated sus- 

 pension of washed elementary bodies 

 of myxoma virus. In the case of these 

 viruses the methods employed were 

 similar in principle to those used by 

 Griffith in the transformation of pneu- 

 mococcal types. These observations 

 have subsequently been confirmed by 

 other investigators (9). 



The present paper is concerned with 



175 



a more detailed analysis of the phe- 

 nomenon of transformation of specific 

 types of Pneumococcus. The major 

 interest has centered in attempts to 

 isolate the active principle from crude 

 bacterial extracts and to identify if 

 possible its chemical nature or at least 

 to characterize it sufficiently to place 

 it in a general group of known chem- 

 ical substances. For purposes of study, 

 the typical example of transformation 

 chosen as a working model was the one 

 with which we have had most experi- 

 ence and which consequently seemed 

 best suited for analysis. This particular 

 example represents the transformation 

 of a non-encapsulated R variant of 

 Pneumococcus Type II to Pneumo- 

 coccus Type III. 



EXPERIMENTAL 



Transformation of pneumococcal 

 types ill vitro requires that certain cul- 

 tural conditions be fulfilled before it is 

 possible to demonstrate the reaction 

 even in the presence of a potent ex- 

 tract. Not only must the broth medium 

 be optimal for growth but it must be 

 supplemented by the addition of serum 

 or serous fluid known to possess cer- 

 tain special properties. Moreover, the 

 R variant, as will be shown later, must 

 be in the reactive phase in which it has 

 the capacity to respond to the trans- 

 forming stimulus. For purposes of con- 

 venience these several components as 

 combined in the transforming test will 

 be referred to as the reaction system. 

 Each constituent of this system pre- 

 sented problems which required clar- 

 ification before it was possible to ob- 

 tain consistent and reproducible re- 

 sults. The various components of the 

 system will be described in the follow- 

 ing order: (1) nutrient broth, (2) 

 serum or serous fluid, (3) strain of R 

 Pneumococcus, and (4) extraction, 

 purification, and chemical nature of 

 the transforming principle. 



