AVERY, MACLEOD, MCCARTY 



of type derivation all "R" variants of 

 Pneumococcus are characterized by 

 the lack of capsule formation and the 

 consequent loss of both type specificity 

 and the capacity to produce infection 

 in the animal body. The designation 

 of these variants as R forms has been 

 used to refer merely to the fact that 

 on artificial media the colony surface 

 is "rough" in contrast to the smooth, 

 glistening surface of colonies of en- 

 capsulated S cells. 



The R strain referred to above as 

 R36A was derived by growing the parent 

 S culture of Pneumococcus Type II in 

 broth containing Type II antipneumo- 

 coccus rabbit serum for 36 serial passages 

 and isolating the variant thus induced. 

 The strain R36A has lost all the specific 

 and distinguishing characteristics of the 

 parent S organisms and consists only of 

 attenuated and non-encapsulated R vari- 

 ants. The change S -* R is often a re- 

 versible one provided the R cells are not 

 too far "degraded." The reversion of the 

 R form to its original specific type can 

 frequently be accomplished by successive 

 animal passages or by repeated serial sub- 

 culture in anti-R serum. When reversion 

 occurs under these conditions, however, 

 the R culture invariably reverts to the 

 encapsulated form of the same specific 

 type as that from which it was derived 

 (11). Strain R36A has become relatively 

 fixed in the R phase and has never spon- 

 taneously reverted to the Type II S form. 

 Moreover, repeated attempts to cause it 

 to revert under the conditions just men- 

 tioned have in all instances been unsuc- 

 cessful. 



The reversible conversion of S^R 

 within the limits of a single type is 

 quite different from the transforma- 

 tion of one specific type of Pneumo- 

 coccus into another specific type 

 through the R form. Transformation 

 of types has never been observed to 

 occur spontaneously and has been in- 

 duced experimentally only by the spe- 



177 



cial techniques outlined earlier in this 

 paper. Under these conditions, the 

 enzymatic synthesis of a chemically 

 and immunologically different cap- 

 sular polysaccharide is specifically 

 oriented and selectively determined by 

 the specific type of S cells used as 

 source of the transforming agent. 



In the course of the present study it 

 was noted that the stock culture of R36 

 on serial transfers in blood broth under- 

 goes spontaneous dissociation giving rise 

 to a number of other R variants which 

 can be distinguished one from another by 

 colony form. The significance of this in 

 the present instance lies in the fact that 

 of four different variants isolated from 

 the parent R culture only one (R36A) 

 is susceptible to the transforming action 

 of potent extracts, while the others fail 

 to respond and are wholly inactive in this 

 regard. The fact that differences exist in 

 the responsiveness of different R variants 

 to the same specific stimulus emphasizes 

 the care that must be exercised in the 

 selection of a suitable R variant for use 

 in experiments on transformation. The 

 capacity of this R strain (R36A) to re- 

 spond to a variety of different transform- 

 ing agents is shown by the readiness with 

 which it can be transformed to Types I, 

 III, VI, or XIV, as well as to its original 

 type (Type 11), to which, as pointed out, 

 it has never spontaneously reverted. 



Although the significance of the fol- 

 lowing fact will become apparent later 

 on, it must be mentioned here that pneu- 

 mococcal cells possess an enzyme capable 

 of destroying the activity of the trans- 

 forming principle. Indeed, this enzyme 

 has been found to be present and highly 

 active in the autolysates of a number of 

 different strains. The fact that this intra- 

 cellular enzyme is released during auto- 

 lysis may explain, in part at least, the 

 observation of Dawson and Sia (5) that 

 it is essential in bringing about transfor- 

 mation in the test tube to use a small 

 inoculum of young and actively growing 

 R cells. The irregularity^ of the results 

 and often the failure to induce transfer- 



