CATALYSIS: THE GUIDE OF LIFE 133 



definite chemical agents (e.g., LiCl 2 , FeCh, sodium taurocholate); 

 (3) by change in pH; (4) by animal passage, a method much used, 

 involving considerable, though unknown, changes in the culture 

 medium. For example, Alexander-Jackson 85 was able to change 

 two strains of human tubercle bacilli of R form over into S by 

 the addition of 0.0004 per cent of ferric chloride to Bordet-Gengou 

 medium. As a rule, the S form is relatively more virulent than 

 the R form of the same strain. At the laboratory of the New 

 York City Board of Health the virulence of test strains of pneumo- 

 cocci is maintained by daily passage through mice, to give an 

 example of the well-known effect of animal passage, which also 

 exerts changes in the virulence of viruses; e.g., the virulence of 

 smallpox to man is reduced by passage through cattle. 



On the other hand Herald R. Cox 86 found that the virulence of a 

 Dermacentor variabilis strain of Rocky Mountain spotted fever was 

 enhanced, as against guinea pigs, upon growth in the yolk sacs of 

 developing chick embryos. The virulence decreased after about 50 

 passages, and after about 100 passages the strain, while causing slight 

 or no reaction in the guinea pigs, made them "solidly immune to 

 massive doses of highly virulent strains." See, also, R. E. Green 77 

 on the nature of virus adaptations. 



Griffith, 87 working with white mice, discovered the interesting fact 

 that S forms of pneumococci can be transformed from one specific type 

 into other specific types through the intermediate stage of the R form. 

 Since the specificity of the types involves the formation of specific sub- 

 stances, (especially, as Heidelberger 88 has shown specific carbohy- 

 drates), it seems obvious that the catalysts of the pneumococci in 

 Griffith's experiments must have undergone a change; either old 

 catalysts were transformed, or new ones were formed. 



Dawson and Sia, 89 working in vitro, found that R forms of pneu- 

 mococci derived from S forms of one specific type (Type II), can be 

 transformed into the S forms of other specific types (Types I or III), 

 by growth of small inocula of the R form of Type II in media con- 

 taining vaccines prepared from heterologous S cultures. It seems 

 possible that in this case the R form contains an enzymic carrier which 

 has been deprived of its prosthetic group, but which takes up a dif- 

 ferent prosthetic group from the new vaccine-containing medium. 

 That these R forms are more labile is brought out by Sia and Daw- 

 son, 90 who state: "R cultures possessing only slight degree of R stability 

 are most suitable for transformation purposes by in vitro procedures." 



Drs. Oswald T. Avery, Maclyn McCarty and Colin MacLeod, of the 

 Rockefeller Inst, for Medical Research, found that the substance indue- 



