570 A THEORY OF MICROBIC VIRULENCE 



5. The virulence, P.D., agglutinability, etc., showed parallel differences for 

 "Rough" and "Smooth" strains of pneumococci/ 



Experiments with cultures of the colon-typhoid-paratyphoid-enteritidis group, 

 hemolytic and green-producing streptococci, bacteria pathogenic for plants,^ etc., 

 have given similar parallelisms. 



In all cases referred to, the parallelisms between P.D. and virulence obtain within 

 the genus, species, or variety. There is no evidence that P.D., treated as an inde- 

 pendent characteristic, is qualitatively or quantitatively different on all organisms 

 which are virulent to a particular host from all organisms which are not virulent for 

 that host. Hence, it is not permissible to assert that P.D. parallels differences in 

 virulence among different micro-organisms, but only that such parallelisms are demon- 

 strable within the grotips. 



In 1918 Baerthlein^ reported on the association between variations in colony form 

 and morphological, biochemical, and serological characteristics of bacteria. His ob- 

 servations were first fully appreciated and extended by Arkwright'' and by DeKruif,^ 

 and have led to extensive investigations on "Smooth" and "Rough" strains, so called 

 after the smooth and rough appearance of colonies on agar media.'' It has come to be 

 recognized that there are almost invariable parallelisms for "S" and "R" strains be- 

 tween variations in virulence, agglutinability, and in other characteristics. Thus, for 

 example, a typical "S" strain is relatively highly virulent, has so-called "normal" 

 morphology, is motile, encapsulated, and is relatively inagglutinable ; a typical "R" 

 strain is slightly or not at all virulent, shows a large proportion of "involution" forms, 

 is non-motile, is without capsule, and is easily or spontaneously agglutinated. Jacob- 

 son and I have shown^ for pneumococci that when "S"-"R" variations are induced, 

 alterations in virulence are associated with parallel variations in inagglutinability, in 

 production of precipitable (specific soluble) substance, and in the magnitude of the 

 P.D. My observations on the occurrence of "R" and "S" strains in bacteria patho- 

 genic for plants were developed by Sharp and later by Link and Hull.* The parallelism 

 of P.D. and virulence is presented in the report by Sharp.^ 



The indications are that the significance of P.D. in virulence studies on animal 

 hosts extends into similar studies on virulence for plant hosts. 



The earlier notions of discontinuity between "S" and "R" variations have latterly 



I Falk, I. S., Gussin, H. A., and Jacobson, M. A.: /. Inject. Dis., 37, 481. 1925; Falk, I. S., 

 Jacobson, M. A., and Gussin, H. A.: ibid., pp. 495, 499. 1925; Falk, I. S., and Jacobson, M. A.: 

 ibid., p. 507; 38, pp. 182, 188. 1926; /. Bad., 13, 191. 1927. 



= Sharp, C. G.: Bot. Gaz., 83, 113. 1917; Link, G. K. K., and Sharp, C. G.: ibid., p. 145. 1927; 

 Falk, I. S., Sharp, C. G., and Link, G. K. K.: Proc. Soc. Exper. Biol. &- Med., 24, 576. 1927. Cf. also 

 chap, xliv by Dr. Link in this volume. 



3 Bacrlhlein, K.: Centralbl.f. Baklcriol., I. Orig., 81, 369. 1918. 



1 Arkwright, J. A.: J. Path. £r Bad., 24, 36. 192 1. 



s DeKruif, P. H.: J. Exper. Med., 33, 733. 1921; ibid., 35, 631. 1922. 



"^ Cf. the extensive review by Hadley, P.: J. Infed. Dis., 40, i. 1927. Also chap, vii by Dr. 

 Iladlcy in this volume. 



'Jacobson, M. A., and Falk, I. S.: J. Bad., 13, 191. 1927. 



8 Link, G. K. K., and Hull, K. L.: Bot. Gaz., 83, 412. 1927. » Loc. cil. 



