18 INFECTION AND RESISTANCE 



Sawtchenko, 22 by Danysz, 23 and by Walker. 24 The results of Wal- 

 ker are especially instructive. He worked with a typhoid bacillus 

 which he cultivated for a number of generations upon the serum of a 

 typhoid-immune animal, and found that after such treatment the 

 organism had gained in virulence and lost in agglutinability by im- 

 mune serum, and that a larger amount of specific immune serum was 

 necessary to protect animals against it than sufficed for protection 

 against normal typhoid strains not thus cultivated. We will refer 

 to these results more in detail in a later chapter, since the conception 

 will be easier to grasp when we have Considered more fully the 

 mechanism of defence at the disposal of the animal body. 



That this power of gaining resistance against deleterious influ- 

 ences on the part of bacteria is not confined to their resistance to the 

 animal defences alone is well shown by the experiments of Danysz 25 

 upon the immunization of anthrax bacilli against arsenic. In in- 

 oculating series of 50 tubes containing arsenic dilutions (ranging 

 from 1 to 10,000 to 1 to 200) with anthrax bacilli Danysz found 

 that up to 1 to 5,000 the arsenic increased the growth of the bacilli ; 

 in concentrations higher than this growth was inhibited. By grad- 

 ually progressive cultivation of the organisms in increasing concen- 

 trations of arsenic he finally succeeded in obtaining growth in solu- 

 tions five times more concentrated than those in which they would 

 develop at first. 



It is intensely interesting also that Danysz found, both in the 

 case of his serum-resistant and arsenic-resistant strains, that, as 

 they became less sensitive to the deleterious effects of these agencies, 

 they were altered morphologically in that they developed capsules. 

 Similar in significance to this is the very important observation that 

 certain strains of spirochseta pallida may acquire resistance against 

 salvarsan or "606." 26 These so-called arsenic-fast strains are ap- 

 parently unaffected by the injection of this preparation into the 

 patient. 



. * The experiments of Danysz were probably the first to call atten- 

 tion to the possible relationship of bacterial capsule formation to 

 virulence, and this particular phase of the subject has since then 

 been extensively studied. It is a matter of common observation that 

 micro-organisms like the pneumococcus, the anthrax bacillus, some 

 streptococci, and a number of other germs which are capable of pro- 

 ducing capsules under suitable conditions are most virulent in the 

 capsulated stage. As the strains are passed through animals and 

 their virulence increases their ability to form capsules becomes more 



22 Sawtchenko. Ann. Past., Vol. 11, 1897. 



23 Danysz. Ann. Past., 14, 1900. 



24 Walker. Jour, of Path, and Pact., Vol. 8, 1903. 



25 Danysz. Loc. cit. 



26 Oppenheim. Wien. kl. Woch., 23, 1910, No. 37. 



