SYNOPSIS xiii 



itself— nevertheless may benefit strain. Invasion of tissues by virulent 

 saprophyte involves change in foodstuffs — ^relationship between altered 

 metabolism and acquirement of toxicity — possibly a change in excretion 

 following change in assimilation — experimental evidence, (a) B. coli does not 

 attack proteid if carbohydrate present, (6) B. diph. does not yield toxin if 

 much carbohydrate present — suggest toxins may result irom alteration in 

 food material. Altered metabolism of saprophyte facilitates invasion of 

 tissues— this supposed alteration in metabohsm does not always confer 

 toxicity — toxins may be regarded as an excretion or as a secretion — or as 

 product of enzyme— activity of enzyme may be due to adaptation, encouraged 

 by natural selection. 



The value op virulence in classification. Classification according to 

 virulence inconsistent. Non-virulent B. diph. in "carriers" regarded as 

 lineal descendant of virulent Klebs-Loeffler bacillus — other non-virulent 

 B. diph. provoke antitoxin, therefore same species as Klebs-Loeffler bacillus. 

 Non-virulent and virulent pneumococcus regarded as varieties of same 

 species. Non- virulent and virulent B. coli communis thought by some to be 

 different species — cf amoeba coli. Non-virulent "■B. anthracoides" described 

 as different species from virulent B. anthracis. S. erysipelatis and 8. 

 pyogenes formerly regarded as distinct species. Virulence not a specific 

 chai-axiter. (71—93) 



CHAPTER VII 



VARIATIONS IN PATHOGENICITY 



Pathogenicity is power to produce in certain animals certain symptoms 

 and certain lesions — quite distinct from virulence and other characters. 

 Generally regarded as more fixed than other characters — constitutes final 

 appeal in doubtful cases, e.g. Hofmann's bacillus and Klebs-Loeffier bacillus 

 — gonococcus and meningococcus — gonococcus does not cause meningitis 

 nor meningococcus urethritis. Pathogenicity a variable character in all 

 three aspects. 



I. Variation in kind of animal affected. 



II. Variation in symptoms caused. 



(1) Same organism causes different symptoms in different cases. 

 Symptoms may depend upon organs affected — cf lead poisoning— this 

 determined by route of infection and vitality of organs — also by patho- 

 genicity of organism — e.g. tubercle bacillus causes phthisis, osteitis, arthritis, 

 lupus — unlike lead poisoning types remain distinct — skin rarely infected by 

 tuberculous sputum — contrast between gonococcus and meningococcus no 

 greater than between different strains of tubercle bacilli. Fallacy due to 

 pre-existing disease, e.g. nephritis in cerebrospinal fever. 



(2) Pathogenicity can be artificially modified, e.g. that of B. anthracis 

 by ultra violet rays. 



(3) During epidemic different cases exhibit different symptoms. 



