PATHOGENIC MICROBES— INFECTION 117 



by acids ; the cholera vibrio finally suffers from the alkalinity 

 which it itself produces ; the presence of fatty materials is 

 injurious to anthrax bacillus. There exist processes for 

 diminishing virulence : i. The action of a high temperature 

 (Toussaint, Pasteur), e.g., a temperature of 41° to 43°C., 

 instead of the body temperature of 36-37°C. 2. Temperature 

 + aeration (Pasteur's experiments on fowl-cholera and swine- 

 erysipelas). 3. Desiccation (Pasteur : preparation of the spinal- 

 cord of rabbits in the treatment of hydrophobia). 4. Light, 

 pressure, oxygen under pressure. 5. Antiseptics (Roux : 

 carbolic acid, potassium bichromate, etc.). 



Increase of Virulence — Passages. — For this purpose 

 one provides a microbe with the food-conditions which suit it 

 best ; oxygen of the air for the B. diphtheria ; extracts of 

 putrefying meat for the B. of tetanus (Brieger and Cohn). 

 The virulence is augmented by accustoming the bacteria to 

 the body against which they are being prepared ; the feebler 

 individuals are destroyed by the natural defences, and a 

 selection of the strongest members takes place. 



A bacterium may be habituated to the guinea-pig by com- 

 pelling it to live in the peritoneum of this animal enclosed in 

 a collodion sac, which permits the penetration of the nutrient 

 juices while keeping off the leucocytes. It is a culture in the 

 living body. Habituation is chiefly produced by the method of 

 passage, i.e., by inoculating the bacterium in an animal and 

 from this animal into another (generally of the same species). 

 In certain cases a degree of virulence is reached which cannot 

 be exceeded in the species of animal employed ; thus the virus 

 of hydrophobia becomes virus fixe after a certain number 

 of passages through the rabbit. Passages do not perceptibly 

 affect the tubercle bacillus, but have a pronounced influence 

 on the streptococcus. According to Marmorek the strepto- 

 coccus, which required at first a dose of i c.c. to kill, could be 

 brought by passage to kill with a dose of cooooooooooi c.c. 

 Pasteur raised the virulence of the anthrax bacillus by passing 

 it through new-born animals, then through older ones, then 

 adults, and finally through different species. 



Passage does not give the same results in all cases, and 



