268 BACTERIOLOGICAL EXAMINATION OF MATERIAL 
organism is exciting inflammation, however; it would appear that the 
secretion is a favorable medium for the development of the microbe. 
The viruses of poliomyelitis and of lethargic encephalitis may be 
found in the nasal secretion. Their identification has been discussed 
above. 
THE UTILIZATION OF ANIMALS FOR BACTERIAL DIAGNOSIS 
AND EXPERIMENTATION. 
Pasteur's brilliant animal experiments led Koch to formulate his 
Postulates for the etiological relationship of bacteria to disease, although 
in justice to Pasteur it should be emphasized that Koch popularized 
what Pasteur had expressly stated in several communications prior 
to Koch's publication. A rigorous demonstration of the etiological 
relationship of bacteria to specific disease, said Koch, must fulfil the 
following conditions: 
1. A specific microorganism must be constantly associated with the 
disease. 
2. The organism must be isolated from the lesion and cultivated 
outside the body of the host. 
3. A pure culture of the organism must incite the disease when intro- 
duced into a normal animal. 
4. The organism must be isolated from the experimental animal 
again in pure culture. 
Experience has shown that many diseases of man cannot be exactly 
reproduced in experimental animals and Koch's postulates, therefore, 
cannot be fulfilled with exactitude in these instances. Nevertheless, 
experimental animals are indispensable both in diagnostic and experi- 
mental bacteriological laboratories. They are used: 
1. As culture media for certain types of bacteria which grow slowly 
or feebly upon artificial media, particularly when the number of such 
organisms is too small to permit of cultivation under artificial condi- 
tions. The isolation of tubercle bacilli from urine, of glanders bacilli 
from the lesions of glanders are illustrative. 
2. To obtain pure cultures of bacteria from mixtures, as the inocu- 
lation of white mice with pneumonic sputum for the pneumococcus, 
or rubbing mixtures containing plague bacilli upon the shaved abdo- 
men of a guinea-pig to obtain pure cultures of B. pestis. 
3. To study experimentally the lesions incited by specific micro- 
organisms. 
4. To distinguish sharply between closely related bacteria, as for 
example, between bovine and human tubercle bacilli. Thus, rabbits 
are susceptible to infection with bovine, but not with human tubercle 
bacilli. Guinea-pigs are susceptible to infection with both types. 
5. To study the virulence of various microorganisms. 
6. To test the toxicity of bacterial toxins and other products, and 
to measure the potency of curative sera. 
