164 MICROBIOLOGY 



those of tetanus, tuberculosis or glanders, are present in small 

 numbers and in contaminated material, it is necessary to 

 inoculate them into the susceptible guinea pig from whose tis- 

 sues pure cultures may be procured after the disease has de- 

 veloped. By this means septicemia hemorrhagica and the 

 bacillus of hog cholera, and in fact nearly all of the pathogenic 

 bacteria affecting animals, may be obtained in pure culture by 

 this method where it would be quite impossible to succeed by 

 other means. 



2. To determine the pathogenesis and virulence of bac- 

 teria. In the investigation of animal diseases, it frequently 

 happens that pure cultures of some microorganism may be 

 procured from certain diseased processes and because of its 

 association with such lesions may be suspected of standing in 

 a causal relation to the disease. In order to determine whether 

 such relation actually exists, it is necessary to inoculate these 

 organisms into animals. It sometimes happens that one pro- 

 cures a culture of some species of bacteria, such as Bacterium 

 septicemiae hemorrhagicae, but does not know whether or not it 

 is virulent. "While it is known that this species is pathogenic 

 one can not tell without actual inoculation into susceptible 

 animals whether or not the organisms in this particular cul- 

 ture are virulent. In order, therefore, to determine whether 

 a given organism is pathogenic or not and to determine its 

 degree of virulence it is necessary to inoculate animals. 



3. To modify the virulence. In the investigation of ani- 

 mal diseases it is sometimes necessary to modify the virulence 

 of pathogenic organisms. This can be done in many instances 

 by a continuous series of inoculations. Thus an organism like 

 Bacterium septicemiae hemorrhagicae remaining for a certain 

 length of time in the tissues of a partially immunized animal 

 becomes attenuated. In these cases, especially with the septi- 

 cemia hemorrhagica bacteria, it is possible to restore the 

 original virulence by passing the organism through a series of 

 susceptible animals such as rabbits. 



4. To differentiate species. Animals are sometimes used. 

 and necessarily so, to differentiate between species or varieties 



