BACTERIA AND DISEASE; GENERAL CONSIDERATIONS 117 



(d) The organism may be demonstrated microscopically, but 

 will not grow upon the culture-media of the laboratory. Such 

 are some of the protozoa. With a few of these the proof has been 

 perfected by the study of the growth of the organism in an inter- 

 mediate host, for example, the malarial parasite in the mosquito. 



Evidence of the relationship of an organism to a disease may 

 frequently be secured by using the agglutination, precipitation, 

 or some of the other tests discussed in the following chapters. 

 Improvements in staining technic continually reveal new or- 

 ganisms. 



Animal Inoculation. Experimental inoculation and injections 

 are made in the bacteriological laboratory for a number of reasons: 



1. In determining the causal relationships of a specific organism 

 to a disease in accordance with Koch's rules. 



2. In the diagnosis of certain diseases. For example, one of 

 the most satisfactory methods of diagnosing glanders is to inject 

 some of the nasal; discharge from a suspected animal into a male 

 guinea-pig and note the development of acute orchitis and subse- 

 quent general body reaction. 



3. In the isolation of certain pathogenic bacteria. For example, 

 if it is desired to isolate the organism causing tuberculosis from 

 sputum or from milk, it may most readily be accomplished by 

 inoculating the infected material into a suitable animal. All the 

 non-pathogenic organisms will be destroyed and the specific 

 organism may be isolated in pure culture from diseased tissue or 

 lesions of the animal so infected. The animal body is used as 

 a kind of filter for the removal of the non-pathogenic bacteria. 



4. In determining the strength of concentration of certain 

 biological products. As will be seen later, the only way that has 

 been devised for the determination of the strength or potency of 

 certain poisons, such as toxins, and for their antitoxins, is animal 

 injection. The animal is used by the bacteriologist in much the 

 same manner as an indicator is used by the chemist in determining 

 the acidity or alkalinity of a solution. 



5. In the production of certain so-called antibodies, such as anti- 

 toxins, and for the demonstration of certain characteristics of the 

 blood-serum in immunity. 



The animal most frequently used in experimental work is the 



