/p/J"] FAIL 01 Trr.KKn.K B.u 'lu.i OUTSIDE THE ANIMAL BODY 3ll 



(General Methods of Procedure 



There are three points that need special attention: the sample, 

 the manner of exposure, and the length of time required for life 

 to become extinct in the culture exposed. 



It is advisable that the sample of the organism 

 SAMPLE undergoing test be a pure culture. In some cases 



where the organism is readily differentiated from 

 contaminating growth, this is not so essential, as, for example, when 

 pathogenic organisms are mixed with non-pathogenic organisms 

 and the latter are readily eliminated by animal inoculation, or when 

 a colored organism can be readily distinguished by its color char- 

 acteristic. Even in these cases it is usually advisable to work with 

 pure cultures. 



EXPOSURE Much depends upon the manner of exposure; for 



OF SAMPLE instance, if exposure is to light it makes a decided 

 difference whether the culture is exposed directly 

 or in a glass container, or in the presence or in the absence of 

 air. A complete statement as to the manner of exposure is ab- 

 solutely essential to a definite interpretation of the reported re- 

 sults. 



INDEX OF ^ e most difficult point is to determine when life 



DEATH becomes extinct. This is comparatively simple 



with those organisms that readily grow on culture 

 media, when they are exposed in pure culture. Their failure to 

 grow gives a ready index of their death. With tubercle bacilli it 

 it much more difficult to tell just when these organisms are dead. 

 They cannot be cultivated readily from mixed growth since it is 

 very difficult to prevent over-growth with contaminating organ- 

 isms. Another difficulty is that dead tubercle bacilli produce typi- 

 cal tuberculosis in test animals (see page 312). It is therefore 

 necessary to make animal inoculations in all cases where mixed 

 tuberculosis materials are used, and from the original test animal, 

 especially if only a local lesion is produced, a second healthy ani- 

 mal must be inoculated from some of the suspected tuberculous 

 material. Suitable culture media should be seeded with this 

 material from both the original and the secondary test animal. 

 When all these tests prove characteristic for tubercle bacilli, it is 

 evident that these germs are still living and virulent; when no in- 

 fection r or only a local lesion, is produced in either the original 

 or the secondary animal, and no growth occurs on the culture 

 media, the tubercle bacilli are surely dead. In cases where the 

 tuberculosis produced in the original inoculated animal from a 

 small amount of material is severe and generalized, it may be con- 



