DETERMINATION OF THE PATHOGENICITY OF AEROBES viiis-S 



as peculiarities of the organism, the cultural conditions, the age of the 

 culture and the nature of the filtering agent must be considered when 

 testing the toxic properties of bacterial filtrates. As a rule, exotoxins 

 are heat labile and deteriorate on standing. Scarlet fever is the most 

 heat stable of the exotoxins and approaches the endotoxins in this 

 I'espect. Heat stability may assist in diflferentiating the two types, 

 but the final criterion of a true exotoxin is its ability to stimulate the 

 production of a specific antitoxin when injected into a suitable animal. 

 The exotoxin in a filtrate may be neutralized by the addition of im- 

 mune serum and any residual toxic action may then be assumed to be 

 due to other toxic principles. The different organs affected and the 

 type of tissue damage should be recorded. 



The Use of Laboratory Animals 



For a general discussion of the care and use of laboratory animals 

 see, e.g., Meyer (1932), Farris et al (1945), Gumming (1947) and 

 Wadsworth (1947). Animals are necessary, not only for determining 

 the etiology of specific infectious diseases and the pathogenicity of 

 particular cultures of bacteria, but they are also utilized as a means of 

 isolation, to determine specific pathogenic properties, to maintain 

 organisms that grow only in vivo, to increase pathogenicity and to 

 produce antibodies and other agents used in the growth and identi- 

 fication of microorganisms and in the diagnosis and therapy of disease. 



The choice of an experimental animal and the method of injection 

 and recovery of the organism depend upon the bacterial species and 

 the property to be studied. The human animal would be most 

 satisfactory in dealing with diseases of man but he is not available 

 except on rare occasions. This limits the application of Koch's 

 postulates in the case of man, but natural infections and accidental 

 infection of laboratory workers are useful in supplying circumstantial 

 evidence as to the pathogenicity of certain bacteria for man. 



Healthy, previously unused animals should be employed. Several 

 days of observation prior to injection are necessary to insure that the 

 animals are in good condition and to provide a period of acclimati- 

 zation. Following injection the animals should be observed daily 

 for gross abnormalities and symptoms of disease and in certain cases 

 it may be necessary to take daily temperature, pulse, respiration 

 changes, hematology, etc. Large animals may be marked with 

 metal tags in the ears, and the ears of small animals may be tattooed 

 or marked with an indelible pencil. 



Pathogenic bacteria produce different types of lesions in animals 

 which may be specific and equally as important as immunological, 

 serological and biochemical properties. To recognize them the stu- 

 dent should be trained in pathologic technic and should be familiar 

 with the gross and microscopic appearance of normal and diseased 

 tissue. 



Methods of Injection 



Bacteria or their products which cause disease when injected 

 parenterally may fail to do so when placed on the skin or when intro- 

 duced by insufflation or by mouth. Hence the importance of differ- 

 ent routes of injection. 



