226 MANUAL OF MICROBIOLOGICAL METHODS 



Corynehacterium diphtheriae, Clostridium hotulinum, and Clostridium 

 tetani. The endotoxins, sometimes designated as ''gram-negative endo- 

 toxins," or O antigens, are bacterial complexes which are easily released 

 into the culture on autolysis. They are not so readily detectable as the 

 exotoxins and frequently require considerable concentration by chemical 

 methods before they can be demonstrated. 



In establishing the toxicity or toxigenicity of a bacterial species, the 

 selection of the test animal is of vital importance. Whereas the mouse 

 and rat are generally less susceptible than the guinea pig to exotoxins, the 

 mouse is particularly susceptible to the endotoxins of Pasteurella pestis, 

 and the rabbit to the Salmonella or Escherichia endotoxins (especially 

 E. coli strains 0-111 and B-55). 



3. Important components of bacterial pathogenicity are a number of 

 extracellular enzymes and metabolites. Some of these, such as the 

 hemolysins, are possessed by many species. Others, for example, 

 lecithinase (or alpha-toxin of Clostridium perfringens) , are found only in 

 certain organisms. Fibrinolysins, collagenases, coagulases, hyaluroni- 

 dase, and a variety of still uncharacterized cytolysins, such as streptococ- 

 cal leucocidins, all contribute to the virulence of bacteria. However, it 

 should be noted that these toxic substances can rarely be directly demon- 

 strated in the animal dead of bacterial infection. For this purpose, 

 rather, it is usually necessary to perform specific in vitro tests with the 

 bacterial culture. Finally, the presence of extracellular toxicity in a 

 species does not of itself indicate that it is a pathogen. For example, 

 autolysates of some saprophytic bacteria, such as Leuconostoc mesenter- 

 oides, can be shown to be cytotoxic when injected into the skin of rabbits 

 and other animals. 



4. As stated above, bacterial infection is the resultant of a complex of 

 opposing variables on the part of the host and the invading cell. Any 

 determination of pathogenicity, therefore, must be made in healthy 

 animals whose resistance can be assumed to represent the norm for that 

 species. If one injects a bacterial culture into animals which are fatigued, 

 overcrowded, dehydrated, or suffering from an avitaminosis, etc., death 

 of the animal, even with an overwhelming bacteremia, does not neces- 

 sarily signify that the organism is a virulent one. The possibility should 

 always be considered that the resistance of this particular host was so 

 lowered by unfavorable living conditions or some other organic disease 

 that it did not respond in the normal fashion to bacterial invasion. With 

 regard to the strain of animal, it should be borne in mind that certain 

 substrains of rabbits or of mice will vary in their susceptibihty to a par- 

 ticular pathogen. It is not profitable to list each instance here, for this 

 information can be found in appropriate reference texts. 



5. One must maintain a certain sense of proportion in selecting the size 



