CHAPTER 8 

 Salmonella Antigens 



Progress in bacteriology was greatly aided by the development ot 

 methods of staining microorganisms to facilitate their microscopic 

 observation. The staining methods were developed empirically. Al- 

 though their mechanism is still obscure, two of them are now known 

 to detect fundamental and significant differences in the cellular 

 structure of bacteria. These two reactions are the Gram stain and 

 the acid-fast stain. On the basis of their behavior toward the reagents 

 used in the two reactions, all bacterial species may be classified into 

 three broad groups : Gram-positive, Gram-negative, and acid-fast. 

 There also exist intermediate forms with poorly defined staining 

 reactions (Dubos, 1952). 



Endotoxins 



Gram-negative bacteria are characterized by the fact that, when 

 they are dyed with a basic triphenylmethane dye such as gentian 

 violet, the color can be removed by washing with alcohol. Gram- 

 negative bacteria have a number of other features in common. One 

 of the most interesting of these features to the immunologist is their 

 content of endotoxins. These characteristic substances, not released 

 to any great extent into the culture medium as the organism grows 

 (in contrast to exotoxins such as diphtheria toxin), can be obtained 

 by lysing the bacteria or by extracting them with trichloracetic acid, 

 diethylene glycol, etc. They are toxic in animals in very small amounts 

 (of the order of 0.001 microgram per kilogram of body weight), pro- 

 ducing fever, leukopenia, etc. 



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