INFECTION AND INFECTIOUS AGENTS 41 



he named tuberculin. Seibert crystallized out the active principle, 

 and she and Long studied its properties. They are discussed in 

 Chapter XXVI. 



Carbohydrate. — Zinsser (1923), Heidelberger and Avery 

 (1923), and others have obtained specific carbohydrates from 

 pneumocoeci and other bacteria. In some instances the exact 

 chemical composition of the polysaccharide has been determined. 



Protein. — Furth and Landsteiner (1928) isolated from typhoid 

 bacteria specific protein fractions which they designated as Pi 

 and Po. Ando and Ozaki (1930) have shown that scarlet fever 

 streptococci contain nucleoprotein fractions which must be dif- 

 ferentiated in the culture from specific soluble toxins secreted by 

 the organisms. Similar studies on C. diphtheriae have been carried 

 out by Neill and his colleagues (1930). They have apparently 

 shown that all hypersensitive reactions to products of growth in 

 diphtheria cultures are due to a hypersensitiveness to toxin and 

 that no reactions to nucleoproteins occurred. Extensive chemical 

 studies of many other species of bacteria are being reported in the 

 literature and will be referred to in later chapters. 



Effect of Environment on Bacteria. — The cultivation of bac- 

 teria in artificial media frequently leads to interesting changes. 

 They may lose their ability to form pigment, to liquefy gelatin, 

 to form capsules or to produce infection as well as many other 

 characteristics. When the environment is modified properly, 

 many of these lost characteristics may be restored. 



The loss of characteristics mentioned above has been spoken of 

 frequently as degeneration phenomena. Occasionally an organism 

 acquires a new character and retains it. This is called mutation. 

 Neisser (1906) described mutation in a strain of B. coli. His 

 observations were soon confirmed by a number of investigators. 

 Barber (1913) and Jordan (1915) have reported mutation arising 

 from the progeny of a single cell. 



Bacteria are also able to adapt themselves to growth under 

 many favorable conditions. They may become habituated to 

 growth at temperatures normally inhibitory or to develop in the 

 presence of concentrations of chemical agents such as antiseptics 

 or antisera that are destructive to the original culture. 



Theory of Constancy of Characteristics. — Cohn (1875) and 

 Koch (1877) are credited with the concept that bacterial char- 



