52 STUDY AND IDENTIFICATION OF BACTERIA 



greenish zone. Streptococci which are profoundly toxic and which 

 have been isolated from milk-borne epidemic sore throats differ from 

 the ordinary S. pyogenes in being encapsulated, not tending to form 

 chains and producing only slight haemolysis on blood agar. 



Some of the English authorities have introduced biochemical 

 methods of differentiating: the Strep, pyogenes coagulating milk, re- 

 ducing neutral red, and producing acid in lactose, saccharose, and 

 mannite media. 



S. pyogenes does not produce acid in inulin media while the pneu- 

 mococcus does, 



A freshly prepared solution of sodium taurocholate, 5%, added to an equal 

 amount of a twenty-four-hour bouillon culture of S. pyogenes does not disintegrate 

 the cocci or, at any rate, not within a few minutes. The reverse is true of 

 the pneumococcus. 



When we consider the biochemical variations which a single organism, as the 

 colon bacillus, may exhibit, the value of such methods of differentiating may well 

 be questioned. The question of the symbiotic relationship, which, when established 

 between two or more bacteria, may cause harmless organisms to take on virulence, 

 would appear to be a more important consideration. 



Almost without exception, human streptococci are Gram positive. 

 Their colonies are quite small, but distinct and discrete. In appear- 

 ance the colonies of streptococci and pneumococci are practically iden- 

 tical. In a blood-serum throat culture pneumococcus and streptococcus 

 colonies are the smallest, diphtheria ones are quite small and discrete, 

 but slightly flatter. (Always examine the water of condensation for 

 streptococci.) The sarcina and staphylococcus colonies are much 

 larger. 



Streptococcic colonies on blood agar are much more moist and luxuriant than 

 on ordinary agar. A very important point, in judging whether a streptococcus 

 or other organism is pathogenic in a given infection, is to examine smears from the 

 pus or other material in a Gram-stained specimen for information as to abundance 

 and, in particular, phagocytosis of any organism, before plating out. 



Streptococci are commonly the cause of diffuse phlegmonous inflam- 

 mations, while the staphylococci cause circumscribed lesions. Strepto- 

 cocci cause necrosis and do not characteristically produce pus. The 

 importance of the streptococcus as a secondary infection in diphtheria, 

 tuberculosis, small-pox, and even in typhoid fever must always be kept 

 in mind. It is this infection which does not respond to diphtheria 

 antitoxin, and not the diphtheria one. 



