60 STUDY AND IDENTIFICATION OF BACTERIA 



blood and 3 to 6 parts of agar. On this medium Strep, erysipdatis has 

 a hemolytic action, the laking of the red cells bringing about a more 

 or less clear ring surrounding the colony. This organism is often termed 

 S. hcemolyticus. It tends to produce suppurative arthritis in rabbits 

 while S. viridans causes endocarditis and the Pneumococcus an acute 

 sepsis. 



The haemolytic substance is much like a ferment and is found in filtrates of cul- 

 tures. The short-chain streptococci do not have a haemolytic halo. They also 

 have a greenish appearance like the Pneumococcus (S. viridans}. The Pneumococcus 

 has a 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 haemoly- 

 sis on blood agar. 



Some of the English authorities have introduced biochemical meth- 

 ods of differentiating: the Strep, pyogenes coagulating milk, reducing 

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

 media. 



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



The Pneumococcus and Streptococcus mucosus ferment inulin and give greenish 

 colonies. Of streptococci not fermenting inulin we have S. pyogenes, with its haemo- 

 lytic zone around the colony; S. viridans, with its green colony, and S. f&calis, which 

 gives colonies neither distinctly green nor haemolytic. 



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. 



( iWhen 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 



