CHAPTER VII. 



THE GENUS STREPTOCOCCUS. 



The genus Streptococcus has been denned by its 

 parasitic life, the occurrence of the cells in chains, posi- 

 tive reaction to the Gram stain, faint effused growth on 

 media, high fermentative power for most carbohydrates, 

 but not for inulin, and the general failure to liquefy gelatin 

 or reduce nitrates. The single cells may be elongated 

 or lance-shaped but are more often spherical, or even 

 flattened at right angles to the chains in which they occur. 

 They may appear in long chains of a hundred or more 

 elements, or in pairs and short chains of four to six cells. 

 Chains are best developed in broth cultures; but some 

 strains display the tendency to chain formation much 

 more markedly than others. Chains are frequently 

 made up of successive pairs of flattened cells, — prob- 

 ably as the result of active division. Double chains 

 and irregular plate-like groups sometimes appear. Cocci 

 isolated freshly from the mouth are particularly apt to 

 show this grouping, and occasionally exhibit elongated 

 rod-like cells. Large cells sometimes appear in chains 

 of smaller ones. Vincent (1902) has even described a 

 branching streptococcus which he isolated from a pleuritic 

 exudate, and which he records as retaining its habit of pro- 



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