THE GENUS STREPTOCOCCUS 141 



minute dose will kill a rabbit in a few hours. The same 

 culture "may produce at one time merely a passing local 

 redness, at another a local suppuration, at another a 

 spreading erysipelatous condition, or again a general 

 septicaemic infection, according as its virulence is arti- 

 ficially increased" (Muir and Ritchie, 1903). This 

 extreme variability explains the association of closely 

 related streptococci with such widely different diseases 

 of the human subject as erysipelas, phlegmons and spread- 

 ing inflammation, joint diseases, endocarditis, anginas, 

 puerperal infections, and general septicaemias. 

 The genus has been defined above as follows: 



Genus Streptococcus (Billroth) Winslow and 

 Rogers. Parasites. Cells normally in short or long chains 

 {under unfavorable cultural conditions, sometimes in pairs 

 and small groups, never in large packets) . Generally stain 

 by Gram. On agar streak, effused translucent growth, often 

 with isolated colonies. In stab culture, little surface 

 growth. Sugars fermented with formation of large amount 

 of acid. Generally fail to liquefy gelatin or reduce nitrates . 



On the whole, the genus Streptococcus is well marked 

 and easily distinguished. Its relation to Diplococcus, into 

 which it grades most closely, has been somewhat fully 

 discussed already. It is distinguished from the rest of 

 the Paracoccaceae, on the other hand, by its characteristic 

 morphology and sparse growth on artificial media. Three 

 of Cohn's original species of micrococci, M. Vaccine w 

 M. diptericus, and M. septicus, were probably representa- 



