60 RELATIONSHIPS OF THE COCCACE^ 



temperature relations, and pigment formation were also 

 recorded on this medium under conditions to be 

 described below. 



Biochemical Reactions. 



Action upon milk. Milk is a favorable nutrient medium 

 for bacterial growth because of its rich food properties, 

 and in many groups its reactions are important. It has, 

 however, no specific diagnostic value for the Coccaceae, 

 since all the changes it undergoes are correlated with 

 those which occur in the sugar broths, and with the gen- 

 eral activity of the organism. No coagulating enzymes 

 and no casein-digesting enzymes have been observed in 

 any of our cultures and we have never seen gas forma- 

 tion in a pure culture of cocci. These various reactions 

 have at times been recorded by individual observers. 

 Thus Dyar (1895) described a coccus which coagulated 

 milk in alkalin solution. Leichmann (1896) reports the 

 isolation of a coccus which, when grown in milk, formed 

 carbonic acid gas and levo-rotary lactic acid. The great 

 weight of negative evidence on the other side makes 

 it probable that these observations were due to errors 

 of technique, and makes it quite certain that such proper- 

 ties are at least exceptionally rare. As a rule the only 

 changes which the cocci effect in milk are the production 

 of acid or alkali, coagulation, and decolorization of the 

 litmus. 



Decolorization has no significance, except that it 

 indicates general activity of the organism. When a 



