BACTERIA 



35 



grains ; it forms appreciable amounts of acid together with yeast, 

 and its optimum temperature lies below 30 C. 



(2) Sphere Forms. The Streptococci are related to the Strepto- 

 bacteria, the points of similarity having been mentioned above. 

 They usually grow out into long chains when cultivated in broth, 

 but in milk and on solid media they may present varied appear- 

 ances. They grow well in milk, and always form dextro lactic 

 acid, though very seldom more than J to f per cent., i.e., not much 

 more than is required to coagulate milk. They are better able to 

 grow on the surface of solid media than the rod forms, being less 

 strictly anaerobic in character, but they do not form spreading 



FIG. 37. Streptococcus cremoris. (Streptococcus lacticus.} 



colonies. With the exception mentioned below, the Streptococci 

 show very little tendency to hydrolyse casein, and completely lose 

 the power to do so if not cultivated in milk. Their optimum 

 temperature is as a rule 30 C., and many strains do not grow above 

 37 C. Sc. thermophilus, however, grows best at 40 C. ; it is irregular 

 in shape, and is easily isolated from milk which has been kept at a 

 fairly high temperature. The greatest interest attaches to Sc. 

 cremoris, which is used for ripening cream in butter making ; it 

 forms long chains in milk and grows best at 25 to 30 C., but not 

 at blood heat. Slime-producing strains are represented by the 

 " bacterium of long milk," and Sc. Hollandicus. The former of 

 these was first described by Gerda Troili-Petersson under the name 



32 



