194 AGRICULTURAL AND INDUSTRIAL BACTERIOLOGY 



be well established that at least one, perhaps several, species 

 of nonpathogenic streptococci are the most common organ- 

 isms instrumental in bringing about lactic acid fermenta- 

 tion of milk. 



Streptococcus lacticus has spherical cells sometimes 

 slightly elongated, occurring in chains of greater or less 

 length. It is Gram-positive, does not produce spores and is 

 nonmotile. Apparently there are some strains, perhaps dis- 

 tinct varieties, capable of producing capsules. Certain of 

 these capsulated types are responsible occasionally for the 

 development of ropiness in milk, and for the sliminess 

 experienced in certain cultures or starters. Streptococcus 

 lacticus grows well, but never luxuriantly, on many of the 

 laboratory media. Its growth is greatly increased in amount 

 by the presence of suitable sugars which it may ferment. 

 Media prepared from whey are frequently found to be use- 

 ful. Lactose agar plates poured from souring milk will 

 show the organisms developing as small pin-point colonies, 

 frequently lying somewhat below the surface of the medium. 

 If litmus or some other suitable indicator is present, the 

 medium surrounding the colony will be noted to have 

 become intensely acid. Upon agar slants, the colonies are 

 usually more or less separated or discrete, at first scarcely 

 visible, and at last dewdroplike in appearance. In ordinary 

 broth the organism does not grow well unless sugar is 

 present. Usually the medium clears rapidly by sedimenta- 

 tion. Gelatin is not liquefied. 



The common sugars, dextrose, sucrose and lactose, are all 

 fermented with the production of lactic acid but never with 

 the formation of gas. There is some confusion in the liter- 

 ature as to the type of acid produced, but pure cultures in 

 certain sugars, at least, frequently produce the dextro type 

 only. When grown in milk sufficient acid is usually formed 

 from the lactose (.5 to 1.25 per cent) so that coagulation or 

 development of an acid curd occurs. The curd so formed is 



