468 MILK 



essential factors influencing the shape of the cell, chain formation, 

 and capsule formation. 



Acid Formation and Coagulation in Milk. — As a general rule, 

 pathogenic streptococci produce less acid than saphophjrtic strep- 

 tococci. Sometimes acid production is so small that milk does 

 not coagulate for days, if at all. If streptococci with small acid- 

 producing power are cultivated persistently on carbohydrate 

 media, preferably dextrose media, the quantity of acid produced 

 may increase materially. This is not always true, some strains 

 responding with greater readiness than others. Vice versa, high 

 acid producers may partially lose this property if cultivated on 

 carbohydrate-free media. The more persistently a strain has 

 become habituated to albuminous food, the less the power to 

 ferment carbohydrates. In this respect strains also differ in 

 susceptibility to changed environment. Therefore acid formation 

 and coagulation are not criterions for differentiation. 



Relation of Amount of Acid Formed from Different Carbo- 

 hydrates. — By cultivation of a large number of strains of strepto- 

 cocci from various sources Gordon and Andrews and Horder have 

 attempted to differentiate strains. They used Ktmus in the me- 

 dium as indicator of acid production. Winslow and Palmer 

 studied the amount of acid formed by titration with normal al- 

 kali. An extensive literature has accumulated, which, however, 

 has not led to entirely uniform results. Attacking the problem from 

 a different point of view, the writer has shown that by passage 

 through animals, the fermentation properties of two strains of 

 Streptococcus lacticus could be altered. In fact, the two strains, 

 isolated from ice-cream and certified milk, did not have identical 

 fermentation reactions to start out with, one fermenting mannite, 

 the other faihng to do so. Otherwise they had the typical reac- 

 tions of Streptococcus lacticus. Considerable irregularity was 

 noted after the strains had been passed throtigh rabbits and 

 guinea-pigs. Therefore acid formation from carbohydrates can- 

 not be regarded as a permanent basis for differentiation of strains 

 of streptococci, but may be of sufficient constancy to determine 

 the immediate source from which the strain was isolated. That 

 is to say, a strain isolated from a certain source will show definite 

 fermentation reactions until it has been under a different environ- 

 ment for some time. After this, fermentation reactions may be 

 altered. 



Agglutination of streptococci has been investigated by a nimi- 

 ber of workers. The majority agree that agglutination with ho- 

 mologous strains is, as a rule, pronounced, but -agglutination with 

 other strains variable and sometimes negative. When polyvalent 

 sera are prepared the reaction is different within wide limits with 



