MILK-BORNE INFECTIONS 467 



factors that have been studied are morphology, acid formation 

 and coagulation in milk, acid formation from different carbo- 

 hydrates, agglutination, hemolysis, relation to free oxygen, and 

 virulence. Streptococci have been classified by various authors 

 according to one or more of these properties. 



The fundamental question is whether streptococci represent 

 one species or a multiplicity of species. Both views have cham- 

 pions. However, the essential difference between the two views 

 is not as great as might appear. It is difficult to determine accu- 

 rately the meaning of "bacterial species." It has been shown that 

 some strains of streptococci can adapt themselves with greater or 

 less readiness to environmental conditions, and in doing so alter 

 some properties materially. A brief summary of the present state 

 of our knowledge on this subject follows. 



Morphology. Some streptococci form long chains; others, 

 short chains. Experiments have shown that the character of 

 the culture-medium has some influence on this property. As a 

 rule, absence of carbohydrate in the food is conducive to long 

 chain formation. Liquid media favor long chain formation. 

 Media containing carbohydrate, as milk, for example, tend to 

 shorten chains. There are exceptions, of course. The general 

 tendency only is indicated. Some strains preserve certain prop- 

 erties with greater tenacity than others. The shape of the indi- 

 vidual cell is subject to variation. Some strains are represented 

 by strictly spherically shaped cells, others have cells of more or 

 less elongation. The long diameter of elongated cells may be 

 parallel to the axis of the chain or transverse. The chain itself 

 may be continuous or may be so interrupted as to give the appear- 

 ance of being composed of diplococci. The fundamental causes 

 for these differences in morphology are obscure. Probably the 

 rapidity of cell division has some influence. It has been recorded 

 that cultures which originally were composed of one form, to all 

 appearances, would later show some of the other forms, mixed 

 with the original one. Transition forms are, therefore, common, 

 but under what conditions they appear is not well understood. 



Capsule formation has been observed in some strains of strep- 

 tococci. It is sometimes assumed that passage through animals 

 develops the property of forming capsules. This is unquestion- 

 ably true with some strains, while others do not form capsules 

 after repeated passage. It has not been conclusively demon- 

 strated that capsule formation is always the result of passage 

 through animals, and that it has a definite relation to virulence. 



It is obvious that morphology gives no clue for differentiation 

 of types. The composition of the medium and the particular 

 environment which has surrounded the strain in the past are 



