MILK-BOBNE INFECTIONS 469 



different strains used for immunization. Specificity of agglutina- 

 tion reactions in general is not sufficiently pronounced to serve 

 the purpose of differentiation. 



Hemolysis. — Hemolytic properties of streptococci have also 

 been widely investigated. Saprophytic strains, as a rule, do not 

 hemolize; pathogenic strains frequently do. However, the prop- 

 erty is not always constant. Some strains retain hemolytic 

 power with considerable tenacity through many generations on 

 artificial media; others fluctuate or lose the property entirely. 

 Passage through animals in many cases increases hemolytic power 

 or may change a non-hemolytic strain into a hemolytic one. 

 Environment seems to play as great a role in this reaction as in 

 others. Hemolytic strains of streptococcus have been found in 

 milk by several investigators, and the hemolytic property has 

 been considered of sufficient constancy to warrant differentiation. 

 This has been true especially in connection with studies of the 

 etiology of septic sore throat in man. These studies have led 

 several authors to the conviction that septic sore throat is caused 

 by infection from human beings. Smith and Brown have iso- 

 lated hemolytic streptococci from human throats, and Krum- 

 wiede and Valentine have isolated similar hemolytic streptococci 

 from the udder of a cow. The latter authors assume that the 

 cow was infected with these organisms from human beings. Davis 

 believes that septic sore throat may be caused by streptococci 

 derived from persons handling the milk, who first cause infection 

 of the udder and thus communicate the virus to human beings. 

 Both Davis and Mathers were able to produce intense mastitis 

 in cows by injecting hemolytic streptococci directly into the milk 

 ducts. The latter author states that while hemolytic streptococci 

 injected into the milk ducts produce severe and persistent mastitis, 

 when cultures of streptococcus lacticus are similarly injected they 

 also cause lesions, but of less severity and duration. 



Davis found hemolytic streptococci in 23 out of 92 samples 

 of milk, and stated that they are more common in raw than in 

 pasteurized milk. These hemolytic streptococci are, as a rule, 

 not virulent for animals, and produce reactions in milk which are 

 identical with those produced by Streptococcus lacticus. The 

 author thinks that "there are both hemolytic and non-hemolytic 

 lacticus strains, both of which are non-virulent, active acidifers 

 and coagulators of milk," and are readily distinguished from 

 hemolytic streptococci of human origin. 



Virulence of some bacteria for a particular species can be 

 increased by passage through animals of the same species, but 

 virulence for other animals may decrease at the same time. Non- 

 virulent bacteria may become virulent by passing through ani- 



