470 MILK 



mals. The writer has repeatedly shown that Streptococcus 

 lacticus by rabbit and guinea-pig passage may become eminently 

 virulent. Rosenow has stated that streptococci from milk re- 

 sembled morphologically and culturally the streptococcus of 

 rheumatism, and that other strains from milk showed selective 

 preferences for joints, muscles, gall-bladder, and other organs. 

 Virulence is variable and cannot be depended upon for differentia- 

 tion. Virulence is sometimes expressed by a "virulence num- 

 ber." The virulence number is determined by the phagocj-lic 

 property of leukocytes. A suspension of leukocytes is mixed 

 with a suspension of the organisms to be tested. The number of 

 leukocytes out of 100 in which no bacteria are found expresses 

 the virulence number. If the number is higher than 60, strep- 





/ \ 



} 





5^?^ (3:a>%-^ 







Fig. 194. — Sediment from mastitis milk twentj'-four hours old (Ernst) : a, Mas- 

 titis streptococci; b, descendants of streptococci; 1, 2, 3, 4, cellular elements. 



tococci are considered virulent. The result refers naturally only 

 to the species from which the leukocytes were obtained. There- 

 fore the test is only relative and of little practical value. 



It is obvious that methods of differentiation between strepto- 

 cocci have yielded unsatisfactory results. The same thing is 

 true of mastitis streptococci. Not only that different strains of 

 mastitis streptococci cannot l)e separated from each other by 

 their properties but also that it is not always possible to distin- 

 guish mastitis streptococci from saprophytic ones, including Strep- 

 tococcus lacticus, which is nearly always present in milk. 



If it is borne in mind that some properties of certain strains of 

 streptococci persist for a reasonable period when conditions are 

 not materially altered, it would seem that mastitis streptococci 



