248 MANUAL OF BACTERIOLOGY. 



nature will only be revealed on bacteriological examination. 

 They are, however, as a rule, milder than genuine diph- 

 theria. The pseudo-membranous affections of the throat 

 which occur in scarlet fever and measles are generally caused 



FIG. 58. 













Streptococcus pyogenes in Pus, Gram's Stain. (X 1000.) 



by the streptococcus pyogenes, although those diseases may 

 be complicated by genuine diphtheria. Streptococci are very 

 commonly present in the throat in scarlet fever, 1 and some- 

 times occur in the blood. Some observers believe that scarlet 

 fever is caused by streptococci. Streptococci are very often 

 found in the pustules of small-pox, and may also appear in 

 the blood. 



The streptococcus pyogenes is pathogenic for mice and 

 rabbits, but the virulence is very variable. That may some- 

 times be increased by passing through a number of animals 

 in succession, but is rapidly lost in artificial cultures. It is 

 said that the virulence is best maintained when cultures on 

 gelatin, after forty-eight hours' growth, are kept in a cool 



' Weaver, American Medicine, April 18, 1903. 



