CH. vi] VARIATIONS IN VIRULENCE 83 



the streptococcus pyogenes is inoculated with it, but not to 

 the same degree if the diphtheria bacillus is inoculated 

 alone. 



In this connection Miss Williams' experiments (1902) are 

 of interest. She grew two strains of avirulent (but morpho- 

 logically typical) diphtheria bacilli with a strain of virulent 

 streptococci in broth, transplanting every three or four days 

 for 90 successive generations, without producing any change 

 in the virulence of either organism. 



The virulence of the bacillus of malignant oedema is 

 markedly increased if this organism is inoculated together 

 with B. prodigiosus, and that of the streptococcus if it is 

 inoculated with B. coli communis (Muir and Ritchie). 



Klein (1903-4) also found that the injection of many 

 organisms subcutaneously (B. coli, B. Gaertner, B. enteritidis 

 sporogenes, and others) enhanced the virulence of organisms 

 such as B. typhoms or F. cholerae growing simultaneously in 

 the peritoneal cavity. 



The exalted virulence thus produced applies to the par- 

 ticular species of animal employed for passage and may, as 

 we have shown, apply to another species also but this is not 

 necessarily the case. Indeed, the virulence towards other 

 species may be markedly diminished. For example, the virus 

 of rabies becomes attenuated by passage through monkeys 

 (Mohler and Washburn, 1906). The bacillus of swine erysipelas, 

 isolated by Loeffler, after passage through the rabbit shows 

 exalted virulence towards this animal but attenuated virulence 

 towards pigs (Adami, 1892). Duguid and Burdon-Sanderson 

 found that the virulence of anthrax bacilli for bovine animals 

 was diminished after passage through a number of guineapigs. 

 Pasteur found that, if swine plague were inoculated from rabbit 

 to rabbit, the organism became more virulent for the rabbit 

 but less virulent for pigs (Muir and Ritchie). Streptococci, on 

 being inoculated through a series of mice, acquire increased 

 virulence for these animals but become less virulent for rabbits 

 (Knorr, ibid.). 



Many other examples might be given. A familiar one is 

 the preparation of the calf lymph for "vaccination" where 



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