CH. VI] VARIATIONS IN VIRULENCE 75 



could, they found, be converted by "passage" into a highly 

 virulent strain which would not grow at a temperature below 

 37° C. By artificial culture the reverse change could be 

 brought about In one experiment a single inoculation 

 into an animal sufficed to bring about the conversion of 

 one type into the other, the relationship between virulence 

 and the temperature at which growth would occur being 

 constant. 



(b) Another abnormal condition of growth which tends 

 to modify the virulence of organisms is the presence of weak 

 antiseptics. Thus Chamberland and Roux (quoted, Muir and 

 Ritchie) found experimentally that B. anthracis lost virulence 

 if grown on a medium to which carbolic acid had been added, 

 in the proportion of 1 to 600, or a minute quantity of Pot. 

 bichromate. A virulent strain of B. diphtheriae is promptly 

 attenuated by the addition of iodine trichloride to the medium 

 (Mohler and Washburn, 1906). 



In the living body certain secretions play a similar r&le. 

 Leiitscher (1911) proved that the saliva had a bactericidal 

 effect on the pneumococcus and he attributes the diminished 

 virulence of the pneumococci found in the mouth to this 

 agency. Savage showed, by experiment on himself, that the 

 streptococcus mastitidis, which causes mastitis in cows, had 

 its virulence greatly reduced by 2 or 3 days' residence in the 

 mucous membrane of the human pharynx. • 



On the other hand the addition to a medium of certain 

 substances may cause heightened virulence. The bacillus 

 of "blackleg,'' rendered avirulent by exposure to a high 

 temperature, has its virulence completely restored if lactic 

 acid is added to the medium in which it is growing (Mohler 

 and Washburn, 1906). Many organisms, also, which lose their 

 virulence rapidly on ordinary culture media, maintain it for 

 long periods when grown in the secretions of the body — for 

 example, in urine (vide p. 19). 



(c) The presence or absence of oxygen is another factor 

 of importance. For example, Haffkine (quoted Hankin, 1892) 

 found that the cholera spirillum lost virulence considerably 

 when grown in a current of sterile air, while Hueppe (quoted 



