82 VARIATIONS IN VIRULENCE [ch. vi 



inserted into the body cavity and allowed to remain there for 

 a certain time. 



By the last named method, for example, Martin (1898) 

 increased the virulence of a slightly virulent strain of B. 

 diphtheriae. 



Eyre and W^sl^^ourn (1899) showed that the saprophytic 

 pneumococcus, found in the mouths of healthy persons, could 

 by "passage" be made as virulent as the parasitic type isolated 

 from an acute lobar pneumonia. The number of inoculations 

 required varied considerably in different cases. In most of the 

 experiments a series of eight or ten rabbits sufficed. In one 

 case virulence only reached its height after no less than 

 53 passages. In another case a single inoculation was sufficient 

 to convert an avirulent organism into a highly virulent 

 one. They noted that strains in which virulence was easily 

 raised were able to maintain their exalted virulence on 

 suitable media for a long time, while those strains which 

 very slowly acquired virulence, quickly lost it on artificial 

 culture. 



Mohler and Washburn (1906) mention that the virulence 

 of the virus of rabies is increased by passage through rabbits 

 and that the cholera organism, after passage through guinea- 

 pigs, becomes much more virulent towards pigeons. 



Other animals than those named may be utilised for the 

 purpose of "passage." For example, Salter (1899), by means 

 of five successive passages through the goldfinch, raised the 

 virulence of the "pseudo-diphtheria bacillus" sufficiently to 

 render it fatal to guineapigs, 



The process of "passage" may be even more effective if 

 it be made to alternate with culture on ordinary media. 

 Marmorek (quoted, Muir and Ritchie) showed that the viru- 

 lence of the streptococcus was enormously increased by grow- 

 ing it alternately in a mixture of human blood serum and 

 bouillon and in the body of a rabbit. 



11. Inoculation with a living or dead cultwre of some 

 other organism in many cases intensifies the result. Thus, 

 Klein (1903-4) states that the virulence of the diphtheria 

 bacillus is greatly increased by inoculation into an animal if 



