CH. vi] VARIATIONS IN VIRULENCE 81 



growth but which could not be made to grow otherwise 

 (Allen, 1910). 



That symbiosis is an important factor in determining not 

 only the growth but also the virulence of a strain of bacteria, 

 is abundantly proved both by experiment and observation. 



It is said that a dog will not succumb to the infection of 

 tetanus unless it is infected simultaneously with pyogenic cocci 

 and in man it is recognised that the prognosis is more serious 

 if the tetanus gains access to the body by means of a sup- 

 purating wound. Some authorities explain these facts by 

 assuming that the tetanus bacillus can only multiply in the 

 body in the presence of pus-forming cocci (Marshall Ward 

 and Blackman, 1910), but analogy with other phenomena of 

 the same kind certainly suggests that it is a question of altered 

 virulence. 



Sanarelli observes that B. coll communis in typhoid stools 

 was highly virulent. Muir and Ritchie state "guinea-pigs 

 may resist the subcutaneous injection of a certain dose of the 

 typhoid bacillus, but if at the same time a sterilised culture of 

 the bacillus coli be injected into the peritoneum they quickly 

 die of general infection." These authors attribute the pheno- 

 menon to the diminished vitality of the animal, but here 

 again analogy suggests that increased viability or heightened 

 virulence on the part of the typhoid bacilli may be factors of 

 no less importance. 



Other examples of symbiosis influencing virulence will be 

 given in discussing "passage." 



10. The successful invasion of the animal body by bacteria, 

 leading to their increased virulence, may be brought about 

 experimentally. 



Pasteur was the first to discover that virulence could be 

 "exalted" by " artificial passage" through an animal or series 

 of animals. Rabbits or guinea-pigs are those commonly em- 

 ployed for the purpose and inoculation may be made into the 

 blood stream or the peritoneal cavity and a cultivation made 

 subsequently from the heart's blood or the peritoneal fluid ; 

 or the. organisms may instead of being introduced directly 

 into the peritoneum be shut up in a closed sac which is then 

 D. 6 



