PATHOGENICITY FOR EXPERIMENTAL ANIMALS 197 



given strain of Pneumococcus are the serological type; the vital 

 condition of the coccus at the time of trial as shown by the posses- 

 sion of a capsule; the mass of culture injected; the site chosen for 

 inoculation ; the species, and even the variety and the individual 

 idiosyncrasies of the animals selected. There also enters the ques- 

 tion whether the malignancy of a culture is due to a uniform viru- 

 lence of all the cocci in a culture or to the presence of a few or- 

 ganisms of especially high virulence accompanied by other organ- 

 isms possessing less or no invasive power. 



Pneumococci of the various serological types as they exist in 

 the lesions, exudates, or secretions of infected animals, or when 

 freshly isolated from these sources, are pathogenic for other ani- 

 mals of the same species and, depending upon the serological type, 

 may be virulent for animals of a different species. When a strain 

 is passed serially through the bodies of animals of a given species 

 it may acquire an elevated infectivity for other members of that 

 species. Virulence may reach a permanent zenith or may decline 

 but, virulent though an organism may be for the species used, it 

 by no means follows that it is correspondingly pathogenic for 

 animals of a different species. 



During propagation outside the body the conditions of cultiva- 

 tion effect profound changes in the integrity of Pneumococcus, 

 which, in turn, affect its pathogenic powers. Conditions that serve 

 to maintain the organism in the highest state of metabolic activity 

 with maximal production of capsular substance favor increase and 

 maintenance of virulence. Conditions that, on the contrary, induce 

 dissociation and degradation of the organism, by being inimical 

 to capsule formation, lessen or destroy virulence. 



FRESHLY ISOLATED STRAINS 



In considering some of the earlier reports dealing with the 

 pathogenicity of strains isolated from cases of lobar pneumonia 

 and other pneumococcal infections, it should be remembered that 

 many of the studies were conducted at a time before pneumococci 



