264 president's address — section l. 



Roseiiow's " virulins " are of a somewhat different nature, op- 

 posing merely the ingestion of the bacteria by phagocytes. Work- 

 ing with various strains of pneumococci he found thein all non- 

 phagocytable when firet recovered from cases of pneumonia, and at 

 iho same time virulent for rabbits and guinea pigs. The virulence 

 was in proportion tO' their inability to absorb " opsonin " from 

 serum, aiid as the virulence was lessened by cultivation so' the 

 power to adsorb opsonin increased, and at. the same time the 

 power to resist phagocytosis was lost. Extracts of the virulent 

 organisms in saline contained a substance "virulin " which neu- 

 tralized the opsonic effect of serum. 



This is in line with the old observation of Marchand that viru- 

 lent streptococci were not taken up^ by phagocytes in the presence 

 ui normal serum, whilst non-virulent ones were; and further that 

 Ihe virulent organisms killed by heat and washed M^ere' still re- 

 fractory. This appears to be due to seme antiopsonic property 

 of the bacteria. 



One factor in the virulence of an organism related probably to 

 those just considered is the property of capsule formation, which 

 IS seen in a large number of species of bacteria. This property 

 occurs in most cases in the organism as first isolated from the 

 animal body, e.y., in anthrax, but is speedily lost on artificial 

 media unless the medium contain blood serum. With the loss 

 of a capsule theie is a loss of virulence, and it has been asserted 

 by Preisz that anthrax bacilli which have lost the poAver of cap- 

 sule formation are no longer capable of infecting. A similar 

 parallel loss of capsule and virulence is seen in the case of pneuiuo- 

 coccus, and streptococcus. On the other hand Friedlander's 

 Lacillus is a capsulated organism retaining its capsule in artificial 

 culture, and along with this retention there is little diminution 

 in virulence. Capsule formation is a function of bacteria in acute 

 cases at an early stage of the infection. The capsular material 

 appears to act as an irritant to phagocytes and to be able to 

 repel them {i.e., to causes a. condition of negative chemiotaxis). 



Another factor of virulence to be mentioned is that of si/ri'/val 

 hy selection. This probably explains in large part the effects of 

 frequent repeated passage through the animal body of any 

 bacterial strain. The various defensive forces attack these in- 

 vading germs and annihilate the weaker individuals, but the 

 stronger ones survive. By a repetition of this process the strain 

 recovered from the last of a series of animals is possessed of high 

 powcTS of resistance, or in other words, it has become highly viru- 

 lent. But this is one of many possible adaptations which, with 

 a varying environment, bacteria undergo, therebv surviving and 

 remaining virulent. 



One last adaptation is that of resistance to leucocytic digestion 

 on the part of certain virulent organisms. A typical example is 

 afforded by the gonococcus, which ingested by polymorphonuclear 



