BACTERIA IN DISEASE. 157 



of bacteria. It is important, therefore, to keep the resisting 

 power of the tissues at the highest possible point. Injury of 

 the tissues disposes the part to infection; so. do strangulation 

 and necrosis. In operating, it is to be remembered that hy- 

 peremic and edematous parts are more likely to become infected ; 

 so also are anemic regions. An infarct of the lung which was 

 originally sterile may be infected with bacteria through inhala- 

 tion, and undergo suppuration or gangrene. The presence of 

 foreign bodies in the tissues disposes to infection. Injection 

 of the staphylococcus pyogenes aureus into a rabbit's tissues is 

 not always followed by suppuration, but if a foreign body, 

 like a piece of sterilized potato, be inserted at the same time, 

 infection is much more likely to occur. When lesions are pro- 

 duced in the internal viscera of animals by cauterization or 

 crushing and bacteria then injected subcutaneously or into the 

 blood, the bacteria lodge in the lesions and multiply.* 



Amount of Infectious Material. — A large number of bac- 

 teria introduced into the body simultaneously will be more likely 

 to produce infection than a small number. This factor is of 

 less importance with organisms whose virulence is very constant 

 than with those of more variable virulence. 



Variability in the Virulence of Bacteria. — The occur- 

 rence of an infectious disease depends very largely upon the 

 virulence of the bacteria. Any species of pathogenic bacteria 

 may vary in virulence at different times. In some cases the 

 virulence is not easily lost, as with the anthrax bacillus; in 

 others the virulence is maintained in cultures only with difficulty, 

 as in the case of the micrococcus lanceolatus (of pneumonia) 

 and the streptococcus pyogenes. As a rule, the virulence is 

 likely to be diminished in old cultures. It may sometimes be 

 preserved better in the ice-chest than at the room temperature. 

 The virulence of the anthrax bacillus becomes diminished if it 



* Cheesman and Meltzer. Journal Experimental Medicine. Vol. III. 



