1 68 MANUAL OF BACTERIOLOGY. 



remembered that hyperemic 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 in- 

 fected with bacteria through inhalation, and undergo sup- 

 puration or gangrene. The presence of foreign bodies in 

 the tissues disposes to infection. Injection of the staphy- 

 lococcus 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 produced 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. 1 



Amount of Infectious Material. A large number of 

 bacteria 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 lan- 

 ceolatus (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 is cultivated at 

 42 C. Exposure to light and to oxygen tends to weaken 

 the virulence; and also cultivation upon unfavorable media, 

 such as those containing a small proportion of carbolic acid 

 or certain other chemical germicides. 



1 Cheesman and Meltzer, Journal Experimental Medicine, Vol. III. 



