1 82 MANUAL OF BACTERIOLOGY. 



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 dif- 

 ficulty, 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 is cultivated at 42° to 43° C. Exposure to light 

 and to oxygen tends to weaken the virulence; and also cultiva- 

 tion upon unfavorable media, such as those containing a 

 small proportion of car-bolic acid or certain other chemical 

 germicides. 



In laboratory work the virulence is usually maintained best 

 by inoculating the bacteria from time to time into susceptible 

 animals. Bacteria coming freshly from infected animals are 

 likely to be highly virulent. The virulence may be increased 

 by beginning with an especially sensitive animal like a very 

 young guinea-pig, and progressively inoculating into less sensi- 

 tive animals. The infection of relatively insusceptible animals 

 may sometimes be produced by the injection of a very large 

 dose of the bacteria. The addition of the toxic products of the 

 bacteria, which may be obtained by using large doses of cul- 



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



