96 BACTERIOLOGY. 



duced. In the majority of cases in man the number of 

 bacteria received is comparatively small; but by the 

 rupture of an abscess into a cavity or into the circula- 

 tion, or by the opening of the intestinal contents into the 

 peritoneum, the quantity introduced may be enormous. 



The Degree of Virulence Possessed by Bacteria. Bac- 

 teria as found in nature differ, as has already been stated, 

 as to the amount of poison they produce and the ease and 

 rapidity with which they grow in any nutritive sub- 

 stance. Both of these properties not only vary greatly in 

 different members of the same species, but each variety 

 of bacteria may to a large extent be increased or dimin- 

 ished in virulence. The specific poisons produced by 

 bacteria can be best studied in diphtheria and tetanus. 

 We note, first, that different individual bacilli of diph- 

 theria and tetanus have, when freshly obtained, wide 

 variations in the amount of toxin which they produce 

 i.e., a diphtheria bacillus obtained from a case of diph- 

 theria will produce in suitable nutrient broth a poison 

 of such strength .that 1 c.c. will kill an average sized 

 guinea-pig, while the poison from another bacillus will 

 kill with a much less quantity, or 0.005 c.c. Further, 

 the bacilli obtained from some sources retain their power 

 of producing poison, when grown on artificial media, for 

 years unaltered, while others lose much of this in a few 

 months. This is equally true of the tetanus bacilli. 



The power to produce toxin can be taken from bacilli 

 by growing them under adverse circumstances, such as 

 cultivation at the maximum temperature at which they 

 are capable of development. Some bacilli are easily 

 attenuated; others are robbed of their virulence only 

 with great difficulty. Increase of toxin-production is 

 more difficult, and it is only possible to obtain it to a 



