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 ditficulty. Increase of toxin-production is 
more difficult, and it is only possible to obtain it to a 
