388 PYOGENIC BACTERIA. 
quickly proves fatal. But cultures of Streptococcus pyogenes, after 
it has been carried through successive generations in artificial media, 
injected beneath the skin of a rabbit, usually produce no result, or 
at most an abscess of moderate dimensions. 
It seems probable that the micrococcus isolated by Fligge from 
necrotic foci in the spleen of a case of leucocythemia, and described 
by him under the name of Streptococcus pyogenes malignus, was 
simply a very pathogenic variety of the streptococcus of pus. He 
was not able to differentiate it from Streptococcus pyogenes by its 
morphology or growth in culture media, but it proved far more 
pathogenic when tested upon animals. Mice inoculated subcutane- 
ously with a minute quantity of a pure culture died, without excep- 
tion, in three to five days. A large abscess was formed at the point 
of inoculation, and the blood of the animal contained numerous cocci 
in pairs and chains. Rabbits inoculated in the ear showed at first 
the same local appearances as result from inoculations with strepto- 
coccus of pus and of erysipelas, but after two or three days symp- 
toms of general infection were developed, and death occurred at the 
end of three or four days. At the autopsy the cocci were found in 
the blood, and frequently there were purulent collections in the 
joints containing the same microérganism. Krause has also de- 
scribed a streptococcus which only differs from Streptococcus pyo- 
genes of Rosenbach and Passet by the greater virulence manifested 
by its cultures. 
The fact that pathogenic bacteria may attain an intensified de- 
gree of virulence by cultivation in the bodies of susceptible animals 
was demonstrated by Davaine many years ago, and is fully estab- 
. lished by the experiments of Pasteur and others. It is true of the 
anthrax bacillus, of the writer’s Micrococcus Pasteuri, and of other 
well-known pathogenic microédrganisms. The reverse of this—at- 
tenuation of virulence as a result of cultivation in artificial media— 
is also well established for several pathogenic species. Now it 
appears that the attenuated streptococcus is far less likely to give 
rise to erysipelas or to puerperal infection than is the same micro- 
organism as obtained from a case of one or the other of these infec- 
tious diseases. The same is probably true also of Staphylococcus 
aureus and other facultative parasites which are found as sapro- 
phytes upon the surface of the body and upon exposed mucous mem- 
branes in healthy persons. And it is not improbable that attenuated 
varieties of these micrococci which find their way into open wounds, 
or into the uterine cavity shortly after parturition, if they escape 
destruction by the sanguineous discharge, acquire increased patho- 
genic power from their multiplication in it, as a result of which they 
are able to invade the living tissues. But it appears probable that 
