222 MODES OF ACTION. 
parasite, may be so modified, by cultivation for successive genera- 
tions in a culture medium containing glycerin, that it will finally 
grow in ordinary beef infusion, thus showing a tendency to adapt 
itself to a saprophytic mode of life. 
Some of the saprophytic bacteria are indirectly pathogenic by 
reason of their power to multiply in articles of food, such as milk, 
cheese, fish, sausage, etc., and there produce poisonous ptomaines 
which, when these articles are ingested, give rise to various morbid 
symptoms, such as vomiting, gastric and intestinal irritation, fever, 
ete. Or similar symptoms may result from the multiplication of 
bacteria producing toxic ptomaines in the alimentary canal. No 
doubt gastric and intestinal disorders are largely due to this cause, 
and may be induced by a variety of saprophytic bacteria when these 
establish themselves in undue numbers in any portion of the ali- 
mentary tract. In Asiatic cholera the same thing occurs, but with 
more fatal results from the introduction of the East Indian cholera 
germ discovered by Koch. This is pathogenic for man, because it is 
able to multiply rapidly in the human intestine, and there produces a 
toxic substance which, being absorbed, gives rise to the morbid pheno- 
mena of the disease. The spirillum itself does not enter the blood or 
invade the tissues, except to a limited extent in the mucous coat of 
the intestine, and the true explanation of its pathogenic power is no 
doubt that which has been given. 
Other microérganisms invade the tissues and multiply in cer- 
tain favorable localities, but have not the power of developing in the 
blood, in which they are only found occasionally and in very small 
numbers or not at all. Thus the typhoid bacillus locates itself in the 
intestinal glands, in the spleen, and in the liver, forming colonies of 
limited extent, and evidently not finding the conditions extremely 
favorable for its growth, inasmuch as it does not take complete pos- 
session of these organs. The symptoms which result from its pre- 
sence are doubtless partly due to local irritation, disturbance of func- 
tion, and, in the case of the intestinal glands, necrotic changes 
induced by it. But in addition to this its pathogenic action depends 
upon the production of a poisonous ptomaine which has been isolated 
and studied by the German chemist Brieger (typhotoxin), 
Certain saprophytic bacteria, when injected beneath the skin of a 
susceptible animal, multiply at the point of inoculation and invade 
the surrounding tissues, giving rise in some instances to the forma- 
tion of a local abscess, in others to an infiltration of the tissues with 
bloody serum, and in others to extensive necrotic changes. These 
local changes are due not simply to the mechanical presence of the 
microorganisms which induce them, but to chemical products evolved 
during the growth of these pathogenic bacteria. Indeed, their patho- 
