34 
BACTERIOLOGY. 
cocci are frequently present, but do no harm unless the 
vitality is lowered. The stomach is generally free 
from, bacteria, due to the acid in its secretions. If 
however there is any disturbance of digestion and the 
secretions are no longer acid, the bacteria swallowed 
in the food may cause fermentation and other dis¬ 
orders. The intestine harbors great numbers of bac¬ 
teria, chiefly the colon bacillus) and others closely 
allied to it. They are, in health, not only harmless, 
but of much benefit in breaking down the food into 
substances that can be absorbed for nutriment of the 
tissues. Under conditions of lowered resistance or 
when injury to the intestines has been done, they may 
cause infection. 
After infection has taken place it may remain 
localized in the form of a boil or abscess, or it may 
spread so that the blood contains the infecting organ¬ 
ism. When infections become generalized the condi¬ 
tion is called septicemia, and when there is added to 
this scattered areas of pus formation throughout the 
body the condition is called pyemia. Toxemia is the 
condition caused by the poisons of bacteria, either in 
locaj or general infections. 
How do bacteria produce injury to the tissues? 
In two ways: The multiplication of bacteria in the 
tissues may cause injury in a mechanical way by 
obstructing the very small blood-vessels, causing the 
necrosis or death of the tissue. The absorption of the 
necrotic material gives rise to the symptoms of infec¬ 
tion. Much greater injury is produced by the absorp- 
