142 BACTERIOLOGY. 
PATHOGENESIS AND CULTIVATION OF BACTERIA. 
Into what two clinical classes may all bacteria be divided ? 
Into pathogenic or disease-producing organisms, and non-pathogenic 
or those that do not produce disease. This classification is not abso- 
lute, however, as an organism that may at one time produce disease, 
may be so modified by external influences that it loses its pathogenic 
property and vice versa. 
To what is the action of pathogenic bacteria thought to be due? 
To the excretion of certain poisonous substances called ptomaines. 
How may pathogenic bacteria be deprived of their poisonous 
qualities ? 
By successive transplantations in artificial culture media they become 
less and less virulent until finally they may become entirely innocuous. 
Also by growing them at high temperatures they quickly lose their 
poisonous qualities. 
In what way may the invasion of pathogenic bacteria be checked 
in the body ? 
In blood plasma there is some chemical material, the nature of which 
is not well understood, that certainly has the power of resisting the 
attacks of bacteria. Some authors attribute this power to the alka- 
linity of the blood plasma. 
What other theory has been brought forward to explain the resist- 
ing powers of the body against these pathogenic invaders ? 
The phagocytic theory. It is thought that the white cells of the 
blood are ever ready to attack invading bacteria. That they surround 
and devour them, or that they become so thickly massed about the 
invaders as to cut off their supply of nutriment. 
Do all forms of bacteria affect the living tissues of the body ? 
No. Many forms may be found upon the skin and mucous mem- 
branes, throughout the alimentary tract and even in open wounds, 
which seem to give rise to no more disturbance than so much inorganic 
dust would. These organisms may all be classed as non-pathogenic. 
How do pathogenic or disease-producing bacteria affect the living 
tissues ? 
The cells of the tissue in which they may be present may become 
swollen and lose their nuclei, and be changed into granular masses or 
be totally disintegrated. The intercellular substance may soften and 
disintegrate. The walls of the bloodvessels in their vicinity may die, 
