90 BACTERIOLOGY. 
Still further removed from the saprophytic bacteria 
are those which grow in the blood and most living 
tissues as readily as in the most suitable artificial 
media. Thus a streptococcus which has passed 
through a number of animals or human beings will, 
when introduced into the circulation or the tissues, 
develop as rapidly and generally as in bouillon, and 
produce death within twenty-four hours, every drop of 
blood being crowded with bacteria. 
Finally, there are bacteria which, in so far as we 
know, find the bodies of human beings or animals the 
only fit soil for their growth. These are the true 
parasites. The leprosy bacillus grows only in man; 
neither the food nor the conditions suitable for the de- 
velopment of this micro-organism outside of the body 
have as yet been discovered. The spirillum of relapsing 
fever is another good example of this group. 
Following rather closely the schematic separation of 
bacteria according to their relation to disease we might 
classify them as : 
1. Strict saprophytes, or bacteria which grow readily 
in suitable dead organic material, but not in the body 
under ordinary conditions. 
a. Bacteria which in their growth produce no sub- 
stances which are poisonous to the body, or at least none 
capable of absorption. 
6. Bacteria which produce in their growth in dead 
organic matter sufficient poisons to cause sickness if 
they are absorbed into the animal body. 
2. Facultative Saprophytes, These are bacteria which 
can develop either as parasites or saprophytes. The 
different varieties vary as to the amount of poison 
which they produce. Some grow luxuriantly in dead 
