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. 



b. 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 



