PARASITIC BACTERIA. 131 



teria which cause diphtheria, tetanus, typhoid, 

 tuberculosis, and many other diseases, produce, 

 even when growing in common culture media, 

 poisons which are of a very violent nature. These 

 poisons when inoculated into the bodies of ani- 

 mals give rise to much the same symptoms as 

 the bacteria do themselves when growing as para- 

 sites in the animals. The chief difference in the 

 results from inoculating an animal with the poison 

 and with the living bacteria is in the rapidity of 

 the action. When the poison is injected the poi- 

 soning symptoms are almost immediately seen ; but 

 when the living bacteria are inoculated the effect 

 is only seen after several days or longer, not, in 

 short, until the inoculated bacteria have had time 

 enough to grow in the body and produce the poi- 

 son in quantity. It has not by any means been 

 shown that all pathogenic germs produce their 

 effect in this way, but it has been proved to be 

 the real method in quite a number of cases, and 

 is extremely probable in others. While some 

 bacteria perhaps produce results by a different 

 method, we must recognise the production of poi- 

 sons as at all events the common direct cause of 

 the symptoms of disease. This explanation will 

 enable us more clearly to understand the relation 

 of different bacteria to disease. 



PATHOGENIC GERMS WHICH ARE NOT STRICTLY 

 PARASITIC. 



Recognising that bacteria may produce poi- 

 sons, we readily see that it is not always neces- 

 sary that they should be parasites in order to 

 produce trouble. In their ordinary growth in 

 Nature such bacteria will produce no trouble. 



