BACTERIA IN NATURE 15 



gests a treatise on bacteria pathogenic for man and animals. 

 A further analysis of the powers of the disease pro- 

 ducing forms, reveals the interesting fact that these organ- 

 isms may be divided according to their pathogenesis into 

 three quite distinct groups, namely : 



(1) Those that are restricted in their pathogenesis to 

 the human species. 



(2) Those that are restricted in their pathogenesis to 

 -certain species of the lower animals. 



(3) Those that are pathogenic for man and one or more 

 species of the lower animals. 



This classification has reference to natural infection. A 

 number of bacteria that in the natural course of events 

 attack but one species of animals or man, are capable of 

 causing tissue disturbances in other species when they are 

 brought into their tissues. Thus the bacterium of glanders 

 naturally affects the equine family only, but it is patho- 

 genic for guinea pigs when these animals are inoculated with 

 the organism. 



Human bacteriology therefore deals with the life his- 

 tory and properties of the bacteria that produce disease in 

 man. Animal bacteriology includes a study of the species 

 of bacteria that are the cause of diseases in the lower ani- 

 mals. Each necessarily overlaps the other and experimen- 

 tally the use of animals plays an important part in the 

 study of all pathogenic bacteria. 



The lower bacteria have, within certain limits, a uni- 

 form structure, and the fundamental methods employed in 

 their study are the same. The principles of bacteriology, 

 therefore, have a general application in the study of all bac- 

 teria, but for the more exact determination of the properties 

 of different species or groups of organisms, such for exam- 

 ple as the nitrifying bacteria, special methods are necessary. 



