14 BACTERIA 



beyond all question by taking cultures from any of the 

 specific disease products, growing them in suitable media 

 for several generations, and inoculating these on living 

 subjects, when the original disease is reproduced. But 

 pathogenic and non-pathogenic are only relative terms. 

 The organisms, usually harmless, under certain conditions 

 become harmful ; while those that are pathogenic by 

 cultivation and otherwise sometimes lose their toxic power, 

 and live and reproduce themselves amongst dead vegetable 

 and animal matter. The bacilli of anthrax, hog cholera, 

 and tetanus illustrate these varying states of activity and 

 change of habit. Microbes which have had their activity 

 reduced or destroyed may, however, under certain con- 

 ditions regain it. 



Both pathogenic and non-pathogenic bacteria are divided 

 into three classes, each distinguished by the form assumed : 

 (a) Micrococci or round cells, such as the sarcina found in 

 the stomach, or the cocci which arrange themselves in 

 clusters or in chains, and cause strangles in horses, and pus 

 formation in all animals ; (6) Bacilli or rod-shaped bacteria, 

 as those of anthrax and glanders, as well as the short ovoid 

 bacillus of pneumonia ; (c) Spirilla or thread-like bacteria, 

 as of relapsing fever in man and the comma-like organisms 

 of cholera. 



In recent years, as the result, in great measure, of the 

 search for pathogenic bacteria, and aided by methods of 

 precision devised for the detection of bacteria in the tissues 

 and for their cultivation outside the body, a large number 

 of pathogenic protozoal and other animal organisms have 

 been described, their history and the methods and channels 

 by which these organisms pass through various phases of 

 development in various hosts. Following Manson and 

 Ross's work on the intermediate phases of development 

 of the malaria organism of man in certain anopheline hosts 

 (mosquitoes) came Bruce's observations on the Nagana 

 or tsetse-fly disease of Southern Central Africa, by which 

 he was able to prove that the trypanosome associated 

 with this condition found an intermediate host in the 

 tsetse- fly (Glossina morsitans). He demonstrated that 

 the tsetse-fly might carry the trypanosome from an animal, 



