DEFINITIONS AND LIMITS 85 



numerous, chemically the most active, and economically the most 

 important members of the phylum Bacteriacese. They are rarely 

 pathogenic, that is, they rarely initiate disease in man or the lower 

 animals. Whenever they are found associated with morbid processes 

 their presence is usually to be explained on the ground that they are 

 secondary invaders. 



A smaller group of bacteria are parasitic, that is, they exist upon 

 the bodies of living plants, animals or men. Many of them are rarely 

 met with in Nature far removed from their respective hosts. Their 

 activities are not usually in opposition to those of their host and their 

 presence is therefore unnoticed. They may become invasive, how- 

 ever, whenever the natural barriers, which ordinarily suffice to keep 

 them out, are impaired. 



From the parasitic bacteria there has been gradually evolved a 

 small but formidable group of organisms, the pathogenic bacteria, 

 whose activities are in partial opposition to those of their host. The 

 pathogenic bacteria, like the parasitic bacteria, require a living host, 

 but they differ from the parasitic forms in that they actually invade 

 their hosts and induce progressive disease from host to host. 



There are no sharply definable limits between these three groups 

 of bacteria, the saprophytic, parasitic, and pathogenic; the latter 

 appear to have arisen from the former by a process of evolution. 

 Certain general modifications in the general types of chemical activity 

 manifested by these groups are discernible, however, which are partly 

 the result and partly the cause of their change in environment as 

 they have passed from a saprophytic to a parasitic existence. Promi- 

 nent among these modifications and activities is a gradual decrease 

 in the intensity with which the parasitic and pathogenic bacteria act 

 upon their environment. 



The essential function of the saprophytic bacteria in Nature is to 

 effect a rapid, deep-seated degradation of organic matter to simple 

 compounds; these organisms decompose a relatively large amount 

 of substance in a relatively short time. They are chemically active 

 and many of them form highly resistant spores which enable them 

 to survive prolonged periods of environmental vicissitude. The 

 habitually parasitic bacteria, on the other hand, which exist upon 

 the bodies of living animals, and the progressively pathogenic bacteria 

 which develop within the tissues of animals are not subjected to 

 extremes of temperature and food supply; they rarely or never form 

 spores. The chemical activity of these organisms is usually much 



