12 MICROBIOLOGY 



From the localized habitat of these species, such as the intes- 

 tine for Bacillus coli, large numbers of individuals are car- 

 ried to other places where they multiply if the life condi- 

 tions are suitable, or perish if they are not. The colon 

 bacilli which normally multiply in the intestinal tract and 

 are expelled with the contents, continue to multiply if by 

 chance they are brought into a suitable environment. If, 

 however, they are under conditions unfavorable for their 

 multiplication they soon die. This is equally true of the 

 disease-producing forms which are eliminated from infected 

 individuals. 



Among bacteria, there are a few species that have 

 acquired, if they did not possess it in the beginning, the 

 power to live and multiply in the living tissues of the body. 

 These are spoken of as the parasitic or paratrophic (Fischer) 

 bacteria. They are also spoken of as pathogenic bacteria. 

 However, but a small percentage of the species that thrive 

 on the different mucosae or in the deep layers of the skin are 

 capable under usual conditions of producing morbid changes. 

 The majority of bacteria have for their source of food sup- 

 ply dead organic matter either of animal or plant origin. 

 Such organisms can not exist in living tissues. These are 

 designated as saprophytic or metatrophic (Fischer) bac- 

 teria. 



The role of bacteria in nature. Bacteria play a very 

 important part in nature's economy. Through their func- 

 tional activities the highly complicated tissues of plants and 

 animals ore resolved into simple compounds such as car- 

 bonic acid (C0 2 ), water (H 2 0) and ammonia (NH 3 ), in 

 which form they may be appropriated as nourishment by 

 the more highly organized plants. In this way they are able 

 to remove as scavengers dead organic matter by converting 

 it into food necessary for the life of higher vegetation. This 

 is important, as chlorophyl plants do not possess the power 

 to obtain their carbon and nitrogen directly from such 

 highly organized substances as serve for the nutrition of 

 bacteria. As the production of the simple compounds by 



