76 CHEMICAL ENVIRONMENT 



Those bacteria whose food consists of dead material are 

 spoken of as saprophytes, while those whose natural habitat, 

 without reference to their food, is in or on other living 

 organisms are called parasites. The host is the organism in 

 or on which the parasite lives. Parasites may be of several 

 kinds. Those which neither do injury nor are of benefit to 

 the host are called non-pathogenic parasites or commensals; 

 many of the bacteria in the intestines of man and other ani- 

 mals are of this class. Those which do injury to the host 

 are called pathogenic or disease-producing, as the organisms 

 causing the transmissible diseases of animals and plants.^ 

 Finally, we have those parasites which are of benefit to and 

 receive benefit from the host. These are called symbionts 

 or symbiotic parasites and the mutual relationship symbiosis. 

 Certain of the intestinal bacteria in man and especially in 

 herbivorous animals are undoubted symbionts, as are also 

 the "root-tubercle bacteria" already mentioned. 



It is also evident that all parasites that may be cultivated 

 outside the body are for the time saprophytic, hence the terms 

 strict parasites and facultative parasites, which should require 

 no further explanation. 



The changes which the above-mentioned tj-pes of food 

 material undergo in the various anabolic and katabolic pro- 

 cesses within the cell are as yet but very slightly known. 

 Nevertheless there are a nimaber of reactions brought about 

 by bacteria acting on various food materials, partly within 

 but largely ^vithout the cell which are usually described as 

 "physiological activities" or "biochemical reactions." Some 

 of these changes are to be ascribed to the utilization of 

 certain of the elements and compounds in these materials 

 as tissue builders, some as energy-yielding reactions and still 

 others as giving rise to substances that are of direct benefit 

 to the organism concerned in its competition with other 

 organisms. 



Though all of the twelve elements already mentioned are 



' The term pathogenic is also applied to certain non-parasitio saprophj'tio 

 bacteria whose produota cause disease conditions, as one of the organisms 

 causing a type of meat-poisoning in man {Bacilhis holulimis) and certain 

 fungi found on spoiled fodder or grain. 



