The foregoing brief review of the chemical composition of the bacterial cell illustrates the variety of compounds which necessarily occurs, but affords no definite clue as to the source of the elements which enter into these compounds. These elements come from the material which the organism uses as food. Under this term are included elements or compounds which serve as building material, either for new cell substance or to repair waste, or as sources of energy. An organism which is capable of making use of an element in the free state is said to be _prototrophic_ for that particular element. Thus aërobes and facultative anaërobes are prototrophic for O. The “root-tubercle bacteria” of leguminous and other plants and certain free living soil organisms are prototrophic for N.[8] On the other hand, if the element must be secured from compounds, then the organism is _metatrophic_ in respect to the element in question. Should the compound be inorganic, the term _autotrophic_ is applied to the organism and _heterotrophic_ if the compound is organic. It is very probable that anaërobes, exclusive of a few nitrogen absorbers, are metatrophic for all the elements they utilize. With the exception of the anaërobes it seems that all bacteria are _mixotrophic_, that is, prototrophic for one or two elements and auto- or heterotrophic for the others.[9] 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 animals 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.[10] 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 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.