CHAPTER VIII.
CHEMICAL ENVIRONMENT (Continued).
GENERAL FOOD RELATIONSHIPS. METABOLISM.
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.
The changes which the above-mentioned types of food material undergo in the various anabolic and katabolic processes within the cell are as yet but very slightly known. Nevertheless there are a number of reactions brought about by bacteria acting on various food materials, partly within but largely without 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 essential for the growth of every bacterium, two of them are of especial importance for the reason that most of the “physiological activities” to be described in the next chapters are centered around their acquisition and utilization. These elements are carbon and nitrogen. Some few of the special activities of certain groups have to do with one or the other of the remaining nine, as will be shown later. But generally speaking when a bacterium under natural conditions secures an adequate supply of carbon and nitrogen, the other elements are readily available in sufficient amount.