Carbon is necessary not only because it is an essential constituent of protoplasm but because its oxidation is the chief source of the energy necessary for the internal life of the cell, though nitrogen and sulphur replace it in this function with a few forms. This latter use of carbon constitutes what may be called its respiratory function. Bacteria like other organisms in their respiration utilize oxygen and give off carbon dioxide. The amount of the latter given off from the cell in this way is very small as compared with that which is frequently produced as an accompaniment of other reactions (see [Fermentation], next chapter). But there is no doubt of its formation and it has been determined by a few investigators. On account of this use of carbon, bacteria require relatively large amounts of this element. One group of bacteria concerned in the spontaneous heating of coal seems to be able to use free carbon from this material. Another group is said to be able to oxidize marsh gas, CH4, and use this as its source of carbon. The nitrite, nitrate and sulphur bacteria mentioned later utilize carbon dioxide and carbonates as their carbon supply, and one kind has been described which uses carbon monoxide. With these few exceptions bacteria are dependent on organic compounds for their carbon and cannot use CO2 as green plants do.
The oxygen requirement is high partly for the same reason that carbon is, i.e., respiration. Oxygen is one of the constituents of protoplasm, and combined with hydrogen forms water which makes up such a large part of the living cell. Anaërobic bacteria are dependent on so-called “molecular respiration” for their energy. That is, through a shifting or rearrangement of the atoms in the compounds used as food the oxidation of carbon is brought about. Enzymes are probably responsible for this action. Carbon dioxide is produced by anaërobes as well as by aërobes, and frequently in amounts readily collected. A carbohydrate is usually though not always essential for the growth of anaërobes and serves them as the best source of energy.
Nitrogen is the characteristic element of living material. Protoplasm is a chemical substance in unstable equilibrium and nitrogen is responsible for this instability. No other of the commoner elements is brought into combination with such difficulty, nor is so readily liberated when combined (all commercial explosives are nitrogen compounds). Bacteria, like other forms of protoplasm, require nitrogen. More marked peculiarities are shown by bacteria with reference to the sources from which they derive their nitrogen than for carbon. Some can even combine the free nitrogen of the air and furnish the only natural means of any importance for this reaction. Some few forms (the nitrite and nitrate formers, [Chapter XI]) obtain their energy from the oxidation of inorganic nitrogen compounds, ammonia and nitrites respectively, and not from carbon. These latter bacteria use carbon from carbon dioxide and carbonates. A great many bacteria can secure their nitrogen from nitrates but some are restricted to organic nitrogen. Many bacteria obtain their carbon from the same organic compounds from which their nitrogen is derived.
Sulphur serves mainly as a constituent of protein compounds in the protoplasmic structure. In some of the sulphur bacteria it is a source of energy, since either free sulphur or H2S is oxidized by them. Some of these bacteria can obtain their carbon from CO2 or carbonates, and their nitrogen from nitrates or ammonium salts.
Whether the iron bacteria, belonging to the genus Crenothrix of the higher, thread bacteria, use this element or its compounds as sources of energy is still a disputed question. The evidence is largely in favor of this view.
Free hydrogen has been shown to be oxidized by some forms which obtain their energy in this way.
Whether there is a special class of phosphorus bacteria remains to be discovered. That phosphorus is oxidized during the activity of many bacteria is undoubted, but whether this represents a source of energy or is the accidental by-product of other activities is undetermined.
Practically nothing is known about the metabolism of the other elements as such.
From the preceding brief review of the relation of certain bacteria to some of the elements in the free state and from the further fact that there is scarcely a known natural organic compound which cannot be utilized by some kind of bacterium, it is evident that this class of organisms has a far wider range of adaptability than any other class, and this adaptability helps to explain their seemingly universal distribution.
As to the metabolism within the cell, no more is known than is the case with other cells, nor even as much. The materials used for growth and as sources of energy are taken into the cell, built up into various compounds some of which have been enumerated and in part broken down again. Carbon dioxide and water are formed in the latter process. What other katabolic products occur it is not easy to determine. Certainly some of the substances mentioned in the next chapters are such products but it is not always possible to separate those formed inside the cell from those formed outside. Perhaps most of the latter should be considered true metabolic products. It would seem that on account of the simplicity of structure of the bacterial cell and of the compounds which they may use as food they would serve as excellent objects for the study of the fundamental problems of cell metabolism. Their minuteness and the nearly impossible task of separating them completely from the medium in or on which they are grown makes the solution of these problems one of great difficulty.