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.
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.
When all of the environmental conditions necessary for the best development of a given bacterium are fulfilled, it will then develop to the limit of its capacity. This development is characterized essentially by its reproduction, which occurs by transverse division. The rate of this division varies much with the kind even under good conditions. The most rapid rate so far observed is a division in eighteen minutes. A great many reproduce every half-hour and this may be taken as a good average rate. If such division could proceed without interruption, a little calculation will show that in about sixty-five hours a mass as large as the earth would be produced.
| ½ hour | = | 2 | ||
| 1 hour | = | 4 | ||
| 2 hours | = | 16 | ||
| 4 hours | = | 256 | ||
| 5 hours | = | 1024 | = | 103+ |
| 15 hours | = | 1,000,000,000 | = | 109 = 0.5 cc. |
| 35 hours | = | 1021+ | = | 500.0 cu.m. |
| About 65 hours | = | 2 × 1042+ | = | 5 × 1020 cu.m. = a mass as large as the earth. |
Such a rate of increase evidently cannot be kept up long on account of many limiting factors, chief of which is the food supply.
The foregoing calculation is based on the assumption that the organism divides in one plane only. If it divides in 2 or 3 planes, the rate is much faster, as is shown by the following formulæ, which indicate the theoretical rate of division:
With two-plane or three-plane division, assuming that each organism attains full size, as was assumed in the first calculation, the “mass as large as the earth” would be attained in about thirty-two and twenty-two hours respectively.
This extraordinary rate of increase explains in large measure why bacteria are able to bring about such great chemical changes in so short a time as is seen in the rapid “spoiling” of food materials, especially liquids. The reactions brought about by bacteria on substances which are soluble and diffusible are essentially “surface reactions.” The material diffuses into the cell over its entire surface with little hindrance. The bacteria are usually distributed throughout the medium, so that there is very intimate contact in all parts of the mass which favors rapid chemical action. The following calculation illustrates this:
It is not uncommon to find in milk on the point of souring 1,000,000,000 bacteria per cc.
Assuming these to be cocci of 1µ diameter the volume of these bacteria in a liter is only 0.05 cc. or in the liter there would be 19999 parts of milk and only 1 part bacteria. The surface area of these bacteria is 3141.6 sq. cm. With this large surface exposed, it is not strange that the change from “on the point of souring” to “sour” occurs within an hour or less.
Although large numbers of bacteria can and do cause great chemical changes the amount of material actually utilized for maintenance of the cell is very slight, infinitesimal almost, and yet is fairly comparable to that required for man, as is illustrated by the following computations:
E. Kohn has shown that certain water bacteria grew well in water to which there was added per liter 0.000002 mg. dextrose, 0.00000007 mg. (NH4)2SO4 and 0.0000000007 mg. (NH4)2HPO4. The bacteria numbered about 1000 per cc. Taking the specific gravity at 1 (a little too low) the mass of the bacteria in the liter was about 0.001 mg. Hence the bacteria used 0.002 of their weight of carbohydrate and 0.00007 of ammonium sulphate. A 150-pound (75-kilo) man can live on 375 g. of sugar (0.005 of his weight) and 52.5 g. of protein (0.0007 of his weight). From these figures it can be calculated that the man utilizes about two and a half times as much carbohydrate and about seven times as much nitrogen as the bacterium, relatively speaking.