In certain species of the lower bacteria, under certain circumstances, changes take place in the protoplasm which result in the formation of bodies called spores, to which the vital activities of the original bacteria are transferred. Spore formation occurs chiefly among the bacilli and in some spirilla. Its commencement in a bacterium is indicated by the appearance in the protoplasm of a minute highly refractile granule (or by a number of minute highly refractile granules scattered about throughout the protoplasm which gradually coalesce) unstained by the ordinary methods. This increases in size, and assumes a round, oval, or short rod-shaped form, always shorter but often broader than the original bacterium. In the process of spore formation the rest of the bacterial protoplasm may remain unchanged in appearance and staining power for a considerable time (e.g. B. tetani or, on the other hand, it may soon lose its power of staining and ultimately disappear, leaving the spore in the remains of the envelope (e.g. B. anthracis). This method of spore formation is called endogenous. Bacterial spores are always non-motile. The spore may appear in the centre of the bacterium, or it may be at one extremity, or a short distance from one extremity. In structure the spore consists of a mass of protoplasm surrounded by a dense membrane.
This can be demonstrated by methods which will be described, the underlying principle of which is the prolonged application of a powerful stain. The membrane is supposed to confer on the spore its characteristic feature, namely, great capacity of resistance to external influences such as heat or noxious chemicals. Koch, for instance, in one series of experiments, found that while the bacillus anthracis in the unspored form was killed by a two minutes' exposure to I per cent carbolic acid, spores of the same organism resisted an exposure of from one to fifteen days.
When a spore is placed in suitable surroundings for growth it again assumes the original bacillary or spiral form. The capsule dehisces either longitudinally, or terminally, or transversely. In the last case the dehiscence may be partial, and the new individual may remain for a time attached by its ends to the hinged spore-case, or the dehiscence may be complete and the bacillus grow with a cap at each end consisting of half the spore-case. Sometimes the spore-case does not dehisce, but is simply absorbed by the developing bacterium.
It is important to note that in the bacteria spore formation is rarely, if ever, to be considered as a method of multiplication. In at least the great majority of cases only one spore is formed from one bacterium, and only one bacterium in the first instance from one spore. Sporulation is to be looked upon as a resting stage of a bacterium, and is to be contrasted with the stage when active multiplication takes place. The latter is usually referred to as the vegetative stage of the bacterium. Regarding the signification of spore formation in bacteria there has been some difference of opinion. According to one view it may be regarded as representing the highest stage in the vital activity of a bacterium. There is thus an alternation between the vegetative and spore stage, the occurrence of the latter being necessary to the maintenance of the species in its greatest vitality. Such a rejuvenescence, as it were, through sporulation, is known in many algae. In support of this view there are certain facts. In many cases, for instance, spore formation only occurs at temperatures specially favourable for growth and multiplication. There is often a temperature below which, while vegetative growth still takes place, sporulation will not occur, and in the case of B. anthracis, if the organism be kept at a temperature above the limit at which it grows best, not only are no spores formed, but the species may lose the power of sporulation. Furthermore, in the case of bacteria preferring the presence of oxygen for their growth, an abundant supply of this gas may favour sporulation. Most bacteriologists are, however, of opinion that when a bacterium forms a spore, it only does so when its surroundings, especially its food supply, become unfavourable for vegetative growth ; it then remains in this condition until it is placed in more suitable surroundings. Such an occurrence would be analogous to what takes place un der similar conditions in many of the protozoa. Often sporulation can be prevented from taking place for an indefinite time if a bacterium is constantly supplied with fresh food (the other conditions of life being equal). The presence of substances excreted by the bacteria themselves plays, however/ a more important part in making the surroundings unfavourable than the mere exhaustion of the food supply. A living spore will always develop into a vegetative form if placed in a fresh food supply. With regard to the rapid formation of spores when the conditions are favourable for vegetative growth, it must be borne in mind that in such circumstances the conditions may really very quickly become unfavourable for a continuance of growth, since not only will the food supply around the growing bacteria be rapidly exhausted, but the excretion of effete and inimical matters will be all the more rapid.
We must note that the usually applied tests of a body de veloped within a bacterium being a spore are (1) its staining reaction, namely, resistance to ordinary staining fluids, but capacity of being stained by the special methods devised for the purpose ; (2) the fact that the bacterium containing the spore has higher powers of resistance against inimical conditions than a vegetative form. It is important to bear these tests in mind, as, in some of the smaller bacteria especially, it is very difficult to say whether they spore or not. There may appear in such organisms small unstained spots the significance of which it is very difficult to determine.
The Question of Arthrosporous Bacteria
It was stated by Hueppe that among certain organisms, e.g. some streptococci, certain individuals may, without endogenous sporulation, take on a resting stage. These become swollen, stain well with ordinary stains, and they are stated to have higher Power of resistance than the other forms ; further, when vegetative life again occurs it is from them that multiplication is said to take place. From the fact that there is no new formation within the protoplasm, but that it is the whole of the latter which participates in the change, these individuals have been called arthrospores. The existence of such special individuals amongst the lower bacteria is extremely problematical. They have no distinct capsule, and they present no special staining reactions, nor any microscopic features. By which they can be certainly recognised, while their alleged increased powers of resistance are very doubtful. All the phenomena noted can be explained by the undoubted fact that in an ordinary growth there is very great variation among the individual organisms in their powers of resistance to external conditions.