The Higher Bacteria group includes more highly organised members of the Schizomycetes. They possess filaments, which may be branched, and almost always have septa and a sheath. Perhaps the most marked difference from the lower bacteria is in their reproduction. In the higher bacteria we have what is in fact a flower—terminal fructification by conidia. In this group of vegetables we have the Beggiatoa, Leptothrix, Cladothrix, and, at the top, the Streptothrix. It has been demonstrated that Streptothrix actinomycotica and Streptothrix maduræ are the organismal cause, respectively, of Actinomycosis and Madura-foot, two diseases which have hitherto been obscure.
Pleomorphism. This term designates an irregular development of a species. Different media and external conditions bring about in protoplasm as susceptible as mycoprotein a variety of morphological phases. These may occur in succession, and represent different stages in the life-history of a bacterium, or they may be involution forms resulting from a change of environment, and occurring as "faults" in the species. In the Bacillus coli, B. typhosus, bacillus of Plague, and B. tuberculosis pleomorphism undoubtedly occurs, and is manifest in the change of shape. This is particularly marked in old cultures of the last named. The ordinary well-known bacillus may grow out into threads, with bulbous endings, granular filaments, drumsticks, and diplococcal forms. Speaking generally, the older the culture, the more marked is the variation.
Polymorphism is a term used to define the theory which held that bacteria were one of the intermediate shapes or forms between something lower and something higher in the vegetable kingdom. Neither pleomorphism nor polymorphism is fully understood, and many bacteriologists find shelter from both in the term involution form. What we do know is that the species already named, for example, take on divers forms when placed under different conditions.
Composition. From what we have seen of the diet of micro-organisms, we shall conclude that in some form or other they contain the elements nitrogen, carbon, and hydrogen. All three substances are combined in the mycoprotein or protoplasm of which the body of the microbe consists. This is generally homogeneous, and there is no sign of a nucleus. It possesses a fortunate affinity for aniline dyes, and by this means organisms are stained for the microscope. Besides the variable quantity of nitrogen present, mycoprotein may also contain various mineral salts. The uniformity of the cell protoplasm may be materially affected by disintegration and segmentation due to degenerative changes. Vacuoles also may appear from a like cause, which it is necessary to differentiate from spores. Two other signs of degeneration are the appearance of granules in the body of the cell protoplasm known as metachromatic granules, owing to their different staining propensities, and the polar bodies which are seen in some species of bacteria. Surrounding the mass of mycoprotein, we find in most organisms a capsule or membrane composed, in part at least, of cellulose. This sheath plays a protective part in several ways. During the adult stage of life it protects the mycoprotein, and holds it together. At the time of reproduction or degeneration it not infrequently swells up, and forms a viscous hilum or matrix, inside which are formed the new sheaths of the younger generation. It may be rigid, and so maintain the normal shape of the species, or, on the other hand, flexible, and so adapted to rapid movement of the individual.
Normal and Pleon-Forms of the Bacillus of Tubash.
Here, then, we have the major parts in the constitution of a bacillus—its body, mycoprotein; its capsule, cellulose. But, further than this, there are a number of additional distinctive characteristics as regards the contents inside the capsule which call for mention. Sulphur occurs in the Beggiatoa which thrive in sulphur springs. Starch is commoner still. Iron as oxide or other combination is found in several species. Many are highly coloured, though these are generally the "innocent" bacteria, in contradistinction to the disease-producing. A pigment has been found which is designated bacterio-purpurin. According to Zopf, the colouring agents of bacteria are the same as, or closely allied to, the colouring matters occurring widely in nature. Migula holds that most of the bacterial pigments are non-nitrogenous bodies. There are a very large number of chromogenic bacteria, some of which produce exceedingly brilliant colours. Among some of the commoner forms possessing this character are Bacillus et micrococcus violaceus (violet); B. et M. aurantiacus (orange); B. et M. luteus (yellow); M. roseus (pink); many of the Sarcinæ; B. aureus (golden-yellow); B. fluorescens liquefaciens et non-liquefaciens (green); B. pyocyaneus (green); B. prodigiosus (blood-red).
Motility. When a drop of water containing bacteria is placed upon a slide, a clean cover glass superimposed, and the specimen examined under an oil immersion lens, various rapid movements will generally be observed. These are of four kinds: (1) A dancing stationary motion known as Brownian movement. This is molecular, and depends in some degree upon heat and the medium of the moving particles. It is non-progressive, and is well known in gamboge particles. (2) An undulatory serpentine movement, with apparently little advance being made. (3) A rotatory movement, which in some water bacilli is very marked, and consists of spinning round, with sometimes considerable velocity, and maintained for some seconds or even minutes. (4) A progressive darting movement, by which the bacillus passes over some considerable distance.
The conditions affecting the motion of bacteria are but partly understood. Heating the slide or medium accelerates all movement. A fresh supply of oxygen, or indeed the addition of some nutrient substance, like broth, will have the same effect. There are also the somewhat mysterious powers by which cells possess inherent attraction or repulsion for other cells, known as positive and negative chemiotaxis. These powers have been observed in bacteria by Pfeiffer and Ali-Cohen.
