Fig. 31.—Beggiatoa alba: a from a fluid containing abundance of sulphuretted hydrogen; b after lying 24 hours in a solution devoid of sulphuretted hydrogen; c after lying an additional 48 hours in a solution devoid of sulphuretted hydrogen, by this means the transverse walls and vacuoles have become visible.
Beggiatoa (parallel with the Blue-green Alga Oscillaria). Long filaments formed of cylindrical cells which are attached by one of the ends, but which are nearly always free when observed. The filaments, like those of Oscillaria, describe conical figures in their revolutions, the free filaments slide upwards and parallel with one another; sheaths are wanting; strongly refractive sulphur drops are found in the interior. The Beggiatoas are the most prevalent Sulphur-bacteria. They occur, very commonly in large numbers, wherever plant or animal remains are decaying in water in which sulphuretted hydrogen is being formed; thus, for example, B. alba (Fig. [31]) occurs frequently as a white covering or slimy film on mud containing organic remains. B. mirabilis is remarkable for its size and its strong peristaltic movements. The Sulphur-bacteria oxidize the sulphuretted hydrogen, and accumulate sulphur in the shape of small granules of soft amorphic sulphur, which in the living cell never passes over into the crystalline state. They next oxidize this sulphur into sulphuric acid, which is immediately rendered neutral by absorbed salts of calcium, and is given off in the form of a sulphate, thus CaCO3 is principally changed into CaSO4. In the absence of sulphur the nutritive processes are suspended, and consequently death occurs either sooner or later. The Sulphur-bacteria may exist and multiply in a fluid which only contains traces of organic matter, in which organisms devoid of chlorophyll are not able to exist. The Beggiatoas very frequently form white, bulky masses in sulphur wells and in salt water, the traces of organic material which the sulphur water contains proving sufficient for them. The cellulose-fermentation, to which the sulphur wells in all probability owe their origin, mainly procures them suitable conditions for existence. The CaCO3 and H2S, formed during the cellulose fermentation by the reduction of CaSO4 is again changed into CaSO4 and CO2 by the Sulphur-bacteria (Winogradsky, 1887).—Other Sulphur-bacteria, the so-called purple Sulphur-bacteria, e.g. B. roseo-persicina, Spirillum sanguineum (Fig. [23]), Bacterium sulfuratum, etc., have their protoplasm mixed with a red colouring matter (bacterio-purpurin) which, like chlorophyll, has the power, in the presence of light, of giving off oxygen (as proved by T. W. Englemann, 1888, in oxygen-sensitive Bacteria). The three purple Sulphur-bacteria mentioned, are, according to Winogradsky, not pleomorphic kinds but embrace numerous species.
Many Spirilli (Spirillum tenue, S. undula, S. plicatile, and others) are found prevalent in decaying liquids.
Bacteria (especially Bacilli) are the cause of many substances emitting a foul odour, and of various changes in milk.
Parasitic Bacteria live in other living organisms; but the relation between “host” and parasite may vary in considerable degree. Some parasites do no injury to their host, others produce dangerous contagious diseases; some choose only a special kind as host, others again live equally well in many different ones. There are further specific and individual differences with regard to the predisposition of the host, and every individual has not the same receptivity at all times.
The harmless parasites of human beings. Several of the above mentioned saprophytes may also occur in the alimentary canal of human beings; e.g., the Hay-bacillus, the Butyric-acid-bacillus, etc.; but the gastric juice prevents the development of others, at all events in their vegetative condition. Sarcina ventriculi, “packet-bacterium,” is only known to occur in the stomach and intestines of human beings, and makes its appearance in certain diseases of the stomach (dilation of the stomach, etc.) in great numbers, without, however, being the cause of the disease. It occurs in somewhat cubical masses of roundish cells (Fig. [25]).
Less dangerous parasites. In the mouth, especially between and on the teeth, a great many Bacteria are to be found (more than fifty species are known), e.g. Leptothrix buccalis (long, brittle, very thin filaments which are united into bundles), Micrococci in large lumps, Spirochæte cohnii, etc. Some of them are known to be injurious, as they contribute in various ways to the decay of the teeth (caries dentium); a Micrococcus, for instance, forms lactic acid in materials containing sugar and starch, and the acid dissolves the lime salts in the external layers of the teeth: those parts of the teeth thus deprived of lime are attacked by other Bacteria, and become dissolved. Inflammation in the tissues at the root of a tooth, is probably produced by septic materials which have been formed by Bacteria in the root-canal.
Dangerous Parasites. In a large number of the infectious diseases of human beings and animals, it has been possible to prove that parasitic Bacteria have been the cause of the disease. Various pathogenic Bacteria of this nature, belonging to the coccus, rod, and spiral Bacteria groups, are mentioned in the following:—
Pathogenic Micrococci. Staphylococcus pyogenes aureus produces abscesses of various natures (boils, suppurative processes in internal organs). The same effects are produced by—
Streptococcus pyogenes, which is the most frequent cause of malignant puerperal fever; it is perhaps identical with—