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If a nutrient solution remains clear after seeding, the species present is one with pronounced chain or thread growth and without self-movement. Forms which require oxygen for their growth develop a film on the surface of the, as a rule, clear liquid. If the latter is uniformly turbid, the forms are isolated, living and sometimes motile; in this case also the oxygen-absorbing species form a film.

Spore Formation.-Endospores are seen as strongly refractive granules in the interior of the cells; usually each cell contains one spore, seldom two. They are formed by the contraction of the contents which become gradually thicker and more refractive and, finally, bounded by an independent wall. The latter is usually smooth; only one species is known in which a membrane of special structure with longitudinal ridges is to be observed (Arth. Meyer). With certain species the cell assumes a special shape during spore formation, becoming, for example, spindle-shaped (Fig. 137, B, f, h). The spore has a much greater power of resistance to external influences than the vegetative cell; aniline dyes are taken up by it much more slowly than by the remaining plasma; on the other hand these colours are obstinately retained when a decolorising reagent is afterwards employed. Extreme degrees of cold and heat, besides complete drying, are endured by the spores without death ensuing. On the contrary, light appears, in the generality of cases, to be the deadliest enemy of bacterial spores. The spores of several species resist a boiling temperature for some hours; their germinating power may even be increased by this means.

Through the great resisting power of bacterial spores, which is more marked in the dry than in the moist state, sterilisation is found to be difficult in many cases, e.g., in order to sterilise water it must be boiled under pressure, or, if this is not convenient, raised to boiling temperature

several times at intervals (see p. 78). Wort is often not sterile after boiling; that this is the case can be seen if a sample is added to yeast water, when frequently a growth of bacteria makes its appearance. That no growth appears in flasks of wort after simple boiling may be due to the fact that the culture medium is not favourable for the germination of the spores present. Wort is indeed a liquid in which many species of bacteria, even in vigorous condition, are not able to grow.

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FIG. 137.-Clostridum butyricum, Prazmowski. A, Vegetative cells. B, Sporebearing cells, with the exception of a and b. In f and h, the cells are swollen to spindle shapes, the formation of the spore being here complete. Two spores

have grown in g. C, Germinating spores. (After Prazmowski.)

As soon as the spore is ripe it is able, under favourable conditions, to germinate. Before germination proceeds the spore swells out and thereby loses its brilliancy, the skin then bursting and a small knob protruding. This protrusion can take place either at the pole of the spore (Fig. 137, C), the new cell continuing its growth lengthwise from the spore, or the spore skin bursts in the middle and the young cell grows

in a direction perpendicular to the longer axis of the spore. These are the two commonest types of germination of bacterial spores; and other ways are quite exceptional.

The causes of the production of spore formation are to be sought in unfavourable nutrimental conditions, accumulation of injurious products of growth, etc. Like the saccharomycetes, certain bacterial forms require free access of air in order to form spores.

3.-VARIATION.

Hansen's experiments with the acetic acid bacteria: Bacterium aceti, Bact. Pasteurianum and Bact. Kützingianum, have shown that temperature is a factor for influencing shape. His conjecture that the results found by him have a more general application was confirmed by the author's experiments with four other species. Henneberg came to the same conclusion from experiments with Bact. oxydans and Bact. acetosum. The investigations of Hansen demonstrated that the species named appear with three different cell forms, viz., sometimes as short rods in chains (Figs. 139 a, 144, 145 and 146), sometimes as threads (Figs. 138, 139 and 140), and at other times as pear-shaped or globular swellings (Fig. 140). When young, vigorous cells are seeded in a favourable culture medium rich in extract, e.g., "double" beer (Danish beer with little alcohol and high extract) or wort, at a temperature between 5° and 34° C., the chain form with the short rods appears; if, now, such a growth be inoculated in the culture medium of a new flask and kept at 40° to 40° C., the cells are quickly changed into threads (Figs. 138 and 139). The latter can attain a length of 500 μ, while the cell seeded only measured 2 μ. If the thread form be now brought again to 34° C., the length continues to increase; here and there globular, spindle, or pear-shaped swellings may occur, whereupon the threads begin to divide

again and to assume the chain form (Fig. 140). By Nageli and others such swollen forms were regarded as not belonging to normal development, but as an indication that the cells are about to die. It is shown by Hansen's above-mentioned

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FIG. 138. Bacterium Pasteurianum, Hansen. The thread form after twenty-four hours in "double" beer at 40° to 40° C. 1900. (After Hansen.)

investigations that, under certain culture conditions, these peculiar forms appear normally and indicate a vigorous growth.

Beyond the above-mentioned investigations on acetic

acid bacteria, only few and incomplete contributions to this subject are as yet to be found. Macfadyen and Blaxall found that some thermophilic bacteria investigated by them also formed long threads at high temperatures (approaching 70° C.).

It is generally known that among organisms different conditions can produce identical or nearly corresponding

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FIG. 139.-Bacterium Pasteurianum, Hansen. Development of the thread form by cultivation on "double" beer agar-agar in a Böttcher's chamber at about 40° C. a, A chain consisting of 8 members; a', the same after 6; a", after 10; a", after 20 hours. b, A five-membered chain; b', after 5; b", after 9 hours. c, Development after 10; d, after 21 hours. The times are reckoned from the beginning of the experiment. 1999. (After Hansen.)

forms of development. It is therefore not remarkable that various authors have stated that the change of shape referred to can also be obtained as the result of the influence of factors other than temperature; H. Buchner has found, in regard to Bacillus subtilis, that the chemical composition of the culture medium was responsible for similar changes of shape.

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