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stances can attract or repel motile bacteria ; this property is known as positive or negative Chemotaxis.

Shape of the Cell.—Some of the cell shapes which occur most frequently are as follows: Spherical bacteria are

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Fig. 136.–Form and size of various bacteria. ,1. Micrococcus of various sizes.

2. Diplococci. 3. Streptococci. 4. Micrococcus tetragonus. 5. Sarcina ventriculi, package form. 6. Staphylococci. B, 1, 2, 4. Long rods. 3. Short rods. 5. Connected chains of long and short rods. 6. Long threads. 999 (A, 5.78%) (After P. Baumgarten.)

known as cocci (Fig. 136, A), rod-shaped ones, i.e., those which are at least double as long as they are broad, as bacilli or long rods (Fig. 136, B, 1, 2, 4, 5), and those of which the length and breadth are more nearly equal, as bacteria in the true sense) or short rods (Fig. 136, B, 3, 5). If a Bacillus displays (as the result of spore formation) a more or less decided club shape it is called Clostridium (Fig. 137); if curved it is called Vibrio, and if it is screw shaped, Spirillum. The very long thread-like bacteria are known as Cladothrir, Streptothrir, Crenothrix, etc. It is common for the same species to occur in different forms.

Most bacteria are very small, and a powerful magnification is required in order to observe them. The smallest cells are not more than 1 u long. Especially large forms are, e.g., Bacillus oxalaticus, whose rods may be 30 y long and 4 u thick, and Bacterium megatherium, whose rods are 10 u long and 2:5 u broad.

2.-METHODS OF REPRODUCTION. Cell Fission. — As above mentioned, the vegetative multiplication of bacteria proceeds by division or fission; many forms have only this one method of multiplication, whilst others can, in addition, form endospores. Rod bacteria become elongated, and divide by means of a septum ; the latter never occurs lengthways; spherical bacteria do not alter their original appearance before division has taken place. In the latter, division can take place in several directions. The new-formed cells often remain in union with the mother cells, whereby long chains may be formed.

By seeding on various nutrient gelatines the species produce colonies, which have a more or less varied macroscopical appearance, as regards both shape and condition of the surface (slimy, dry). In this connection the method by which the colonies were started is also of influence, whether it was by stab or streak cultures or giant colonies from the seeding of a drop. It has already been said that the colour of the colonies may vary.

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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1010 OcOnf Fig. 137. ---Clostridum butyricum, Prazmowski. A, Vegetative cells. B, Spore

bearing 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 M, while the cell seeded only measured 2 p. 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

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