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THE MORPHOLOGY OF BACTERIA.

BACTERIA are the smallest and simplest of living creatures, and can only be seen with the aid of high magnification by the microscope. They are the lowest members of the vegetable kingdom, and differ from unicellular animal organisms in the fact that, unlike the latter, they are nourished through the cell wall by substances held in solution. They are also peculiar in their mode of growth, and in forming spores (or seeds). Their relation to the vegetable kingdom can be easily seen from the following table :

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(grow in filaments and may have special elements of reproduction), e.g., sulphur and ironcontaining bacteria, streptothrix actinomyces, etc.

Lower forms

bacteria proper

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A study of the above table enables us to formulate a definition of bacteria, which would be: minute unicellular organisms multiplying by transverse fission, and containing no chlorophyll.

In structure they resemble an ordinary cell with its protoplasm and a limiting membrane, but no nucleus has been demonstrated. The protoplasm consists of a structureless albuminous substance, which in its reaction to aniline dyes resembles the nuclei of ordinary cells. The living substance may be broken up by vacuoles; or it may contain iron, sulphur and various pigments.

The cell-membrane is composed of a substance allied to cellulose. It sometimes swells and gives rise to a gelatinous envelope, which cements together adjacent bacilli. A colony (zooglæa) is thus formed; but each cell remains. distinct, and there is no attempt at a division of labour.

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In form the bacteria are usually either spheres, rods or spirals; although atypical forms are not uncommon. old cultures they readily undergo degeneration or "involution," and various irregular and distorted forms are produced.

When the bacterial cell is in the form of a sphere it is called a coccus or micrococcus. On the other hand, when the cell is not iso-diametrical, but exhibits a difference between length and breadth, it is a bacillus. If the rod is bent like a German comma it is called a vibrio. When the rod is longer and spirally twisted we get a spirillum; while numerous turns like those of a corkscrew give rise to a spirochote.

The mode of reproduction is very characteristic. The cell elongates and then divides in the middle into two halves, each of which repeats the process. It is to be noted that these organisms are not generated, neither do they die. They merely divide. The protoplasm of the parent still exists in the daughter cells, and, barring accidents, is immortal.

If the daughter cells, instead of being detached, remain connected with one another, they give rise to character

istic groupings. Thus, the cocci may be arranged in pairs (diplococci); or in clusters like a bunch of grapes (staphylococci); or in the form of a chain (streptococci). If the division in the case of cocci takes place in two directions we get tetrads; and if in three directions we have sarcina, a common content of diseased stomach.

Among bacilli, owing to the relation between the transverse and longitudinal diameters, there is room for a wider range of morphological characters. Thus they

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differ not only in size and mode of grouping, but also in form.

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The bacteria are so minute in size that a special standard of measurement has to be employed. This is the micromillimetre, or 1.000 of a millimetre, and is represented by the Greek letter μ. It is equivalent to 25.000 of an inch. Bacteria may vary in size from 0.3 to 30 μ. μ An average-sized coccus would not be more than 1 μ in diameter; so that 25,000 of such individuals placed in a row would make a chain one inch long. Even the comparatively largesized anthrax bacillus is only 3 to 10 μ long and 1 to 1.2 μ

broad, so that its volume would have to be increased eight million times before it attained to the bulk of an ordinary cigarette.

Being devoid of chlorophyll, micro-organisms can only feed on organic matter, which is presented to them in a state in which it can be readily absorbed. The sources of food being, however, plentiful, the rate of their multiplication is simply enormous. Under favourable circumstances bacteria have been observed to divide in about half an hour, and it is calculated that in one day a single cholera germ would produce a progeny of sixteen hundred trillion. In nature, however, it is hardly likely that so great an increase ever takes place. Lack of food and improper physical conditions very often bring about a rapid diminution in their number.

Spore Formation.-Under the influence of conditions, some of which favour and others retard their growth, bacilli enter into a resting stage, and assume a shape endowed with great powers of resistance. The protoplasm shrinks from the cell membrane, and gathers into one or more rounded masses or spores. Each spore, when ripe, appears as a brilliant body surrounded by a highly resistant spore wall. The capsule finally bursts, and the spherical body is set free.

Like the seeds of higher plants, spores resist desiccation and heat, and may retain their vitality for months or years. They successfully resist a temperature of 100° C. for a short time; whereas ordinary bacteria succumb to a temperature of 60° C. They may be said to be the hardiest form of living substance known.

When the conditions are again favourable for germination the membrane bursts, and the contents grow out as a bacillus.

Pathogenic organisms do not form spores so long as

they are enclosed within the body; but they may do so in their saprophytic or external phase of existence. The question whether a certain organism forms spores is an important one from the hygienic standpoint, for it gives us a measure of the resistance of the organism, and therefore of the risk of infection. Non-sporing organisms are easily destroyed, and their infection readily controlled.

Sporulation must not be confused with reproduction. The former is a guise for the preservation of the species; the latter is concerned with its propagation. A spore gives rise to a single individual; but an organism divides into two.

Locomotion.-Bacteria, like all solid particles when suspended in fluids, exhibit the so-called Brownian movements, which consist of oscillations, without any actual movement of translation. This phenomenon is not to be confused with the true locomotion seen in the case of many bacilli or vibriones. Thus if a young culture of a motile organism (e.g., the cholera germ) be examined under the microscope in a so-called "hanging-drop preparation, the organisms are seen to be dancing and shooting across the field with an amazing rapidity.

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The organs of locomotion are slender whip-like appendages called cilia or flagella, which, like oars, propel bacteria through water. The disposition of flagella is characteristic. There may be numerous processes distributed over the whole surface (as in the typhoid bacillus); or there may be only one or two at the poles (as in the cholera vibrio); or there may be numerous polar flagella (as in the spirillum rubrum).

Variability of Bacteria.-Micro-organisms, like all the lower members of animal and vegetable kingdoms, dis

1 See p. 131.

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