The science of life : fully illustrated in tone and line and including many diagrams

THE LOWLY AND MINUTE

§ 7

Bacteria

The smallest and most humbly organized creatures that we have hitherto considered are the single-celled animals and plantsamoeba, the flagellates, the lower alge, and the yeasts. But there are smaller and simpler organisms even than these. For the bodies of most bacteria are incredibly small. The common bacillus that is found in infusions of dead organic matter the world over is a short rod about one-thousandth of a millimetre (one twenty-five-thousandth of an inch) across and five or eight times as long. ‘That may be called an average bacterlum. Some kinds of bacteria are longer than this and some are very rauch smaller, but most are of about this size. Imagine such a bacillus enlarged to the size of a cigar, half an inch across by four inches long ; a man magnified to the same extent would be fifteen miles in height.

There is little room for specialized structures in such a tiny interior, and indeed its organization is far simpler than that of any of the cells that we have hitherto examined. It has none of the various writhing bodies that we noted in a tissue-culture cell. There is not even a nucleus. The chemical substances that characterize the nucleus in animals and plants are diffused through the whole bacterial body, mixed in with the rest of its substance, instead of being confined within a special vesicle. A bacterium is little more than a microscopic blob of protoplasm having a definite shape. Moreover, the life-history of our bacillus is as simple as its anatomy ; the things it can do are very limited. It can grow and proliferate (by simply breaking into two halves) with incredible rapidity. The average length of life of a bacterium that finds itself in favourable surroundings, the time between successive divisions, is twenty minutes, so that in rapidity of multiplication they leave even the protozoa behind. And in many cases it can weather difficult periods, when conditions are unfavourable for its growth, by turning into a resistant, toughwalled spore. There is an important difference, we may note, between the spores of bacteria and those of other creatures. The spores of amoeba, for example, are reproductive devices; each amoeba produces a great number of spores at a time. But the spores of bacteria are in no sense reproductive. Each bacterium converts itself into a single spore. They are devices for resisting unfavourable conditions.

In bacteria, then, the life-history (or, better, race-history, because it is difficult to interpret the things that bacteria do in terms of our own personal experience) consists in an alternation of two phasesthe active phase proliferating with astonishing rapidity, and the passive, resting spore with equally astonishing powers of resisting adverse conditions. Active bacteria are killed at once by the heat of boiling water, but their spores may survive boiling for several hours. Indeed, in order completely to sterilize surgical instruments or fluids for use in bacteriological work it is necessary to expose them to moist steam at about double atmospheric pressure and a temperature of 120° Centigrade in an instrument known as an autoclave—and even under these scalding conditions, some spores survive for a quarter of an hour. At the other extreme, bacterial spores have survived prolonged exposure to the temperature of liquid air (—1go° C.) for six months. The creatures can live for very long periods in this spore stage, perfectly passive, waiting as 1t were in a trance for an opportunity to emerge and grow and divide. They have been kept for three years in this condition and have been fully capable of revival at the end of that time. Taking the normal life-span of a bacterium as twenty minutes, these spores slept during the time of over a hundred thousand generations of active bacteria. It is as if a man had passed into a trance ages ago, before ever the Pyramids were built—even before anything at all was built, for he might have been one of the earliest flint-chipping men — and woke blinking and rubbing his eyes at the present day to go about his accustomed occupations.

In this quiescent, resistant state bacterial spores wait on the ground, in the sea, in the air for an opportunity to hatch and grow. They have been found frozen in icebergs. They have even been found in hailstones, which shows that they float very high in the air. If they are lucky enough to settle in a suitable spot they grow and proliferate, in some species with almost miraculous speed. A flask of sterilized broth, inoculated with bacteria by dipping an infected needle into it, may become visibly turbid in twentyfour hours’ time. Every cubic inch of the broth will now contain some eighty thousand million organisms. .

A striking peculiarity of bacteria 1s that they have no trace of a sexual stage in their life-history. Even in the lowest protozoa and alge there are primitive kinds of sexual union, blendings of cell with cell. .

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