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Multiplication of Bacteria

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MULTIPLICATION OF BACTERIA Mode of a bacterial cell is about to divide, the material of the cell is gradually increased till its vol ume is practically doubled, spherical forms become oval and rod forms stretch to nearly double their length, the cell then be comes constricted at the middle and the constriction deepens till finally the contents are held in two compartments separated by a wall formed at the line of constriction. The two new cells remain adhering for some time and, sooner or later, they separate and form two new individuals or daughter cells which are the exact counterpart of the mother cell and of each other. Bacteria in view of this method of growth are by some spoken of as the fission fungi and are designated by the Greek word Schizomycetes.

For many bacterial species fission represents the only form of reproduction. Some species are better favoured, however, in the possession of an alternative process, that of sporulation or multi plication by spores. Spores are formed in some organisms only when the conditions for growth by the method of fission described above have become unsatisfactory, it may be drought, or excessive heat or cold, or the presence of some poisonous chemical sub stance, in others sporulation follows in a regular sequence in time according to the temperature to which they are exposed. The first visible sign of spore formation is the appearance of a light spot in the body of the organism viewed under the microscope in the unstained condition. This is due to a change in the refraction of the light rays by some newly formed substance or physi cal state. This differentiated portion of the contents of the cell increases in size and soon appears as a rounded, or more usually oval, body enclosed by a wall, at first thin but quickly becoming thicker until at maturity the spore coat is of con siderable thickness. Having formed its spore the rest of the cell disappears and sometimes it is only with difficulty that any trace of it can be found. These spores are highly resistant to heat and drought and serve to tide the organism over a pe riod of stress, so much so that sporulating organisms have been known to remain alive and viable for longer than ten years and in all probability they can exist very much longer, while organisms capable of multiplication only by the method of sim ple division are usually dead in 12 months and sometimes much less than that. When more suitable conditions return, germina tion of the spore takes place in one of two ways, either the spore wall softens, thins down, becomes plastic and the cell reverts to the ordinary type, or the wall of the spore thins and breaks down at a certain spot either at the end or the middle of the cell. Through the ruptured wall the contents emerge, leaving behind the empty spore shell, and assume the typical active form.

Sterilization.

Methods of sterilization have to be chosen in full appreciation of the high resistance to heat of the bacterial spore cell. The ordinary cells are destroyed at temperatures of 50° to 6o° C., hence a single boiling of a fluid or even Pasteurization (application of heat of 6o° C.) is sufficient to destroy them or most of them. The spores, however, require very prolonged heat ing at these temperatures for their destruction and in order to produce sterility of a liquid in a reasonably short space of time temperatures above 12o° C., obtained by steam under pressure, are employed. Dry spores withstand even higher temperatures; but 15o°, maintained for 20 minutes, will usually destroy them.

Rate of Multiplication.

Given suitable conditions for growth the rate of multiplication of bacteria is very rapid ; when ever experiments have been conducted to determine this rate, under conditions purposely made most favourable, it has been found that a division of the cell is repeated every 20 or 3o minutes.

Rate of Growth of Bacillus mycoides at different temperatures. H. Marshall Ward.

Temperature. Time of one Division.

C. Mins.

5-6° 00 300-400 200 100 70-80 60-70 50 40 3o--35 00 A little consideration of what this means will bring out the importance of bacteria in everyday household affairs.

Assuming that conditions are conducive to a rate of one divi sion every 3o minutes, a single individual cell will have produced four cells at the expiry of the first hour, 16 at the end of two hours, 64 at three hours. At the end of eight hours these 64 will have become approximately 64,00o, and at the end of 15 hours there will be roughly i,000,000,000. It can be calculated that these would occupy a space of r cu.mm. Such a mass of bacteria, easily visible to the naked eye, is called a colony and it may here be said that it is no unusual thing in bacteriological practice to obtain colonies of this size as the result of one night's growth. Returning to our calculation the cubic millimetre will have reached the proportion of 65c.c. in 23 hours or enough solid bacteria to fill an egg cup, and at the end of 35 hours the progeny of this single cell will occupy a space of r,000 cubic metres. F. Lohnis, who was responsible for this calculation, states it would require a goods train of ioo wagons to transport this mass of bacteria. In practice these conditions are not met with in nature or at any rate not for long, but that such conditions may actually arise temporarily is shown by experiments in which milk stet ilized by long heating was inoculated and incubated at the tem perature at which it leaves the cow, that is, about 36° C. In the foregoing table it will be seen that the increase in number at 36° C. corresponds very closely to the figure calculated for a doubling every 3o minutes. Even at the temperature of rather cool air, 12.5° C., the rate of growth is sufficiently astonishing and emphasizes the difficulties to be overcome in maintaining a clean milk supply. Conditions for bacterial growth, fortunately for man's position in the world, never remain favourable for such unrestricted growth very long. In order to produce any mass of bacteria there must be an equivalent weight of suitable food ma terial. Besides shortage of food there are other factors ; running parallel with increase in numbers comes an accumulation of sub stances, the by-products of bacterial activity, which, unless re moved, and in nature they seldom are removed, militate against bacterial growth. It is very noticeable when cultivating bacteria in the laboratory upon plates of nutrient media that growth which in the initial stages of incubation is extremely rapid, so that a visible colony may appear in six to ten hours, gradually becomes less rapid and usually comes to a complete standstill after 3o or 36 hours. Many of the by-products of bacterial growth are acid in character and acids generally are unfavourable to growth. The result of such an accumulation of acids is seen every day in the souring and later the curdling of milk caused by bacterial conver sion of milk sugar into lactic acid. The effect of this accumulation of acids in milk is shown in experiments following the course of bacterial growth in milk at 3o° C., during the whole of 24 hours. Beginning with a milk containing 370,000 organisms per c.c., at the end of six hours there were 226 million, and at r 2 hours 8,070 million, at 18 hours 3 2, 243 mil lion. That 32,000 million organ isms may be present in a cubic centimetre, i.e., in less than a third of a teaspoonful, helps one to realize the extreme smallness of bacteria. After this time the injurious effect of the accumulation of acid made its appearance, growth was not merely stopped but many of the organisms were actually killed and at 24 hours the number of living organisms had been reduced to 2,286 million.

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