Before we bring out our microscope to examine these lowly plants, we will first of all kill a myth which has survived, in the non-scientific mind, since the eighteenth century and then describe briefly the life history of a typical bacterium. Now for the myth. Bacteria are so minute, their activities so great and the results of their activities so far reaching, that it is hardly surprising to learn that, even at the present day, bacteria are supposed simply “to happen.” We talk of thunder turning milk sour, but thunder can no more sour milk than can a passing train or an air raid or a burst in a water main. True, milk turns sour more quickly in thundery weather than in frosty weather, because, when thunder threatens, the air is warm and the milk-souring bacteria increase more rapidly in warm weather than in cold. We must remember always that bacteria are living beings and in common with all other living things they must have parents. What probably took place at the beginning of the world we cannot consider here but one thing is certain that, at the present day, no living matter is produced from non-living matter; “life from life” is the only theory that will stand scientific tests and this has been the case ever since the simplest microscopes were thought of and thousands of years before that. Any substance, however liable to decay, if rendered germ free and kept germ free, will retain its fresh condition indefinitely. Could bacteria or germs, call them what you will, simply happen it would be useless attempting to fight against them.
Bacteria are everywhere. In the water we drink, in the milk, butter, cheese and in dust. We cannot avoid them, try as we will; it is fortunate, therefore, that the majority are harmless. You may be surprised that, with this ubiquity, you have never seen one. When, however, you learn that most of them are only about twenty-five thousandths of an inch long and that a thousand million of them could be packed comfortably into a little box, whose sides measured but a twenty-fifth of an inch in length, it is not really so surprising after all. Being so small, the activities of a single bacterium are insignificant; that “union is strength” was never better exemplified than amongst these lowly plants. There are no male and female bacteria, in the majority of cases they increase by splitting, in fact they are often called splitting plants. The change may be watched under the microscope. The plant elongates somewhat, it becomes narrower and narrower in the middle, it develops a waist in fact; finally the two halves part company and each one leads a separate existence as a bacterium. This splitting progresses at an extraordinary rate. A celebrated scientist once wrote: “Let us assume that a microbe divides into two within an hour, these two into four in the next hour, these again into eight in the third hour and so on. The number of microbes thus produced in 24 hours would exceed 16 1/2 millions; in two days they would increase to 47 trillions, and in a week the number expressing them would be made up of 51 figures. At the end of a day (24 hours) the microbes descended from a single individual would occupy one fortieth of a hollow cube with edges one twenty-fifth of an inch long, but at the end of the following day would fill a space of twenty-seven cubic inches, and in less than five days their volume would equal that of the ocean.” It is hardly necessary to add that these alarming figures represent what would happen if no accident befell the bacteria, they show the enormous vitality possessed by the smallest of all plants. Even allowing for misadventure their increase is alarming; actual tests, with a sample of milk containing originally 153,000 bacteria per cubic inch, show that the cubic inch contained after one hour, 539,750; after two hours, 616,250; after seven hours, 1,020,000; after nine hours 2,040,000 and after 25 hours 85,000,000 individuals.
The writer whom we have just quoted calculated that a single bacterium weighs about 0.000,000,000,024,243,672 of a grain, that forty thousand millions weigh one grain and that two hundred and eighty-nine billions weigh a pound. The descendants of one bacterium weigh 1/2666 of a grain, after twenty-four hours; more than a pound after two days, and sixteen and a half million pounds after three days. The assumption in this case, also, is that no harm comes to any of them; the mortality amongst bacteria is, clearly, very great.
Sometimes, owing to external conditions, such as lack of food certain bacteria produce spores. The power of spore formation is not possessed by all bacteria and those which are able to bring it about are difficult to kill for the spores, which contain the living material of the bacterium are surrounded with walls which will resist boiling, drying, freezing and all manner of ill treatment. The spore formation of bacteria is very simple, all or part of the living contents of the bacterium becomes surrounded by a tough wall and remains so surrounded till circumstances are favourable, when the wall bursts, its contents escapes and becomes a bacterium, capable of founding a new colony by the method of splitting we have already described.
Now let us try to find out what sort of plants we are to look for, when we are searching for bacteria, under our microscope. They exist in many forms, to which special names have been applied, and it is unfortunate that, very often, their external form varies according to their state, thus a bacterium may be spherical when young and rod shaped when older. Some bacteria are spherical and are known as Cocci or Micrococci, from Greek words meaning a berry or a little berry respectively; sometimes these spherical bacteria occur in pairs, then they are called Diplococci (double berries); or in chains, Streptococci (chain berries); or in bunches, Staphylococci (grape berries). They may resemble short rods, when they are called Bacteria, a name, by the way, which is also applied generally to all microbes; they may, on the other hand, have the appearance of longer rods and then they are called Bacilli. Some of these longer rods may be curved or even corkscrew shaped when they are known by the name of Spirilla. Rather fearsome names some of these we fear and we wished to avoid long names, but they appear over and over again in books and papers relating to bacteria so we are compelled to introduce them to our pages. Many bacteria possess no power of movement, others swim rapidly, by the aid of the lashing movement of little whip-like structures with which they are furnished.
After all this preamble, which we hope has cleared up certain misconceptions regarding bacteria and has given the reader some insight into their habits, we may proceed to the examination of some of the plants themselves. At the outset we have a confession to make. Bacteria can only be studied seriously, by those who possess very expensive and elaborate apparatus; considerable technical skill is required to prepare the plants for examination—many of them indeed can only be seen after they have been stained and lastly, to trifle with the disease-causing members of the family may lead to dangerous if not fatal results.
Having issued our warning let us see what we can do in the way of microscopic investigation. The easiest subject with which to make a start is the Hay Bacillus, Bacillus Subtilis, not because it is the largest of the bacteria by any means, but because it is very easily obtained. Each plant measures about five thousandths of an inch in length, so we shall require a high magnification to examine it. Having obtained a small quantity of hay, we must boil it in water for about three-quarters of an hour and then set it aside for some hours. In due course the water will contain hundreds upon hundreds of bacteria or, speaking more correctly, of bacilli. For our work, we shall require a special kind of microscope slide; instead of the piece of plain glass we have been accustomed to use we must obtain one with a circular portion, hollowed out from the centre. Having done so, we take a clean glass rod and, with it, transfer a drop of the water, containing the bacilli, to the centre of a clean coverslip. Invert the coverslip so that the drop is on the lower surface and place it over the hollow portion of the slide, in such a manner that the drop still remains suspended from the coverslip; this is known as the hanging-drop method and requires some little skill to accomplish satisfactorily. When our slide is prepared, with a magnification of at least one thousand diameters, we may reasonably hope that our trouble will be rewarded.
At first we shall probably see nothing. We recall that we had some difficulty in examining starch grains, on account of the fact that they were colourless. This time we are dealing with a far more difficult subject. When our eyes become accustomed to the light, however, we shall be conscious that there is something moving in our drop of water. The Hay Bacillus is one of the moving forms, each individual is furnished with a number of little whips whose lashings enable it to travel through the water. The whips cannot be seen in unstained bacilli; experience, however, tells us that they are there, for all these lowly plants which show movement are seen when stained, to possess the little whips. The process of staining kills the plants so that we cannot see the little whips in action.
Having detected that movement is taking place, a little adjustment of focus and a further search will reveal the bacilli to us, as little rod-like, colourless individuals. We shall see their cell contents if they are sufficiently highly magnified and also their cell walls. We may even observe them splitting, each one into two individuals. We must keep our sample of water for later examination. In fact, we may examine drops from day to day, in exactly the same manner. After a short lapse of time we shall notice that the bacteria have increased to an alarming extent and also that they no longer swim about. At this period they tend to arrange themselves in chains lengthwise, their cell walls also lose their clear cut appearance and become jelly like, yet withal they may still continue to split up.