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- The Story Of Germ Life - 4/26 -

out. In short, the bacteria after two centuries of study appear to us about as they did at first. They must still be described as minute spheres, rods, or spirals, with no further discernible structure, sometimes motile and sometimes stationary, sometimes producing spores and sometimes not, and multiplying universally by binary fission. With all the development of the modern microscope we can hardly say more than this. Our advance in knowledge of bacteria is connected almost wholly with their methods of growth and the effects they produce in Nature.


There has been in the past not a little question as to whether bacteria should be rightly classed with plants or with animals. They certainly have characters which ally them with both. Their very common power of active independent motion and their common habit of living upon complex bodies for foods are animal characters, and have lent force to the suggestion that they are true animals. But their general form, their method of growth and formation of threads, and their method of spore formation are quite plantlike. Their general form is very similar to a group of low green plants known as Oscillaria. Fig. 17 shows a group of these Oscillariae, and the similarity of this to some of the thread-like bacteria is decided. The Oscillariae are, however, true plants, and are of a green colour. Bacteria are therefore to- day looked upon as a low type of plant which has no chlorophyll, [Footnote: Chlorophyll is the green colouring matter of plants.] but is related to Oscillariae. The absence of the chlorophyll has forced them to adopt new relations to food, and compels them to feed upon complex foods instead of the simple ones, which form the food of green plants. We may have no hesitation, then, in calling them plants. It is interesting to notice that with this idea their place in the organic world is reduced to a small one systematically. They do not form a class by themselves, but are simply a subclass, or even a family, and a family closely related to several other common plants. But the absence of chlorophyll and the resulting peculiar life has brought about a curious anomaly. Whereas their closest allies are known only to botanists, and are of no interest outside of their systematic relations, the bacteria are familiar to every one, and are demanding the life attention of hundreds of investigators. It is their absence of chlorophyll and their consequent dependence upon complex foods which has produced this anomaly.


While it has generally been recognised that bacteria are plants, any further classification has proved a matter of great difficulty, and bacteriologists find it extremely difficult to devise means of distinguishing species. Their extreme simplicity makes it no easy matter to find points by which any species can be recognised. But in spite of their similarity, there is no doubt that many different species exist. Bacteria which appear to be almost identical, under the microscope prove to have entirely different properties, and must therefore be regarded as distinct species. But how to distinguish them has been a puzzle. Microscopists have come to look upon the differences in shape, multiplication, and formation of spores as furnishing data sufficient to enable them to divide the bacteria into genera. The genus Bacillus, for instance, is the name given to all rod-shaped bacteria which form endogenous spores, etc. But to distinguish smaller subdivisions it has been found necessary to fall back upon other characters, such as the shape of the colony produced in solid gelatine, the power to produce disease, or to oxidize nitrites, etc. Thus at present the different species are distinguished rather by their physiological than their morphological characters. This is an unsatisfactory basis of classification, and has produced much confusion in the attempts to classify bacteria. The problem of determining the species of bacteria is to-day a very difficult one, and with our best methods is still unsatisfactorily solved. A few species of marked character are well known, and their powers of action so well understood that they can be readily recognised; but of the great host of bacteria studied, the large majority have been so slightly experimented upon that their characters are not known, and it is impossible, therefore, to distinguish many of them apart. We find that each bacteriologist working in any special line commonly keeps a list of the bacteria which he finds, with such data in regard to them as he has collected. Such a list is of value to him, but commonly of little value to other bacteriologists from the insufficiency of the data. Thus it happens that a large part of the different species of bacteria described in literature to- day have been found and studied by one investigator alone. By him they have been described and perhaps named. Quite likely the same species may have been found by two or three other bacteriologists, but owing to the difficulty of comparing results and the incompleteness of the descriptions the identity of the species is not discovered, and they are probably described again under different names. The same process may be repeated over and over again, until the same species of bacterium will come to be known by several different names, as it has been studied by different observers.


This matter is made even more confusing by the fact that any species of bacterium may show more or less variation. At one time in the history of bacteriology, a period lasting for many years, it was the prevalent opinion that there was no constancy among bacteria, but that the same species might assume almost any of the various forms and shapes, and possess various properties. Bacteria were regarded by some as stages in the life history of higher plants. This question as to whether bacteria remain constant in character for any considerable length of time has ever been a prominent one with bacteriologists, and even to-day we hardly know what the final answer will be. It has been demonstrated beyond peradventure that some species may change their physiological characters. Disease bacteria, for instance, under certain conditions lose their powers of developing disease. Species which sour milk, or others which turn gelatine green, may lose their characters. Now, since it is upon just such physiological characters as these that we must depend in order to separate different species of bacteria from each other, it will be seen that great confusion and uncertainty will result in our attempts to define species. Further, it has been proved that there is sometimes more or less of a metamorphosis in the life history of certain species of bacteria. The same species may form a short rod, or a long thread, or break up into spherical spores, and thus either a short rod, or a thread, or a spherical form may belong to the same species. Other species may be motile at one time and stationary at another, while at a third period it is a simple mass of spherical spores. A spherical form, when it lengthens before dividing, appears as a short rod, and a short rod form after dividing may be so short as to appear like a spherical organism.

With all these reasons for confusion, it is not to be wondered at that no satisfactory classification of bacteria has been reached, or that different bacteriologists do not agree as to what constitutes a species, or whether two forms are identical or not. But with all the confusion there is slowly being obtained something like system. In spite of the fact that species may vary and show different properties under different conditions, the fundamental constancy of species is everywhere recognised to-day as a fact. The members of the same species may show different properties under different conditions, but it is believed that under identical conditions the properties will be constant. It is no more possible to convert one species into another than it is among the higher orders of plants. It is believed that bacteria do form a group of plants by themselves, and are not to be regarded as stages in the history of higher plants. It is believed that, together with a considerable amount of variability and an occasional somewhat long life history with successive stages, there is also an essential constancy. A systematic classification has been made which is becoming more or less satisfactory. We are constantly learning more and more of the characters, so that they can be recognised in different places by different observers. It is the conviction of all who work with bacteria that, in spite of the difficulties, it is only a matter of time when we shall have a classification and description of bacteria so complete as to characterize the different species accurately.

Even with our present incomplete knowledge of what characterizes a species, it is necessary to use some names. Bacteria are commonly given a generic name based upon their microscopic appearance. There are only a few of these names. Micrococcus, Streptococcus, Staphylococcus, Sarcina, Bacterium, Bacillus, Spirillum, are all the names in common use applying to the ordinary bacteria. There are a few others less commonly used. To this generic name a specific name is commonly added, based upon some physiological character. For example, Bacillus typhosus is the name given to the bacillus which causes typhoid fever. Such names are of great use when the species is a common and well-known one, but of doubtful value for less-known species It frequently happens that a bacteriologist makes a study of the bacteria found in a certain locality, and obtains thus a long list of species hitherto unknown. In these cases it is common simply to number these species rather than name them. This method is frequently advisable, since the bacteriologist can seldom hunt up all bacteriological literature with sufficient accuracy to determine whether some other bacteriologist may not have found the same species in an entirely different locality. One bacteriologist, for example, finds some seventy different species of bacteria in different cheeses. He studies them enough for his own purposes, but not sufficiently to determine whether some other person may not have found the same species perhaps in milk or water. He therefore simply numbers them--a method which conveys no suggestion as to whether they may be new species or not. This method avoids the giving of separate names to the same species found by different observers, and it is hoped that gradually accumulating knowledge will in time group together the forms which are really identical, but which have been described by different observers.


There are no other plants or animals so universally found in Nature as the bacteria. It is this universal presence, together with their great powers of multiplication, which renders them of so much importance in Nature. They exist almost everywhere on the surface of the earth. They are in the soil, especially at its surface. They do not extend to very great depths of soil, however, few existing below four feet of soil. At the surface they are very abundant, especially if the soil is moist and full of organic material. The number may range from a few hundred to one hundred millions per gramme. [Footnote: One gramme is fifteen grains.] The soil bacteria vary also in species, some two-score different species having been described as common in soil. They are in all bodies of water, both at the surface and below it. They are found at considerable depths in the ocean. All bodies of fresh water contain them, and all sediments in such bodies of water are filled with bacteria. They are in streams of running water in even

The Story Of Germ Life - 4/26

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