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play considerable variations in their characters. Indeed, at one time, it was maintained that there was no constancy among bacteria, and that the same species might exhibit diverse forms and properties.

Therefore it was argued that there were no different species among them.

That micro-organisms should be so excessively polymorphic could hardly be a matter of surprise, considering how profoundly they are influenced by environment. If the conditions of existence fluctuatemas must frequently be the case in nature—one of two things may happen. Either the organisms change their form and function and adjust themselves to the new conditions, or they may fail to adapt themselves, and so perish. In the first case, the modifications of biological characters may persist through numerous generations, and lead to the production of new varieties. When we remember that there are already about 500 varieties of potato, although it was only introduced into Europe about two or three centuries ago, we need not be surprised at the countless varieties of bacteria which have been cultivated since the beginning of the earth, and can give rise to fifty generations in the course of twenty-four hours.

And yet it must be confessed that the extraordinary variability of bacteria is very confusing.

The same species may give rise to a rod, a thread or a spherical form. The bacillus prodigiosus produces red spots on starchy substances; but when grown at a higher temperature brings about lactic fermentation, without the production of pigment. The organism of pneumonia, i.e., the pneumococcus, causes indigo fermentation ; but is also pathogenic for man. And the remarkable variability of the cholera vibrio is well known (see p. 77).

But notwithstanding these variations there is no doubt that under identical conditions the properties of bacteria

will remain constant. It is no more possible to convert one species into another than it is among higher plants. Pus cocci, however cultivated, can never be changed into cholera vibrios, and all attempts to convert the B. coli into B. typhosus have failed. Again, it is not always possible by selecting the external conditions to produce other species at will, or to suppress a single biological character in any species.

We conclude, then, that bacteria are divisible into species and genera just in the same way as the higher organisms. It is true that they present considerable departures from the form that is deemed “typical”. But just as a cinchona tree may normally show considerable variations both in the amount and variety of alkaloids it contains, a germ may also show atypical characters and be still within the boundaries of the species. Indeed, it may be affirmed that under all circumstances there exists a well-marked form for each species, which represents for the latter the maximum of its growth, the climax of its well-being.

Classification. But although distinct species of bacteria do exist, their differentiation from one another is a matter of considerable difficulty. Hence it is that the classification of bacteria is in such an unsatisfactory state. Organisms have been divided, according to the characteristic effects they produce, into chromogenic (colour producing), photogenic (light producing), arogenic (gas producing), etc. But as the same micro-organism may produce any of these effects it is obvious that a biological classification is impossible. The division of bacteria into saprophytes (which grow on dead matter) and parasites (which grow in living tissues) is based on the assumption that organic matter is their necessary pabulum, and can no longer be entertained. For the discovery of nitrogen

fixiny bacteria (p. 42) has shown us that certain forms of life can dispense with organic and inorganic matter, and live simply on atmospheric nitrogen. To meet this difficulty Fischer has suggested the division of all bacteria into three groups as follows :1. Prototrophic.—These require no organic food, and

include nitrifying, nitrogen-fixing, sulphur- and

iron-bacteria. 2. Metatrophic.— These require organic matter for their

growth, and comprise facultative parasites (i.e., saprophytes capable of taking on a parasitic exist

ence), and most bacteria. 3. Paratrophic bacteria occur only in living tissues

(obligatory parasites). While this mode of classification has its value, the most practical division is that which is based on morphological characters. As has already been stated, the division of bacteria into spheres, rods, and spiral forms is fairly satisfactory, and will be found amply sufficient for our purpose.

THE GENERAL BIOLOGY OF BACTERIA.

ACCORDING to the generally accepted hypothesis of the origin of the earth, it is obvious that the first formed bacteria must have existed under conditions totally different from those which prevail to-day. The atmosphere must then have been poorer in oxygen and light, but of a higher temperature; and there must also have been a notable lack of organic food-stuffs. It is, therefore, interesting to find certain species which still retain their primitive ancestral characters, and can only grow without light, oxygen or organic matter; and others again which thrive only at the boiling temperature. It is important, then, to bear in mind that although bacteria in general can grow when supplied with suitable food and temperature, yet their precise requirements admit of the greatest variations.

A.-CONDITIONS OF GROWTH.

1. Nutrient Media.—Like all higher plants, bacteria require for their food materials whence they obtain the elements of which they are made up. Carbon is derived from carbohydrates, and nitrogen from albuminoids, or more rarely from inorganic compounds. In addition to these sources of nitrogen and carbon, all media must contain salts and an abundance of water. A neutral or faintly

alkaline reaction is generally indispensable. The media usually employed in the laboratory are bouillon, gelatine, agar,

blood serum, etc. Bacteria, however, are exceedingly fastidious in their demands. Thus, while some are easily satisfied with ammonium salts, others refuse to grow unless cultivated in blood serum, and still others refuse to grow at all in any of our artificial media. Again, the range of their requirements is so narrow that trivial differences in the composition of food materials may favour the growth of one organism in preference to the other. Nägeli found that in a neutral saccharine fluid containing bacteria, yeasts and moulds, only the first flourished causing lactic fermentation; but the addition of per cent. of tartaric acid caused the growth of yeasts; while the addition of 4 to 5 per cent. of the same acid brought about the development of moulds.

It is interesting to note in this connection a remarkable property which some micro-organisms possess, i.e., the power of selection, whereby they can discriminate even between extremely similar substances. Mannite and dulcite are two such substances, and the difference between them is so slight that both are represented by the same chemical formula. But there is a class of organisms which can distinguish between them more easily than the chemist; and if grown in their mixture will attack mannite, leaving dulcite untouched. To take another remarkable illustration : if yeast be added to a solution of fructose, which consists of right-handed and left-handed lævulose, it only attacks the left-handed molecules, leaving the former severely alone. This is probably due to the fact that the yeast organisms, from centuries of growth, have had deeply impressed upon them the capacity of acting upon a particular form of lævulose; but they cannot so

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