The greatest advances in our knowledge of bacteria nave been made by their artificial cultivation. Following the example of Koch the bacteria are now cultivated not only in fluids, but on solid media, where they can be kept much more free from contamination and may be more completely observed in their growth. The cultivation takes place on the surface of nutrient substances, of which the chief are potatoes and meat-juice. Fluid media are rendered solid by the addition of gelatine or agar-agar. As the latter substance does not melt at the temperature of the body, it is exceedingly useful in cultures which require a higher temperature than that of the ordinary air.

Pure cultivations are made by first sterilizing the medium and inoculating afterwards, taking great precautions against contamination. It is possible to propagate many individual forms through successive generations and on different media, so as to observe their habits. The growing and multiplying bacteria appear as visible layers on the surface or in the substance of the medium, and the individual species present, as a rule, variations in mode of growth, colour, and otherwise, sufficient to enable a practised eye to discriminate the form which is under observation. In their growth in media which have been stiffened with gelatine, they often produce liquefaction. When one sees the visible characteristic layers of bacteria on the surface of potato or other medium one has a vivid conception of the reality of these forms which individually are mere microscopic objects.

The methods of cultivation will be found in text-books of Bacteriology, such as those by Klein, Crookshank, Fraenkel, Schenk, Sternberg, and others. It may be noted here that by an ingenious application of Koch's gelatine method a quantitative estimate can be formed of the number of bacteria in a given amount of air, earth, or water. This is done chiefly by means of so-called Plate cultivations. The gelatine solution is heated gently so as to melt it, mixed with a determinate quantity of the substance to be estimated, and then poured, in a thin layer, on a glass plate, where it solidifies. The bacteria develop and form centres of growth which can be counted and so give an approximate estimate of the numbers in a given quantity of material. It has been estimated, for example, that in the water of the River Clyde at Glasgow there are 1,500 bacteria in every drop, while even in Loch Katrine water, which is exceedingly, pure, there is about one in every drop (Maylard).


An important question has been raised as to whether the bacteria always retain their forms, and accordingly whether the genera can be formed on the basis of their outward configuration. It has been asserted that under varying circumstances not only the form but the physiological characters may vary. It is true that according to the stage of development, and the conditions of nutrition, the individual forms do undergo certain variations in size and shape. This is, to a certain extent, true of all living things. But it is also true that when the more perfect modern methods of cultivation are used, the identity of the different forms can be absolutely secured, and that in such case they have a definite configuration which is the constant expression of their complete state of development. As in the case of higher plants we can here recognize indications of genera and species, although the objects are too minute and the science too recent to permit of a permanent classification.

The alleged cases of alteration of form and characters arise largely from imperfections in the observations. It may be acknowledged, however, that in the case of some of the bacteria, especially members of the diphtheria group, a very strong case has been made out for polymorphism. In the construction of genera and species here as in higher plants, attention will be paid to the whole life-history of the bacteria, and not merely to their outward form. We are still far from a complete system on such a basis.


The recent literature of Bacteriology is of enormous extent. It is given for the most part fully in the general works mentioned below. General work* - Cohn, Biol, der Pflanzen, vol. iii., 1879; Zopf, Zur Morphologie der Spaltpilzen, 1882 and 1885; Be Bary, Vorlesungen liber Bacteria, 1885, and Fungi, mycetozoa and bacteria (transl.), 1887; Hueppe, Formen der Bacteria, 1886; Klein, Microorganisms and disease, 3rd ed., 1886; Ceookshank, Bacteriology, 4th ed., 1896; Woodhead and Hare, Pathological Mycology, 1885; Cornil et Babes, Les bacteries, 1885; Fraenkel, Bakterienkunde, 1887 (also transl.); Flugge, Die Micro-organismen, 1896; Baumgarten, Path. Mykologie, 1890; Sternberg, Bacteriology, 1896; Gdnther, Bakteriologie, 1898; Schenk, Elements of bacteriology (transl.), 1893; Muir and Ritchie, Manual of Bacteriology, 2nd ed., 1899; Fischer, Vorlesungen iiber Bakterien, 1897; Hewlett, Manual of Bacteriology, 1898; Bowhill, Bacteriological Technique, etc., 1899. Limits of temperature - Pictet and Young, Comptes rendus, xcviii., p. 747; Coleman and M'Kendrick, Proc. of Roy. Instit., 1885. Products of Bacteria - Aitken, Animal Alkaloids, 1887; Brown, Animal alkaloids, 1887; Brunton, On disorders of digestion, 1886; Brieger, Ueber Ptomaine, 1885, and Untersuchungen, 1885; Gautier, Toxines, 1896.