THE relation of minute vegetable organisms to disease has of late years assumed a high degree of importance, so much so that a special science has been created with its chairs and laboratories, and a literature which has already reached very large dimensions.

The minute organisms have received the names of Bacteria or Microbes, these terms being used in a general sense to include the whole group. The department of pathology which deals with them is designated Bacteriology. The reader is referred to special works on the subject for the details of methods of investigation; we are here chiefly concerned with the relation of the microbes to disease.

I. General Considerations

Definition And Classification

The bacteria are the lowest forms of organic life. They consist of minute round, oval, or cylindrical cells, so small that they require high powers of the microscope for their detection. Like the fungi, they are, except in one or two doubtful cases, devoid of chlorophyll. But they are even lower than the fungi, as they multiply almost entirely by division. It is from this that the name Schizomycetes is derived, a name equivalent in its meaning to Fission-fungi. There is an approach to reproductive organs only in a few forms in which spores are produced under certain conditions, but even in them the multiplication is mainly by fission.

It is not possible at present to make a proper scientific classification of the microbes. The working out of that may be left to the botanists. For our purposes the classification of Cohn is sufficient. He distinguished them according to their external form, into the globular, the short rods, the longer rods, and the spiral forms. The rod-shaped forms may be taken together, so that we have three forms, the globular or oval Coccus or Micrococcus, the rod-shaped Bacillus and the spiral Spirillum.

Structure And Mode Of Growth

The bacterium, of whatever form, is a cell, consisting of cell-contents and membrane, but without a nucleus. The Cell-contents consist of an albuminous substance, which in its reaction to staining agents closely resembles the nuclei of ordinary animal and vegetable cells. This is more particularly the case with the basic aniline dyes, which the bacteria, as well as the nuclei of cells, usually absorb greedily, but the bacteria, in many cases, retain the colour more firmly than the nuclei, some of them even in the presence of acids. The contents sometimes present granules of starch, and, in one form, of sulphur. The Cell-membrane is composed of a substance allied to cellulose. It is difficult to demonstrate, but by the use of iodine the contents may be made to shrink so as to display the membrane, which may be now stained of a different colour (Crookshank). The cell-membrane sometimes in its outer parts swells up or otherwise produces a Gelatinous envelope, which forms a cement between the individual bacteria, uniting them in various numbers. Some bacteria are possessed of Flagella Or lashes, by the active movement of which they move about. As the bacteria themselves are very small, and the flagellum is much finer, the latter is very difficult of observation, but it may be rendered visible by special modes of staining.

The bacteria multiply, as already mentioned, by fission, and tbey do so, under favourable circumstances, with enormous rapidity, so that, as has been calculated, a single individual will multiply a million-fold in twenty-four hours. In multiplying, the individual members may remain for a time united, and so give rise to specific appearances. Thus some of the cocci divide in succession in the same direction, and often appear in twos, diplococcus, or elongated into chains, hence streptococcus. On the other hand they may divide in no determinate direction, and so form groups which resemble bunches of grapes, hence the term staphylococcus. Again the individual cocci sometimes divide a second time at right angles to the first so as to produce four, hence the term tetragenus. In others the division occurs in three planes, so that one coccus produces eight arranged in a cube, as in Sarcina. The bacilli also may remain united, and so form chains or threads or even spiral forms in the case of curved bacilli.

The gelatinous envelope which frequently surrounds bacteria may unite them into large groups or colonies, to which the name Zoogloea is given. The zoogloea sometimes form membranous aggregations, such as we so often see as a scum on the surface of decomposing fluids. Seen thus in the mass in zoogloea, the bacteria give under the miscroscope a characteristic brownish clouded appearance.

Some bacteria produce Spores, and it is this faculty to which botanists pay special attention, with a view to classification. The formation of spores is to be regarded rather in the light of a " resting " stage in the life history of bacteria than as a mode of reproduction. Two forms of spore-formation are described. In the one the spores form inside the bacteria, in which they are differentiated as round or oval bodies. These are called Endospores. In the other form, the occurrence of which is more than doubtful, individual members of a group of cocci become larger and more prominent, having greater refractive power on light, and so assume the character of spores, while the remaining individuals undergo no such change. These are called Arthrospores. In whatever way produced the spore has a thick membrane and is much more resistant than the ordinary cell. The spores are more difficult to stain than the rest of the cell, but when stained they retain the colour more firmly. This fact may be made use of to effect a double-staining, the spores one colour and the rest of the bacterium another. They are stained brown with osmic acid, from which it has been inferred that they contain some form of fat. Bacteria sometimes present an appearance resembling endospores, but not truly of that character. The beads, so characteristic of the tubercle-bacillus for instance, are not universally regarded as spores, although by some viewed in that way. The great majority of bacteria show no spore-formation of any sort, and even those which bear spores do so only under special circumstances.