STRUCTURE AND CHARACTERISTICS. Each bacte rium is a single non-nucleated cell of protoplasm, and is endowed with the properties which belong to living organisms as such. Bacteria differ from one another in size and shape. It has been estimated that. a single bacterium of av erage size weighs not more than 3booTts-36ocitioo of a grain, and that 1500 of the rod-shaped forms placed end to end would about stretch across the head of an ordinary pin. It requires the highest powers of our microscopes to distin guish, not to say study, many of the forms. .\lany bacteria possess distinct powers of motion, due to the possession of long, hairlike appen dages or cilia, which project from the body of the bacterium, and with a lashing motion propel it through any liquid medium in which it may be. This motility differs greatly in degree for different species, and for members of the same species under different conditions. It is confined mainly to the bacilli or rod-shaped bacteria, but has been observed in a few species of micrococci. Such motile micrococei possess flagella•. Like all living things, bacteria nourish themselves from their surrounding media, taking up external substances as food and building them up into their own cell substance, at the same time giving off waste-products. Like other plants, bacteria differ from one another as regards suitable foods, some growing best in one soil, others in a soil quite different. Under the influence, apparently, of unfavorable nutritive conditions, certain bacilli have the property of passing from their ordinary condition into that known as 'spore formation.' This consists in the formation within the bacillus of a clear round or oval body; the spore, which, when fully developed, re places sompleiely the bacillus. The spore does not represent a phase of reproduction. A single bacillus can form but one spore. It seems to be a defense metamorphosis which the organism undergoes when its environment becomes un favorable. When proper nutritive conditions again obtain, the organism returns to its more common form. When this occurs, a single spore produces but a single bacillus. The spore repre sents an inactive condition, inasmuch as there is no reproduction of spores. A most important
significance of this spore-formation lies in the fact that being a defense-form, spores are much more resistant to the methods usually employed for destroying micro-organisms than are the bacilli from which they develop; and it was the failure to destroy spores that kept alive, for so long a time, the theory of spon taneous generation. Bacteria multiply by simple fission. They elongate, an indentation appears near the middle, and this is followed by complete division, forming two daughter-cells similar to the parent-cell. The different species of bacteria preserve their identity as carefully as do the higher vegetable organisms, a given species of bacteria never producing any but the same spe cies. This reproduction of bacteria is, under favorable circumstances, exceedingly rapid—a bacterium reproducing itself in from 15 to 40 minutes. This would, in 24 hours, result in the production of many millions of bacteria from a single individual. Woodhead says, "Fortunately for us, they can seldom find food enough to carry on this wonderful reproduction. A large number die from want of food, and because of other conditions unfavorable to their existence." Drying kills many species of bacteria; others simply remain inactive. Cold destroys many bacteria, though some are not killed by low temperatures. The typhoid bacillus can exist for many months frozen in a cake of ice, to be come active and dangerous again when the ice is melted and used. Heat, especially moist heat, above a certain point, kills all bacteria ; the application of heat to the destruction of bacteria is known as sterilization. Thus, boiling milk or water, or cooking meats or vegetables, destroys any bacteria that may have been present. The same applies to surgical dressings, such dressings being spoken of as sterile, or, because of the absence of infectious material, as aseptic. Cer tain chemical substances known as germicides, disinfectants, etc., are very inimical to bacteria, e.g. solutions of mercuric chloride, of carbolic acid. etc., and surgical dressings in which such chemicals are used arc known as antiseptic dressings.