BACTERIA (plu•. of \eo-Lat. bacterium-, Gk. bak t dimin. of Loki ron, staff, stick). A name applied to vege table organisms—the most minute organisms known. name was given to the first-dis covered of these bodies because of their shape, but is now used to designate this entire class of organisms, although many well-known bae teria are not 'little rods' at all, but are round or oval in shape.
The first recorded observation of the bodies we now recognize as bacteria was made about the middle of the Seventeenth Century by Antony van Leenwenhoek, a Holland lens-grinder, who reported his discoveries to the Royal Society of London in 1683. Continuing his investigations, Leenwenhoek discovered the presence of bacteria in the mouth and in the intestinal evacuations; and it is interesting to note that there followed these discoveries a germ theory of disease no whit less far-reaching, if less accurate, than that which exists at the present day. In 1773 O. F. Muller established two genera, Monas and Vibrio. Not much progress was made, however, until about 1838, when Ehrenberg and Dujardin included bacteria in their investigations of minute organisms. They referred the forms which they described to the animal kingdom, classifying them among the Infusoria, and plac ing a large number of them under the general title of Vibronia. Nor was this reference sur prising. The means at the disposal of these early investigators were extremely crude. They saw these minute moving bodies, and considered them animal in nature: for at that time natu ralists had hardly begun to realize that animals have no monopoly of motility, though it is now a commonplace observation that the simplest plants exhibit a power of locomotion that is almost wholly lost in the higher forms. They were believed to develop spontaneously. It was not until the experiments of Cohn that the doc trine of spontaneous generation as applied to bacteria was overthrown, and Harvey's law, 'Oninc virum ex vivo' ('All life comes from life') was accepted as having universal application.
Bacteria are found almost everywhere. They are present in air, water, soil, in most foods and drink, and are regular inhabitants of the mouth, stomach, and intestines, and of the superficial layers of the skin. The rancidity of butter, the putrefaction of cheese, the gamy flavor and high odor of meat, the yellowness and blueness of milk kept in imperfectly cleaned vessels. the excessive staleness of bread, and many other similar conditions in food, are largely due to the presence- of bacteria. So-called bloody stains on bread, meat, paste, etc., have also been traced to the presence of a brightly colored micro-or ganism. In the same way the bitterness and ropiness of bad wine is brought about.
The air of the country, but especially that of cities, contains large numbers of bacteria. M.
:Miguel, in his laboratory at INIontsouris. in Paris. constructed for the purpose of studying atmospheric germs, found that at ,Alontsouris there were on the average about SO bacteria to the cubic litre of air. The maximum abun dance was in the autumn, the minimum in winter. The number increased at night, and was reduced by a heavy rainfall. The direction of the wind made a great difference. Thus,when the wind blew from the mountains, very few bacteria were found in the air; while, when the wind came from the direction of the city, the number was greatly increased.
Water. even that which is usually considered pure, contains bacteria in abundance. This is true of spring-water and of water from artesian wells. Stagnant water, and the water of sewers, of course, contain bacteria in immense numbers. The sulphurous springs of the Pyrenees contain an abundant bacterium (Beggiatoa), which ac cumulates sulphur in its body, and is especially abundant in the scum that floats on the sur face. Some bacteria, called ehromogenie (color forming) on account of time bright pigment which they produce, sometimes ()emi• in water, and have given rise to superstitious accounts of bloody rain. The red color of stagnant pools in autumn has been known for many years to be due to the presence of a micro-organism described by Ehrenberg as 'Ophidomonas sanguinea,' but now known as a species of Spirillmn. Water serves as an excellent means for the transporta tion of bacteria, and many of the epidemics of infectious diseases are undoubtedly due to an infection of the water-supply.
As already noted, bacteria were at first placed in the animal kingdom. To Cohn (1353) is due the credit of first definitely proving, on the grounds of their structure and life-history, the fact that bacteria are plants. Niigeli (1857) confirmed the conclusions of Cohn, correcting certain details, and classifying bacteria not among the algae, but as a parallel class of fungi. (See SCIIIZOMYCETES.) Bacteria belong in that group of the vegetable kingdom whose mem bers live on dead or live animal or vegetable matter. They are subdivided into two groups, according as they live on dead animal or veget able matter, or on live animal or vegetable matter. The former are termed saprophytes, the latter parasites. The great importance of this distinction will appear when it is remem bered that when bacteria are living on, i.e. destroying, dead matter, they are doing good. They are decomposing dead animals and plants into their constituent elements, and returning them to the mineral kingdom again to furnish food for plant-life. In such way all decay and decomposition take place. But if they live on, i.e. consume, live animals or vegetables, they are doing harm in the sense of destroying life; and it is to this group that all harmful bacteria be long.