THEMA..
Bacteria may be grown artificially on various substances which furnish them suitable food. These are known as culture media. Among the more common of these are bouillon, milk, and potato. In order to make a medium which is solid at ordinary temperatures, but which may be liquefied by slight heating, gelatin or agar is often added to the bouillon. Thus, if a drop of fluid containing bacteria which we wish to study he added to warm melted gelatin or agar bouillon, the latter may be poured out in a thin layer on a cold plate, where it immediately hardens. thus fixing the bacteria and preventing their moving about in the medium. If the dilu tion be sufficient to separate the bacteria well, as growth occurs, minute specks are seen dotting the surface of the plate, and each one of these specks marks the reproduction area of a single bacterium. It is called a colony, and each such colony consists of a single species of bacteria. By taking a bit from one of these spots or colonies and transferring it to some other sterile medium, there results a growth which is free from all life other than the single species of bacteria. This is known as a 'pure culture.' Many different species of bacteria so closely resemble one another in appearance that they cannot be differentiated by the micro scope. Each species has, however, certain peculiarities of growth and development. Thus recourse is often had to cultivation upon dif ferent media, where their life-histories may be studied and the identity of a particular species determined. For the very careful study of bac
teria, ninny different modifications of the culture media have been devised. The medium is some times colored by litmus, to show any reaction the growth of the bacteria may produce. Sugar is sometimes added to the medium, which is then placed in a crooked tube closed at one end. so that if the species causes fermentation it may be noted by the collecting of gas at the closed end of the tube. All media are sterilized before using, to destroy any bacteria that may be present. After inoculation the media are kept at room temperature, or more often are placed in an incubator at about blood-heat.
For microscopical study, bacteria may be simply placed under a thin glass cover upon a glass slide. They may be studied unstained, but are more often stained in various ways. to render them more distinct or to bring out certain peculiarities. Some of the bacteria, e.g. the tubercle bacillus (see TUBERCULOSIS ) , depend for their recognition very largely upon their specific reaction to certain stains, and can be differentiated only in this way. They are usually studied with an oil-immersion lens of not less than 1000 diameter magnification. For history and bibliography, see DISEASE, GERA' THEORY or.