[Illustration: FIG. 178. TUBE CONTAINING CULTURE OF BACILLI OF TUBERCULOSIS. TUBE CONTAINING CULTURE OF COMMA BACILLI OF CHOLERA.] For the study of bacteria they are propagated artificially in a test tube--_i. e._, a substance called a “culture” is prepared from some organic material which, like the substances of the human body, is favorable to their propagation. Such culture media are found in beef blood, gelatine, beef extracts, meat broth, milk, etc. An ordinary test-tube is supplied with some of the culture medium, and is then sterilized over the fire to destroy all interfering germs. Material infected with the microbe is then placed in the test-tube by a sterilized platinum wire and the tube closed by raw cotton. It is then placed in an incubator oven and is subjected to a gentle heat. In a little while the microbes begin to develop and increase, forming colonies, in which they swarm by the million, and present the clotted appearance seen in Fig. 178. The separation of different bacteria existing in the same material, so as to isolate each species and get what is called a “pure culture,” has been greatly promoted by Prof. Koch’s method of _plate culture_. In this the propagation of bacteria is effected upon a sterilized glass plate under a bell jar in such a thin layer as to facilitate the segregation of species, enabling them to be counted under the microscope and picked out and sown in another culture to get an unmixed crop of a definite species. Such a culture so multiplies the same microbe, to the exclusion of others, as to permit it to be easily identified and studied. According to the practice in modern municipal health regulations, the test as to when a child recovering from diphtheria is incapable of disseminating the disease is by test culture. A swab of cotton is rubbed against the interior walls of the child’s throat to secure the germs (if present), and the swab is then placed in a “culture” in a test-tube and the tube put in an incubator. If, after the period of incubation, no colonies of the germs develop, it is accepted as evidence that the diphtheria germs are no longer present in the throat, and the child is released from quarantine.