The study of bacteria may be taken up for the disciplinary and pedagogic value of the study of a science; with the idea of extending the limits of knowledge; or for the purpose of learning their beneficial or injurious actions with the object of taking advantage of the former and combating or preventing the latter. Since bacteria are classed as plants, their successful study implies their cultivation on a suitable soil. A growth of bacteria is called a “_culture_” and the “soil” or material on which they are grown is called a “_culture medium_.” In so far as the culture medium is made up in the laboratory it is an “artificial culture medium” as distinguished from a natural medium. A culture consisting of one kind of bacteria only is spoken of as a “pure culture,” and accurate knowledge of bacteria depends on obtaining them in “pure culture.” After getting a “pure culture” the special characteristics of the organism must be ascertained in order to distinguish it from others. The discussion of the _morphology_ of bacteria in Chapters II, III, and IV shows that the morphological structures are too few to separate individual kinds. They serve at best to enable groups of similarly appearing forms to be arranged. Hence any further differentiation must be based on a study of the _physiology_ of the organism as discussed in the chapters on Physiological Activities of Bacteria. The thorough study of a bacterium involves, therefore: 1. Its isolation in pure culture. 2. Its study with the microscope to determine morphological features and staining reactions. 3. Growth on culture media for determining its physiological activities as well as morphological characteristics of the growths themselves. 4. Animal inoculations in certain instances. 5. Special serum reactions in some cases. Since isolation in pure culture requires material for growing the organism, the first subject to be considered is culture media. A culture medium for a given bacterium should show the following essentials: 1. It must contain all the elements necessary for the growth of the organism except those that may be obtained from the surrounding atmosphere. 2. These elements must be in a form available to the organism. 3. The medium must not be too dry, in order to furnish sufficient moisture for growth and to prevent too great a concentration of the different ingredients. 4. The reaction must be adjusted to suit the particular organism dealt with. 5. There must be no injurious substances present in concentration sufficient to inhibit the growth of the organism or to kill it.