In what has gone before it has been indicated that there is not as yet a complete and satisfactory classification of the bacteria. Sufficient is known, however, to enable us to identify many of the forms, although their generic relations may not be fully established. In order to determine the character and kind of a particular bacterium, it is necessary, not merely to prepare a specimen and observe it under the microscope, but also to discover its behaviour when grown as a pure culture. Hence a full description of each form should include a statement of its mode of growth when so cultivated, as well as of its form. In the case of pathogenic bacteria the results of cultivation must be tested, where possible, by experiments on animals, and the position of any pathogenic form can hardly be regarded as established unless according to Koch's postulates it has been (a) found in persons or animals affected by the disease, (b) cultivated on nutrient media through several generations, (c) inoculated in animals with the result of producing the original form of disease, and (d) cultivated outside the body from the affected animal.

In the following enumeration it is not intended to give a complete account of the bacteria. In regard to the saprophytes especially, we shall only refer to those which, from the frequency of their occurrence or from their association with man, are apparently of practical importance. In the nomenclature, the terms descriptive of the forms of the individual cells are used as generic names, while the specific designation refers to some obvious characteristic.

In the grouping of the pathogenic forms a pathological order has been followed, as being consistent with the general character of this work.