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.
Ordinarily, more attention must be paid to the sources of the two elements N and C than to the others, for in general the substances used to furnish these two and the water contain the other elements in sufficient amount. For very exact work on the products of bacteria, synthetic media containing definite amounts of chemicals of known composition have been prepared, but for most of the work with bacteria pathogenic to animals such media are not needed.