13. Formation of specific antibodies as determined by the proper injection of animals with the organism or its products and the subsequent testing of the blood serum of the inoculated animals. For special kinds of bacteria other activities must be determined (oxidation, nitrate and nitrite formation, action of sulphur and iron bacteria, etc.). The first nine activities are determined by inoculating the different culture media already described and observing the phenomena indicated, making chemical tests where necessary. APPEARANCE OF GROWTH ON DIFFERENT CULTURE MEDIA. In addition to those changes that are associated with the manifestation of different physiological activities, many bacteria, show characteristic appearances on the various culture media which are of value in their identification. Too much stress should not be laid on these appearances alone, however, since slight variations, particularly in solid media due especially to the age of the medium, may change decidedly the appearance of a colony. This is true of variations in the amount of moisture on agar plates. Colonies which are ordinarily round and regular may assume very diverse shapes, if there chance to be an excess of moisture on the surface. Also in slope and puncture cultures on the various solid media much variation results from the amount of material on the inoculation needle and just how the puncture is made, or the needle drawn over the slope. These variations are largely prevented by the use of standard media and by inoculating by standard methods. The Laboratory Committee of the American Public Health Association has proposed standard methods for all culture media and tests and for methods of inoculation, and these have been generally adopted in this country for comparative work. Likewise the Society of American Bacteriologists has at different times (1904, 1914, 1917) adopted “descriptive charts” for detailing all the characteristics of a given organism. A committee is at present working on a revision of the 1917 chart to be presented at the 1920 meeting. One of the earlier charts which includes a glossary of descriptive terms is inserted in this chapter. Among the cultural appearances the following are of most importance: [Illustration: FIG. 144.--Broth cultures × 2/3. _1_ uninoculated transparent broth; _2_, broth cloudy from growth of organisms; _3_, broth slightly cloudy with a deposit in bottom; _4_, broth slightly cloudy with a heavy membrane at the surface.] [Illustration: FIG. 145.--A filiform stab or puncture culture. × 3/5.] [Illustration: FIG. 146.--A beaded stab or puncture culture. × 1/2.] [Illustration: FIG. 147.--A villous stab or puncture culture. × 1/2.]