9. Production of enzymes as illustrated in the above activities.
10. Appearance of growth on different culture media.
11. Production of free toxins as determined by injection of animals with broth cultures filtered free from bacteria.
12. Causation of disease as ascertained by the injection of animals with the bacteria themselves, and recovery of the organism from the animals.
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.