If the ring of vaseline is continuous, and the cover well pressed down, the drop is preserved from evaporation, and the bacteria may be examined in their natural condition--best on the edge of the drop.
For illuminating the drop culture, the concave mirror is used, and a small diaphragm without condenser; whereas, for stained preparations, the flat mirror is used in conjunction with the Abbe condenser.
If the cover-glass be carefully removed, and dried under a bell glass, the culture may be preserved in a dry condition, or may be stained and mounted.[74]
[74] The most recent method is to preserve the slides without cover-glasses: a drop of cedar oil is placed directly on the preparation, and, after examination, carefully washed off with xylol. Care should be taken not to touch the preparation with the fingers.
If the dried preparation is on a cover-glass, it should be held in the fingers (prepared side upwards), and passed slowly three times through the flame of a Bunsen burner. By holding the preparation in this way, the exact temperature for proper fixation is obtained.
A drop of fuchsin stain, or gentian violet, is allowed to remain on the preparation for five minutes; wash off the superfluous dye with water, and examine, either in the wet state or after drying and mounting in balsam.
[Illustration: Fig. 15.--B. Coli Commune. Stained to show Flagellae.]
For a detailed account of the technique of staining and mounting, the following works may be consulted:--
Methods and Formulæ. P. W. Squire. (Churchill.)
Taschenbuch für den bakteriologischen Praktikanten. Dr. Rudolf Abel. (Stubers-Verlag, Würzburg.)
Technique Microbiologique. Nicolle and Remlinger. (Octave Doin, Paris.)
Practical Bacteriology. Kanthack and Drysdale. (Macmillan, London.)
The recent researches of Tissier and Metchnikoff have shown that the flora of the intestines, both of men and animals, consist very largely of anaerobic bacteria. These have been overlooked in previous researches, owing to imperfect means of studying this class of organisms. Indeed, in one work on the microbes of the alimentary canal of the dog, no mention was made of them, whereas they are all very active.
Most of these organisms, and the new methods by which they have been isolated, are fully described in a new work entitled “Les Anærobies,” by M. Jungano and A. Distaso, of the Pasteur Institute, Paris.[75]