Fig. 116.—A potato plate. × ½
It is a good plan to wrap each dish separately in paper to retain the lid securely, then sterilize as for potato tubes, and leave plates wrapped until wanted.
It sometimes happens that the natural acidity of potatoes is too great for the growth of many organisms. The acidity is sufficiently corrected by soaking the pieces of potato in a 1 per cent. solution of sodium carbonate for an hour before they are put into the tubes or plates.
Glycerinized potato tubes are conveniently prepared by covering the potato in the tube with glycerin broth, sterilizing and pouring off the excess broth immediately after sterilizing, taking care that the tubes do not become contaminated which is not very probable if the work is quickly done while the tubes are still hot.
Blood Serum Media.—Blood serum, usually from the larger, domestic animals on account of convenience in securing it in quantity, is used in the study of the bacteria causing disease in man and animals. Most commonly the serum is collected from the clotted blood after it has well separated (usually about forty-eight hours is required for this). It is then run into tubes which are plugged with cotton and placed in an apparatus for coagulating the serum by heat. A copper water bath with a tightly closed air compartment or the horizontal autoclave ([Fig. 81]) is sufficient for this purpose, though special forms of apparatus are to be had. It is important that the temperature be raised slowly so that the blood gases escape gradually. Three to five hours or longer should be allowed for the temperature to reach the boiling-point. If the tubes are heated too rapidly, the serum is filled with bubbles and badly torn since the gases are driven off suddenly. Löffler’s serum is made by adding one part of dextrose broth to three parts of serum and then coagulating as above. The solidified serum in either case is best sterilized discontinuously, though with care the autoclave at 15 pounds pressure may be used for a single sterilization. This is very apt to cause a greater darkening of the serum and frequently also a laceration of the solid mass by escaping gases.
Blood serum is also used in the liquid state. For this purpose it is best to collect it aseptically; or it may be sterilized discontinuously at a temperature of 55° or 56° on seven to ten consecutive days. Novy has recently suggested dialyzing the serum to free it from salts and thus prevent its coagulation when heated. Whether the removal of the various “extractives” which diffuse out with the salts deprives the serum of any of its advantageous properties remains to be ascertained.
From the discussion of the physiological activities of bacteria in [Chapters IX–XII] it is apparent that a very great variety of culture media other than those described is necessary for the study of special types of bacteria, but such media are beyond the scope of the present work.
The ideal culture media are without a doubt the synthetic media, that is media of definite known chemical composition, so that the various changes due to the growth of bacteria can be accurately determined and thus a means of sharply differentiating closely related organisms be secured. Such media have been prepared and every bacteriologist believes strongly in their future usefulness when media of wider application shall have been devised. An example of this type of culture media is Uschinsky’s synthetic medium, of which the following is one of the modifications:
| Distilled-water | 1000 parts |
| Asparagin | 4 parts |
| Ammonium lactate | 6 parts |
| Disodium phosphate | 2 parts |
| Sodium chloride | 5 parts |