Potato tubes are likewise used for “slant” or “slope” cultures ([Fig. 119], No. 8). Potatoes as “plate cultures” have been referred to. Agar and gelatin are very largely used in the form of “plate cultures” also. For this purpose Petri dishes are first sterilized, then the melted agar or gelatin poured into them and allowed to “set” while the plates are kept horizontal. The melted media may be “inoculated” before they are poured, or a portion of the material to be “plated” may be placed in the dish, then the melted medium poured in and distributed over the dish by tilting in various directions, or the medium after solidifying may be inoculated by “strokes” or “streaks” over its surface, according to the purpose in view in using the plate. The larger sized tubes should be used for making plates in order to have sufficient medium in the plate (No. 9, [Fig. 119]).
For using large quantities of medium, Florence flasks, Ehrlenmeyer flasks, special toxin flasks ([Fig. 122]) or various other devices (Vaughan and Novy’s “mass cultures,” [Figs. 123] and [124]) have been employed.
For growing anaërobic organisms it is evident that some method for removing and excluding the oxygen of the air must be used. A very great variety of appliances have been devised for these purposes. Some are based on the principle of the vacuum, exhausting the air with an air pump; some on replacing the air with a stream of hydrogen; others on absorbing the oxygen by chemical means, as with an alkaline solution of pyrogallic acid, or even by growing a vigorous aërobe in the same culture or in the same container with the anaërobe, the aërobe exhausting the oxygen so that the anaërobe then develops, or finally by excluding the air through the use of deep culture tubes well filled with the medium, or in the closed arm of fermentation tubes. For many purposes a combination of two or more of the above methods gives good results.
In any event the culture medium should have been freshly sterilized just before use, or should be boiled in order to drive out the dissolved oxygen. For most, anaërobes the presence in the medium of about 1 per cent. of a carbohydrate, as dextrose, is advisable.
A description of all the various devices is unnecessary in this work, but the following have answered most of the purposes of general work in the author’s laboratories.
Fig. 123.—Tank with raised lids. (Vaughan.)
Fig. 124.—Tank with lids lowered. (Vaughan.)
Figs. 123 and 124.—Vaughan and Novy’s mass culture apparatus.