Fig. 119.—Culture tubes with media in them. × ⅔. 1 to 7 are the smaller tubes mentioned in the text; 9 the larger tube; 8 is extra large for potato tubes; 1, plain broth; 2, plain milk; 3, litmus milk; 4, gelatin for “stab” or “puncture” culture; 5, agar for “stab” or “puncture” culture; 6, agar for “slope” or “slant” culture; 7, blood serum; 8, potato tube; 9, agar for plating. Note the transparency of the broth and gelatin and the slight opalescence of the agar.
One method of using gelatin and also agar is as “puncture” or “stab” cultures. The tubes (the narrower tubes are to be preferred for most “stab” cultures) are filled one-third full of the medium while it is still fluid, plugged, sterilized and allowed to cool in the vertical position. The medium is then “inoculated” with a straight platinum needle by plunging this into the center of the surface down to the bottom of the tube ([Fig. 119], Nos. 4 and 5).
Fig. 120.—Fermentation tubes. 1, filled ready for use; 2, shows a cloudy growth and the development of gas in the closed arm.
Agar and blood serum are frequently used in the form of “slope” or “slant” cultures. That is, the medium solidifies with the tubes lying on their sides which gives a long, sloping surface on which the bacteria are inoculated ([Fig. 119], Nos. 6 and 7).
Fig. 121.—Method of estimating percentage of gas in a fermentation tube by means of the “gasometer”, the reading is 45 per cent.
Fig. 122.—A toxin flask showing a large surface growth.