A criticism of this medium is that the elements K, Ca, Mg, Fe, Mn, and S which have been shown to be essential are not present if chemically pure salts are used in the preparation.
CHAPTER XVII.
METHODS OF USING CULTURE MEDIA.
The way in which culture media shall be used depends on the purpose in view. By far the larger part of bacteriological work is done with cultures in “bacteriological culture tubes.” Various laboratories have their own special types but all are more or less after the “Board of Health” form. They differ from ordinary chemical test-tubes in that they are usually longer, have no “lip” and have much thicker walls to prevent breakage and consequent loss of the culture as well as danger from pathogenic organisms. The author finds two sets of tubes most serviceable for student use—one size 15 cm. long by 19 mm. outside diameter (No. 9, [Fig. 119]), the other 15 cm. long by 13 mm. (Nos. 1 to 7, [Fig. 119]). Culture tubes are conveniently used in “wire baskets” circular or square in section and of a size to correspond with the length and number of tubes used. These baskets are light, do not break, and if made of good galvanized wire netting do not readily rust ([Figs. 117] and [118]).
Liquid media such as broth, milk, litmus milk, indol and nitrate broths are used in the above-mentioned tubes when small quantities only are to be worked with. The tubes are filled approximately one-third full, then plugged with non-absorbent cotton and sterilized. Cotton plugs are used so much in bacteriological work because they permit a free circulation of air and gases and at the same time act as filters to keep out the bacteria of the air.
Sugar broths or other media in which gas may be produced are used in fermentation tubes (Smith tubes) of the type shown in [Fig. 120] so that the gas may be collected in the closed arm of the tube, measured ([Fig. 121]) and tested if desired.
Fig. 117.—Round wire basket.
Fig. 118.—Square wire basket.