CHAPTER XVII. 

 METHODS OF USING Cl'LTURE MEDIA. 



The way in which culture media shall be used depends 

 on the purpose in view. By far the larger part of bacterio- 

 logical work is done with cultures in "bacteriological cul- 

 ture 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 cul- 

 ture 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. 117), the other 15 cm. long by 13 mm. (Nos. 

 1 to 7, Fig. 117). 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. 115 and 116). 



Liquid media such as broth, mUk, 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 tN-pe 

 shown in Fig.. 118 so that the gas may be collected in tlie 

 closed arm of the tube, measured (Fig. 119) and tested if 

 desired. 



