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 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 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 diam- 

 eter (No. 9, Fig. 116), the other 15 cm. long by 15 mm. 

 (Nos. 1 to 7, Fig. 116). 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. 

 114 and 115). 



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 ijlugs 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 pro- 

 duced are used in fermentation tubes (Smith tubes) of the 

 type shown in Fig. 117, so that the gas may be collected in 

 the closed arm of the tube, measured (Fig. 118) and tested 

 if desired. 



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