172 METHODS OF USING CULTURE MEDIA 



used in the form of "plate cultures"" also. For this purpose 

 Petri dishes are first sterilized, then the melted agar 

 or gelatin poured in to 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 solidify- 

 ing 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. 117). 



For using large quantities of medium, Florence flasks, 

 Ehrlenmeyer flasks, special toxin flasks (Fig. 120) or various 

 other devices (Vaughan and Novy's "mass cultures," Figs. 

 121 and 122) have been employed. 



For growing anaerobic 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 aerobe in the same culture or in the same con- 

 tainer with the anaerobe, the aerobe exhausting the oxygen 

 so that the anaerobe 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 ha^e been freshly 

 sterilized just before use, or should be boiled in order to drive 

 out the dissolved oxygen. For most anaerobes the presence 

 in the medium of about 1 per cent, of a carbohydrate, as 

 dextrose, is advisable. 



A description of all the various de\'ices is unnecessary in 

 this work, but the following have answered most of the 

 purposes of general work in the author's laboratories. 



