204 BA CTEEIOLOG Y. 



FILTRATION OF CULTURES. A variety of filters 

 have been devised for the purpose of filtering liquid 

 cultures and other fluids to obtain sterile filtrates. 

 These filters are usually constructed of un glazed porce- 

 lain or of infusorial earth, and are made in the form of 

 hollow cylinders or bulbs. The best-known forms of 

 bacterial filters are those of Chamberland and of Berke- 

 feld. All the filters used for this purpose require some 

 motive power to force the fluid through the filter. Com- 

 pressed air may be employed to force the fluid through 

 the filter, or atmospheric pressure may be utilized by 

 creating a negative pressure on the distal side of the 

 filter by the use of an air-pump. 



It is always necessary to test the sterility of the fil- 

 trate by making cultures from it into nutritive media 

 and noting whether growth takes place or not. 



CULTIVATION WITHOUT OXYGEN. As we have 

 already learned, there is a group of bacteria to which 

 the designation " anaerobic " has been given, which 

 are characterized by inability to grow in the presence 

 of free oxygen. For the cultivation of the members of 

 this group, a number of devices are employed for the 

 exclusion of free oxygen from the cultures. 



Method of Buchner. The plan suggested by Buchner, 

 of allowing the cultures to develop in an atmosphere 

 robbed of its oxygen by pyrogallic acid, gives very good 

 results. In this method the culture, which is either a 

 slant- or stab-culture in a test-tube, is placed tube, 

 cotton plug, and all into a larger tube, in the bottom 

 of which have been deposited 1 gramme of pyrogallic 

 acid and 10 c.c. of i normal caustic-potash solutions. 

 The larger tube is then tightly plugged with a rubber 



