CHANGES IN THE REACTION OF MEDIA 209 



Examination of Cultures for Bacterial Toxins. — In the sys- 

 tematic study of a pathogenic organism it is necessary to 

 know whether it is capable of producing a soluble toxin 

 when growing in culture-media. This is done by filtering 

 cultures of various ages and. testing the effect of the filtrate 

 upon susceptible animals. 



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 unglazed porcelain or of infusorial earth, and 

 are made in the form of hollow cylinders or bulbs. The best- 

 known forms of bacterial filters are the Chamberland and 

 the Berkefeld. All the filters used for this purpose require 

 some motive power to force the fluid through the filter. 

 Compressed air may be employed to force the fluid through 

 the filter, or atmospheric pressure may be utihzed 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 filtrate 

 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 "anae- 

 robic" 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 dcA'elop 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 — 

 14 



