226 BACTERIOLOGY 



of ammonia in the distillate is shown by the yellow coloration 

 resulting from the addition of the reagent. 



The direct application of the reagent to the culture will 

 give satisfactory results if a great deal of ammonia has been 

 formed. In this instance the mercury in the reagent will 

 be precipitated as mercurous oxide. Another rough test for 

 the formation of ammonia is to place a strip of filter-paper 

 moistened with the Nessler reagent over the mouth of a 

 test-tube containing the culture, and then gently heating 

 the culture. As the ammonia is driven off by the heat, it 

 will react on the reagent on the strip of paper. 



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 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 filtrate 

 by making cultures from it into nutritive media and noting 

 whether growth takes place or not. 



