36 BACTERIOLOGICAL METHODS 



tracts, aquae, etc., must be subjected to processes which will con- 

 centrate the organisms, as by passing the liquid through a filter 

 in which the pores are sufficiently small to leave the organisms 

 behind, as for example a Berkefeld or Chamberland clay tube. 

 In addition to the filter, the centrifuge will be found useful as will 

 be explained later. 



Any liquid containing not more than from 100 to 1,000,000 

 organisms per cc. does not lend itself to direct examination quanti- 

 tatively without concentration. The amount or volume of sub- 

 stance (liquid) to be passed through the filter will depend upon the 

 degree of concentration required. Since the Thoma-Zeiss 

 hemacytometer (with Turck ruling) is to be used in making the 

 counts, the organisms should average at least 4 to 5 in thej^^gg 

 c.mm. areas of the counting apparatus, or 1,000,000 to 1,250,000 

 organisms per cc. Let us suppose that a direct count is to be made 

 of a drinking water which is very pure, having not more than from 

 50 to 500 bacteria per cc. In order to make direct counting with 

 the hemacytometer possible, it would be necessary to pass from 

 20 to 30 liters of the water through the clay filter and thoroughly 

 mix the organisms left in the tube with 10 or even i cc. of 

 filtered sterile water. To filter that amount of water requires 

 too much time unless a large specially constructed apparatus is 

 installed. For practical purposes, i liter is the largest amount 

 of liquid that it will be necessary to filter and reduce to i cc, 

 making a concentration of 1000. For special purposes the i cc. 

 may be further concentrated in the centrifugal tube described in 

 Fig. 3. Weaker concentrates may answer the purpose in some 

 cases, as ten or one hundred, as in sewage, badly contaminated 

 milk and in other liquids in which the number of organisms 

 present may range from 100,000 to 1,000,000 per cc. 



The special centrifugal tube described in Fig. 3 is used as 

 follows : After passing a liter of the liquid to be examined through 

 the clay filter tube and thoroughly washing out the organisms and 

 other particles left in the tube, pour the contents into the special 



