38 BACTERIOLOGICAL METHODS 



centrifugal tube. For washing use about lo cc. of filtered sterile 

 water, adding up to the lo cc. mark if necessary. Centrifugalize 

 at high speed for 30 min., which will throw the bacteria and other 

 solids down into the narrow i cc. end of the tube. 



The following is a brief outline of the method of procedure: 

 Use a Kitasato filter with the usual hydrant suction pump attach- 

 ment. Pass a Hter of the hquid to be examined through the 

 filter, continuing suction until nearly all of the liquid has passed 

 through. Remove the clay bougie and wash down the organ- 

 isms clinging to the sides of the tube with not more than 10 cc. 

 of distilled water which has been filtered and boiled. Place thumb 

 over the opening of the tube and mix the contents thoroughly by 

 shaking for 20 sec, then pour the thoroughly mixed contents 

 into a sterile cylindrical graduate and add sterile distilled and 

 filtered water up to the 10 cc. mark, shake thoroughly and make 

 the counts at once by means of the hemacytometer. This pro- 

 cedure gives a concentration of 100. By means of the special 

 centrifugal tube the concentration may be increased to 1000, 

 as already explained. The method gives approximate results 

 only, the counts as a rule being less than the actual number of 

 organisms present in the liquid, a dift'erence due to three chief 

 sources of error: First, a small number of bacteria (especially 

 the smaller motile forms) will pass through the clay filtering tube; 

 second, some of the smaller bacteria are caught and held in the 

 pores of the clay tube; and third, some organisms will remain 

 cHnging to the inner surface of the tube after the mixed contents 

 are poured out for counting purposes. These sources of error 

 are, however, not great, perhaps not exceeding 8 to 10 per cent., 

 and are on the side of conservative estimates. The clay bougies 

 used should be of the finest quality and should be of uniform and 

 standard thickness. The sources of error by the direct method 

 are perhaps not as great, certainly not greater, than by the usual 

 plating methods and offer some very decided advantages. The 

 concentrates show, in addition to the bacteria, other organisms 



