ISOLATION OF INDICATOR ORGANISMS 2$ 



funnel, which is connected with the filter candle by means of 

 a perforated rubber stopper. The filter flask is connected to 

 an exhaust-pump. In this arrangement the filtration is from 

 within out, so that all the bacteria are deposited upon the 

 inside of the candle. After filtration sterile water is added, 

 and an emulsion made. The porcelain filter tube should hold 

 rather over 10 c.c. ot water. 



The apparatus must be sterilized in steam for several hours 

 before use. 



The actual amounts to select for examination will vary 

 somewhat with the nature of the material being examined. 

 For sewage or milk, for example, only small amounts are 

 required, as these spores are likely to be abundant in such 

 material, while for water 100 or 1000 c.c. are often suitable 

 amounts to examine. As a rule the amounts taken are not 

 nearer spaced than -^, i, 10, ico, 1000 c.c. etc. This wide 

 spacing, while sufficient for some substances, such as water, is 

 not satisfactory when the spores are likely to be fairly numerous. 

 An alternative and more satisfactory procedure is given under 

 milk (Chapter VI). 



Methods of dilution and of recording results. 



As has already been pointed out, numerical estimations, 

 not mere detection, are essential for the indicator organisms 

 discussed in this chapter. Using the methods recommended, 

 this involves the examination of a series of carefully measured 

 quantities of the substance. Considerable attention to detail is 

 required if reliable and comparable results are to be obtained. 

 When solid substances, for example soil or the meat foods, 

 have to be examined, accurate enumerations of the number of 

 bacteria present or of the number of any special organism are 

 very difficult. The most satisfactory procedure devised up to 

 the present is to add definite weights of the substance to bottles 

 or flasks containing definite amounts of sterile water. As a 

 rule this can most conveniently be done by cautious addition 

 of the soil or other substance to the diluting flask, in equi- 

 librium upon a balance, until the addition of the quantity 

 required is registered. The diluting vessels should have glass 



