34 WATER 



Qualitative Exam in at ion* 

 This may be divided into two parts. 



A, The detection and estimation of indicator organisms. 



B. The examination for specific disease-producing bacteria. 



A. The detection and estimation of indicator organisms. 



The most important part of the bacteriological examination 

 of water is the examination for and estimation of organisms 

 whose presence serves as an indication of undesirable con- 

 tamination, such as from excreta or sewage. The value and 

 significance of such indicators has been discussed in Chapter I. 



Examination for B. coli and allied organisms. 



The general principles relating to the isolation of these bacilli 

 together with a consideration of the most suitable methods have 

 been dealt with in Chapter II, so that only the actual procedures 

 recommended for water samples need to be described. The 

 following is recommended as convenient and satisfactory : 



Add 0*1 and roc.c. respectively to tubes of lactose bile salt 

 broth in double tubes. Add 10 c.c. to a similar tube, but con- 

 taining lactose bile salt broth of double strength. To the 

 remainder in the bottle, after all the different amounts of water 

 have been withdrawn for the different parts of the examination, 

 add the contents (about 10 c.c.) of a tube of four times strength 

 neutral red broth. Replace the glass stopper. Four times 

 strength bile salt broth may be used, and, if the examination 

 is for B. coli alone, is preferable, but by using neutral red broth 

 the mixture is also available for the examination for strepto- 

 cocci. 



If a 2-ounce sample is collected, the amount remaining in the 

 bottle will be about 30 c.c. If a large sample of water is collected, 

 then 50 c.c. should be added by sterile pipette to a tube of four 

 times strength neutral red broth large enough to hold the added 

 water. 



The tubes are labelled, incubated at 37 C, and examined 

 after twenty-four and after forty-eight hours. 



