;U A ALAXrAL OF BACTEKlOLOaY 



Exercise 76. Presumptive Test for Bacillus coli 



The presumptive test is designed to give a fairly accurate 

 idea of the existence of pollution without the time-consuming 

 operation of isolating B. coli. Several such tests are in use. 

 Under diverse conditions diverse results are obtained. 



1. Lactose-bile bouillon. Add 1 per cent of lactose to fresh 

 ox bile, place in fermentation tubes, and sterilize in the Arnold 

 sterilizer. Inoculate with samples of suspected water or sewage 

 and incubate at 37.5C. for forty-eight to seventy-two hours. 

 Tubes showing 25 per cent or more of gas are regarded as positive. 



2. Dextrose-beef bouillon. Inoculate fermentation tubes of 

 sterile dextrose-beef broth with 1 cc. samples of the water to 

 be tested. Incubate at 37.5 0. Test for gas after forty-eight 

 to seventy-two hours. If the gas is approximately one third 

 carbon dioxide and two thirds hydrogen, the test is regarded as 

 positive. 



3. Litmus-lactose agar. Although less delicate than the fer- 

 mentation tests, this method is of considerable value. With 

 suitable dilutions pour Petri dishes, using this agar. Count the 

 colonies which redden the agar. A large number of acid-forming 

 colonies is regarded with suspicion. 



Exercise 77. Tests for Fecal Bacteria (Bacillus coli and Others) 



The tests here given are those commonly used. None of them 

 meets the approval of all bacteriologists. They are to be re- 

 garded as presumptive rather than as absolute. 



A. Lactose-bile bouillon. 



1. Inoculate six or more fermentation tubes of sterile lac- 

 tose bile Avith at least 1 cc. of the sample to be investi- 

 gated. Incubate at 37 0. 



2. Measure the amount of gas present in the closed arm 

 after twenty -four and after forty-eight hours. 



3. Make stains from each tube and use (Tram's stain to 

 determine the presence of streptococci. 



B. Lactose-beef bouillon. 



Repeat A, using lactose-beef bouillon. 



