54 A MANUAL OF BACTERIOLOGY 



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 37.5° C. for forty-eight to seventy-two hours. 

 Tubes showiag 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° C. 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 with at least 1 cc. of the sample to be investi- 

 gated. Incubate at 37° C. 



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 Gram's stain to 

 determine the presence of streptococci. 



B. Lactose-beef bouillon. 



Repeat A, using lactose-beef bouillon. 



