78 BACTERIOLOGY FOR NURSES 



forty-eight hours those tubes showing a color change and the pro- 

 duction of gas are presumed to contain the colon bacillus. If, 

 for example, gas appears in the tubes containing 10 c.c. and 1 c.c. 

 of water and not in the tube containing 0.1 c.c. it is assumed that 

 the water contained at least one colon bacillus per c.c. The in- 

 terpretation of the results by the plate method is evident. The 

 number of red colonies developing on the plate containing 1 c.c. 

 of water would give the number of B. coli present per c.c. ; if 

 these should be too numerous to count or if the color of the entire 

 medium be changed, then the colonies on the plate containing 0.1 

 c.c. of water should be counted and the number multiplied by ten 

 to give the total number per c.c. 



It should be remembered that the above are only presumptive 

 or partial tests for the colon bacillus, and although fairly reliable 

 and of value for routine examination other tests are necessary 

 before an organism can with certainty be said to be the colon 

 bacillus. 



Determination Test. In order to determine beyond doubt 

 that B. coli has caused the fermentation of lactose in the tubes 

 a loopful of the culture is streaked on plates containing solidified 

 Conradi medium, and after twenty-four hours' incubation a typical 

 red colony is fished and incubated in a tube of broth. At the 

 end of from twelve to twenty-four hours' incubation about 0.1 c.c. 

 of the broth culture is pipetted into the following media, each 

 of which gives a characteristic reaction when used for the culti- 

 vation of the colon bacillus: gelatin, absence of liquefaction; 

 neutral red lactose peptone water, production of acid and gas ; milk, 

 production of acid and coagulation of the protein ; peptone solu- 

 tion, production of indol. 



Sewage Streptococci. In addition to the many different 

 bacilli normally present in the intestines there are also certain 

 cocci often spoken of as sewage streptococci. They, too, produce 

 pink colonies on Conradi medium. They are much smaller, how- 

 ever, than those of the colon bacillus and can easily be differenti- 

 ated. They are not hardy and quickly die in water ; thus their 

 presence represents recent pollution. 



