68 PHARMACEUTICAL BACTERIOLOGY. 



If the temperature and color differential tests indicate sewage contamina- 

 tion, then the following additional tests should be carried out. 



III. Indol Reaction and Gas Formula. — The indol reaction has already been 

 explained. The gas formula is determined as follows: To sets of four grad- 

 uated fermentation tubes containing glucose bouillon and lactose bouillon, 

 add O.I, 0.2, 0.5, and 10 c.c. of the suspected liquid. If gas formation 

 is observed the presence of colon bacilli may be suspected. If the o.i c.c. 

 tubes show gas formation then the presence of colon bacilli may be assumed. 

 Fill the bulb of a tube, showing gas formation, with a 2-per cent, solution of 

 sodic hydrate, hold thumb tightly over the opening and mix contents by 

 tilting back and forth carefully. The portion of gas absorbed is CO2 

 whereas the unabsorbed portion is supposedly hydrogen. The colon bacillus 

 shows a gas formation of 1/3 hydrogen. Of course the total volume of gas is 

 recorded before the sodic hydrate is added. 



1,^ ^^-^ I 





<=> o 



Fig. 40.— Colon bacillus. This microbe is quite large, in the comparative sense, and 

 is morphologically typical of the group bacillus. The flagellae are few in number and 

 comparatively long. • 



The gas formula with a positive indol reaction is practically conclusive 

 as far as the presence of the colon bacillus is concerned. Add to this the 

 other tests and we have presumptive evidence of sewage contamination, and 

 any article of food or drink showing such contamination is unfit for human 

 consumption. 



The colon bacillus, the bacilli of the hog cholera group and others 

 have the power of reducing neutral red; producing a greenish-yellow 

 fluorescence. For this reaction use glucose bouillon to which has been added 

 I per cent, of a 0.5 per cent, solution of neutral red. In examining milk, the 

 pus cell and leucocyte count is considered important; centrifugalize 10 c.c. 

 of milk for thirty minutes, pour off supernatant milk and mix residue with 



