226 ESSENTIALS OF BACTERIOLOGY 



Milk (litmus) coagulated in forty-eight hours and rendered 

 acid; litmus colored red. 



Peptone Solution. Production oj Indol. (A peptone solution 

 tube is inoculated with the culture and kept together with a 

 control four days at 37 C. Then 2 drops of concentrated 

 sulphuric acid and i centimeter of a o.oi per cent, solution 

 of sodium nitrite are added. The appearance of a pink color 

 at the end of thirty minutes denotes the presence of indol.) 



Presumptive Test. If a water from a well or spring produces 

 gas in the sugar broth and forms acid colonies on litmus- 

 lactose agar, the presumption is strong that there is sewage 

 contamination. If gas-production continues in a series of 

 samples carefully collected for several days or weeks, there can 

 be no doubt of a contamination, and especially if the well or 

 spring is protected from surface water. Algae which grow in 

 service pipes, reservoirs, and deep wells may give rise to 

 non-acid gas fermentation, but all well-water that, without 

 further testing, forms acid colonies on litmus-agar lactose 

 plates and ferments sugar broth, is open to suspicion, and if 

 there is evidence of the presence of typhoid fever or diarrheal 

 diseases, the water should be boiled and subjected to care- 

 ful analysis daily. There may be serious contamination 

 and the chemical tests show no appreciable increase in the 

 chlorids. 



Bacterial Treatment of Sewage. Where sewage is to be 

 rendered innocuous before being allowed to flow into streams, 

 the process of nature has been imitated by the construction of 

 septic tanks in which the sewage remains excluded from the 

 air and subject to the action of the anaerobic bacteria present 

 in the sewage. The organic nitrogen is reduced, and com- 

 pounds of hydrogen and sulphur are formed. The effluent 

 is then filtered through coke-beds, where the aerobic bacteria 

 assist in further purification. 



Sewage is also treated by sedimentation with alum and 

 filtration of the effluent over larger beds, or allowed to percolate 

 through the soil, which is thereby enriched and utilized for 

 agriculture. 



