436 AGEICULTUEAL AND INDUSTRIAL BACTERIOLOGY 



sewage bacteria. If gas is produced in any fermentation 

 tube it is highly probable that intestinal bacteria are pres- 

 ent, although this does not constitute proof. It is custom- 

 ary to inoculate certain differential media from test tubes 

 showing gas and by isolation of pure cultures from 

 separated colonies identify Bacterium coli. 



If Bacterium coli is shown to be present in quantities as 

 small as ten cubic centimeters of water, it is evident that 

 there has been more or less recent pollution by sewage. It 

 is possible that in some cases confusion will arise due to the 

 presence of organisms such as Bacterium a'erogenes not 

 uncommon in soils which simulate in many respects the 

 characteristics of Bacterium coli. Condemnation of surface 

 waters, particularly when only small numbers of gas-pro- 

 ducing bacteria are present, should be made with caution. 



It should be emphasized that water containing Bacterium 

 coli is not condemned for drinking purposes because this 

 organism is capable of causing disease when ingested, but 

 because its presence indicates the presence also of sewage 

 and opportunity for other microorganisms, such as the 

 typhoid bacillus, to gain access to the supply. 



Chemical Studies of Water to Determine Pollution. 

 Organic matter in sewage ordinarily will not become a 

 nuisance, that is, give off disagreeable odors, providing 

 there is sufficient aeration of the water in the stream. When 

 large quantities of organic matter are present, the dis- 

 solved oxygen is quickly used up and changes which take 

 place in the sewage, therefore, are necessarily anaerobic in 

 nature. Anaerobic composition or putrefaction is apt to 

 give rise to malodorous gases. The absence of oxygen, fur- 

 thermore, prevents the growth of most forms of animal life 

 in the streams. It is possible, therefore, by chemical analy- 

 sis and comparison of organic matter found by such analy- 

 sis with the amount of dissolved oxygen to determine some- 

 thing concerning the "putrescibility" of the water. In 



