INTESTINAL OR COLON-TYPHOID GROUP 285 



of sewage or surface wash. This does not mean that every water 

 containing sewage is necessarily harmful, but that the presence 

 of the sewage is an indication of the possible and probable occa- 

 sional presence of pathogenic forms. 



Bacteriologic examination of water is important, for several 

 reasons. Much smaller quantities of contaminating organic mat- 

 ter may be determined by bacteriologic than by chemical means. 

 Its methods may be used in the determination of the potability 

 of water-supplies, in tracing a typhoid or similar epidemic to its 

 source, in determining the efficiency of filters for water-supplies 

 and of different types of sewage-disposal systems. 



Water may be examined bacteriologically, either quantita- 

 tively or qualitatively. In the former a determination of the total 

 number of bacteria present in the water is made; in the latter, the 

 tests are designed to determine the abundance of certain specific 

 bacteria. The qualitative examination may be again divided 

 into determination of normal sewage bacteria, particularly B. coli, 

 and of specific disease-producing bacteria, as B. typhosus. The 

 former is the most useful examination made in determining the 

 potability of a water; the latter is rarely used. 



Quantitative Examination of Water. In general, the greater 

 the quantity of organic and decomposing matter present in water, 

 the greater will be the number of bacteria present. However, 

 it must be noted that changes in the environment, such as tem- 

 perature, may cause great variations in bacterial content, even 

 though the original water be uncontaminated. For example, 

 water from a source quite above suspicion may have less than 

 100 bacteria, to the cubic centimeter. This same water, carefully 

 sampled in a sterile bottle and allowed to stand at room-tempera- 

 ture, may show in twenty-four to forty-eight hours hundreds of 

 thousands to the cubic centimeter. It is important, therefore, 

 that the sample taken for examination shall be typical, and that 

 it be examined immediately, to prevent multiplication of the 

 bacteria present. 



Media Used. Either nutrient gelatin or agar may be used. It 

 should be prepared according to the methods outlined by the 

 American Public Health Association. The gelatin will, in general, 

 give a somewhat higher count than the agar, but, when many 



