6 MASS. EXPERIMENT STATION BULLETN 414 



cal examination of water supplies is designed to detect the presence of bacteria 

 of the colon bacillus {Escherichia coli) group, which has been accepted for many 

 years as an indicator of sewage pollution. The colon bacillus is normally present 

 in large numbers in the large intestines (colons) of human beings and warm- 

 blooded animals, and derives its name from that fact. The bacillus is passed in 

 fecal evacuations and so finds its way into sewage. The colon bacillus itself is not 

 considered harmful when taken internally. It is just an indicator of possible harm 

 from other sources. 



Even if a laboratory examination reveals the presence of colon bacilli, it does 

 not necessarily follow that the water is dangerous to health. There have been 

 few cases of typhoid fever or bacillary dysentery reported in New England in 

 recent years, and germs of these diseases get into water only from infected people. 

 It is wise, however, to err on the side of safety; so a water that yields an unsatis- 

 factory bacteriological test should be regarded as possibly dangerous. Even 

 though water polluted by sewage may not necessarily be dangerous, it should be 

 considered dirty water. No one would want to eat food or drink milk that had 

 been polluted by sewage, and one should be equallj' particular about the water 

 he drinks. Water may be so clear as to be sparkling, and have no taste or odor, 

 but it still may be polluted sufficiently to be unclean or even dangerous. 



Method of Examination 



The bacteriological examination of water depends upon the fact that the colon- 

 bacillus group of bacteria, known to bacteriologists as the coliform group, will 

 ferment lactose (milk sugar) and produce gas. A fluid culture medium is made 

 up of water, beef extract, peptone (a product from digested protein), and a small 

 amount of lactose. This medium is put into test tubes, which are filled about 

 one-quarter full, and smaller test tubes or vials are inverted in the larger ones. 

 The large tubes are plugged with cotton and the medium is sterilized under steam 

 pressure in an autoclave, which is essentially an elaborated pressure cooker. 

 When the medium has been sterilized and then cooled, it is ready for use. 



Water samples should be collected in sterilized bottles. In the laboratory, 

 bottles are sterilized with steam under pressure. If samples are brought into the 

 laboratory in containers prepared in the home, such containers should be held 

 in actively boiling water for at least ten minutes before being used, and should 

 then be carefully stoppered with stoppers or covers that have also been boiled. 

 When samples are taken, care should be used to avoid touching either the mouth 

 of the bottle or the stopper with the fingers or any other object that might cause 

 contamination. Water standing in pipes or pumps should be drained ofif so that 

 samples taken are representative of the source of supply. 



When the samples reach the laboratory, portions of the water are transferred 

 to the test tubes of culture media by means of sterile glass pipettes. The tubes 

 are incubated at 37°C. (body temperature) and examined at the end of 24 hours 

 to see if the bacteria present have produced gas. If gas has formed, it will be 

 trapped in the small inverted vials and will displace the fluid in them. If no gas 

 is observed at the first examination, the tubes are incubated another 24 hours. 

 If no gas appears in any of the tubes by this time, the water is considered satis- 

 factory and is so reported. If, however, gas appears in any of the inverted tubes, 

 contamination is indicated. 



Some estimate of the degree of contamination can be made by considering the 

 percentage of the inoculated tubes that show gas and the quantities of water 

 that have been put into them. For instance, the usual procedure in testing a 



