no nROOMALL : 



water supply from a bacteriological standpoint may vary in 

 extent from the simple counting of colonies to the most 

 detailed determination of species. From the practical side 

 this latter is not necessary, as it is only bacteria indicative of 

 sewage pollution which are of importance. If this class of 

 bacteria is present the water must be viewed with suspicion, 

 for if it contains the ordinary intestinal l)acteria it may also 

 contain the typhoid bacillus if there happens to be a case of 

 that dread disease on the watershed. It has long been recog- 

 nized, although not without question, that the Bacillus coli 

 communis is a typical intestinal bacterium and that its pre- 

 scence indicates danger of sewage contamination. It is for- 

 tunately a bacterium easy to test for without much expertness 

 in bacteriological laboratory practice. 



From the practical sanitary and engineering standpoint 

 the bacteriological examination of a given water may be lim- 

 ited to two determinations — first, the counting of the total 

 number of bacteria irrespective of kind and, second, the 

 testing for the probable presence of the Bacillus coli commimis. 



The method of counting the number of bacteria is so 

 simple and so well known as to need little description. It 

 consists of inoculating plates of nutrient gelatine while soft 

 with measured amounts of the sample (or its dilution) and 

 then allowing the gelatine to harden and the bacteria to 

 develop into colonies visible to the naked eye. Each colony 

 which develops represents one bacterium in the original 

 sample. Of course, it is impossible to set minimum figures 

 for the number of bacteria and in good municipal water 

 supplies they may be found to vary through wide limits. It 

 is seen, therefore, that the simple counting of the bacteria 

 indicates little of itself. A very real value of the determina- 

 tion, however, is in ascertaining the bacterial efficiency of the 

 filters or other purification methods usually used in water 

 supply systems. Here simultaneous bacterial counts inside 

 and outside the filters give their efficiency in very decided 

 terms. Again, daily bacterial counts arc of great service in 



