JOHN F. NORTON 363 



greater portion of recently published material in this field is concerned directly or in- 

 directly with methods of bacterial water examination or the interpretation of results. 

 Other problems are concerned with the chlorination of water supplies as affecting not 

 only the fate of the typhoid bacillus but of that of other possible water-borne patho- 

 gens such as Clostridium welchii; with the survival of organisms of the typhoid-para- 

 typhoid group of bacteria in sewage treatment by means of septic or Imhoff tanks, 

 trickling filters, activated sludge tanks, or chlorination of sewage efifluents; or with the 

 very important questions connected with ground-water pollution. I shall attempt to 

 present the present status of these various problems with no pretense toward citing all 

 of the pertinent literature which has accumulated. 



BACTERIAL COUNTS 



Much of the earlier work in water bacteriology was concerned with the total num- 

 ber of bacteria which could be recovered from a water by means of a solid nutrient 

 medium under aerobic conditions and at various temperatures (usually 20° or 37° C). 

 At the present time such counts are made in the majority of water laboratories as a 

 matter of routine. Their value appears to be largely in connection with the control of 

 water filtration — as an index of the extent of and variation in the contamination of a 

 raw water and as a measure of the efficiency of a filter. It is universally recognized 

 that these counts do not represent the actual total number of bacteria present. The 

 anaerobic organisms are to a large extent excluded, the colonies obtained on the plates 

 do not necessarily arise from a single cell, and many aerobic bacteria do not grow on 

 the media commonly used. These errors, however, do not detract from the value of 

 the procedure when used under suitable conditions and for suitable purposes. Where 

 comparative results are desired the method will continue to be used. As an index of 

 the purity of a water supply the total count, even at 37° C, cannot be given serious 

 consideration. It is not included in the standards recommended by an Advisory Com- 

 mittee of the United States Public Health Service.' 



The chief problem connected with bacterial counts is that of devising a medium 

 which will give uniform results and can be readily and accurately duplicated. Infor- 

 mation on the relation of the chemical composition of meat extracts and peptones to 

 bacterial growth is meager. This is a fundamental problem in the whole field of bac- 

 terial nutrition. The water bacteriologist would be satisfied with something less than 

 a final solution. If he could have at his command relatively simple tests which would 

 insure uniformity in the composition of basic nutrient materials, he would be con- 

 tented. This problem is well recognized and is being attacked. A synthetic medium 

 prepared from pure chemical substances offers another solution. So far, no such me- 

 dium has been found generally useful. 



BACTERIAL SPECIES 



Very few comprehensive investigations have been made of the various species of 

 bacteria found in lakes, streams, and ground waters. Probably the most complete 

 study was that made by Jordan^ in connection with his investigations on the Illinois 



' Ibid. 



^ Jordan, E. O.: /. Hyg., 3, i. 1903. 



