26 Elements of Water Bacteriology^ 



by the ordinary procedure were only from 5 to 50 per cent 

 of those obtained by the use of Heyden's Nahrstoff agar. 

 For practical sanitary purposes, however, otu: methods are 

 fairly satisfactory. Within limits, it is of no great impor- 

 tance that one method allows the growth of more bacteria 

 than another. When we are using the quantitative analy- 

 sis as a measure of sewage pollution the essential thing is 

 that the section of the total bacterial flora which we obtain 

 should be thoroughly representative of that portion of it in 

 which we are most interested — the group of the quickly 

 growing, rich-food-loving sewage forms. In this respect 

 meat-gelatin-peptone appears to be unrivalled; and it is 

 in this respect that such media as Nahrstoff agar fail. 

 MiiUer (1900) showed that the larger counts obtained 

 by plating on the Nahrstoff medium are due to the fact 

 that it specially favors the more prototrophic forms, among 

 the water bacteria themselves. Intestinal organisms and 

 even the ordinary putrefactive germs, when plated in pure 

 culture, show no higher counts on Nahrstoff agar than on 

 gelatin. Gage and Adams (1904) found by plating pure 

 cultures of the common laboratory bacteria, saprophytes, 

 and parasites, that Nahrstoff counts were actually lower 

 than those obtained by the use of gelatin. When sewage 

 and highly polluted waters are examined, counts are 

 slightly higher on Nahrstoff media, yrhile witib purer 

 waters the Nahrstoff numbers are far in excess of those 

 obtained with gelatin. Winslow (1905) foimd the ratio 

 of Nahrstoff agar to gelatin count to be 1.7 to i.o for 



