40 



Elements of Water Bacteriology. 



AVERAGE RELATIVE NUMBER OF BACTERIA ON PEPTON 



AGAR WITH DIFFERENT PEPTONS. 



(GAGE AND ADAMS, 1904.) 



Days . 



Merck's 

 Witte's 



2 



9 



38 



4 

 33 

 S3 



6 



SI 



8 



67 



100 



10 

 89 



12 

 98 



100 



The same authors showed that the composition of the 

 water used exercised a marked selective action upon the 

 development of bacteria. Agar made up with sewage 

 permitted a maximum growth of sewage bacteria and 

 showed no colonies when inoculated with filtered city 

 water. On the other hand agar made up with city 

 water showed loo per cent of the bacterid present in 

 city water and river water, three-quarters of those present 

 in sewage and less than half of those present in sewage 

 effluents. 



Hesse (1904) found that the number of bacteria 

 developing on Nahrstoff agar varied with the composition 

 of -the glass tubes in which the media had previously been 

 sterilized. The more soluble glasses yielded suflScient 

 alkali to the medium to inhibit four-fifths of the bacteria 

 present in certain cases. 



All these facts make it evident that only the strictest' 

 adherence to a standard method can ensure comparable 

 results; the ordinary nutrient gelatin should then in all 

 practical sanitary work be made up from distilled 

 water, meat infusion, pepton and gelatin, in exact ac- 



