350 



PUBLIC HEALTH BACTERIOLOGY 



AVERAGE NUMBER OF BACTERIA PER c.c. OF WATER SAMPLE 

 GROWING ON GELATIN AT 20 C. 



Dr. A. C. Houston (Director of Water Examination to 

 the Metropolitan Water Board, London), in addition 

 to gelatin and agar media, for enumeration, also uses 

 MacConkey's neutral-red, bile-salt, peptone, lactose agar 

 (called rebipelagar for brevity). This is similarly inocu- 

 lated and plated and incubated at 37. The colonies are 

 counted on gelatin on the third day, and on agar and rebi- 

 pelagar after twenty to twenty-four hours. 



The following table, culled from Dr. Houston's reports, 

 serves to illustrate the ratios of the various counts : 



RAW RIVER THAMES WATER. 



AVERAGE NUMBER OF MICROBES PER c.c. IN COMPARABLE SAMPLES, 

 TESTED ON THREE MEDIA ; WITH RATIOS. 



Gelatin at 2o-22 C. ; colonies counted on third day. 

 Agar and Rebipelagar at 37 C. ; counted after twenty to twenty- 

 four hours. 



This further table (curtailed) from Houston's Fifth 



