BACTEEIA OF WATEE AXD SEWAGE 



53 



Exercise 75. Qualitative Study of Bacteria in Surface Waters 



1-2. Same as in the preceding exercise. 



3. Pour plates, using litmus-lactose agar, and incubate them 

 at 87° C. for tlnee dajs. 



4. ^lake differential counts of acid-forming colonies (recognized 

 by the reddening of the litmus) and the non-acid-f ormiag colonies. 



Fig. 28. Fermentation tube and gasometric chart for reading tlie quantity 



of gas evolved 



The student may copy the chart on cardboard for use as indicated 



The small, compact red colonies usually represent streptococci ; 

 verify by making a simple stain. Estimate the proportion of 

 streptococci to the total number of colonies. 



5. Examine gelatin plates made in Exercise 74, to determine 

 the number of liquefying and nonliquefying colonies. The latter 

 are assumed to come from decaying organic matter. 



