146 LABORATORY GUIDE IN BACTERIOLOGY 

 EXERCISE 2. EXAMINATION OF SURFACE WATERS 



1. Secure three samples of surface waters from dif- 

 ferent sources. 



2. Shake the samples and prepare dilutions. 



Dilution i. 1:10; remove 10 c.c. from a dilution flask con- 

 taining loo c.c. sterile water, and replace these 10 c.c. by 10 c.c. 

 of the sample. 



Dilution 2. 1:100; add i c.c. of the sample to a dilution 

 flask. 



Dilution 3. i : 1,000; add i c.c. of the dilution i : 10 to another 

 dilution flask. 



3. Melt a number of dextrose or lactose agar tubes 

 in a water bath and cool to 43 C. 



4. Place i c.c. of sterile litmus solution on each of 

 12 petri dishes. 



5. Place i c.c. of the sample and i c.c. of each 

 dilution on the same petri dishes. 



6. Pour the contents of a tube of the liquefied agai 

 on each of the petri dishes and mix. 



7. After the agar has solidified incubate at 37 C. 



NOTE. If working in pairs, it will be instructive to have one 

 student incubate the agar plates at 37 C., and the other student 

 at room temperature. The period of incubation at 37 C. is 48 

 hours, at room temperature 72 hours. It is also instructive to 

 plate the same samples and dilutions in gelatin, incubating these 

 at 20 C. for 48 hours, and comparing the number of colonies 

 with those appearing on agar. 



8. After the plates have been removed from the 

 incubator count the colonies, using a colony counter. 

 Make differential counts of acid-forming colonies, 

 recognized by the reddening of the litmus, and non- 

 acid-forming colonies. 



