146 LABORATORY GUIDE IN BACTEKIOLOGY 



EXERCISK 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. i:io; remove lo c.c. from a dilution flask con- 

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

 of the sample. 



Dilution 2. i;ioo; add i c.c. of the sample to a dilution 

 flask. 



Dilution 3. 1:1 ,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 agar 

 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 colonics 

 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. 



