8 LABORATORY BACTERIOLOGY. 



BXEECISE II. 



QUANTITATIVE EXAMINATION OF SOILS. 



1. Fresh samples of soil taken with all precautions to 

 avoid contamination are to be used. 



2. Bring the bacteria into suspension by shaking 100 

 grams of each of the soils to be tested in 200 ce. of sterile 

 water for 5 minutes. 



3. Then, by means of sterile pipettes for each transfer, 

 make dilutions of the various infusions thus: 



1 cc. of the infusion into 99 cc. sterile water (a). 



10 cc. of (a) into 90 ce. sterile water (b). 



10 cc. of (b) into 90 cc. sterile water (c). 



10 cc. of (c) into 90 cc. sterile water (d). 



4. Transfer three 1 cc. portions of each (b), (e) and (d) 

 dilution to sterile Petri dishes. 



5. To each of one set of plates add 10 cc. of bouillon agar, 

 liquefied, and cooled to 40° C; to the second set, add 10 cc. 

 of synthetic agar similarly liquefied and cooled; and to the 

 third set,-add 10 cc. of gelatin, liquefied and cooled to 30° C. 



6. Mix thoroughly by tipping the plates back and forth 

 and spread evenly over the dishes. 



7. Incubate for 3^ days at 20° C. 



8. Count by means of a Jeffer's plate with the aid of a 

 hand lens. 



9. Prom the figures obtained calculate the number of 

 bacteria per gram of soil. 



