100 LABORATORY MANUAL OF MICROBIOLOGY 



well-cultivated soil rich in organic matter requires a higher 

 dilution than poor, sandy soil. 



According to the total number of bacteria pour plates from 

 the dilutions 1:10,000, 1:100,000 and 1:1,000,000. In general 

 dilutions of about 1:100,000 are satisfactory. About 5 to 10 

 parallel plates for each dilution should be poured. 



Add to each plate about 10 cubic centimeters of the agar 

 medium (Mediums 4 or 5), melted and cooled to about 45°C. 

 A blank plate or control should be poured with each series. In 

 case the medium is turbid, heat slowly, allowing the deposit to 



Fig. 5. — Colonies of bacteria and fungi on Petri dish. (Lohnis and Fred.) 



settle. Use only the clear portion of the medium for pouring 

 plates. 



Immediately after adding the culture medium, rotate each 

 plate to secure a uniform mixture. Allow agar plates to harden 

 on a level surface for at least 30 minutes. 



Agar plates should be inverted and incubated under a moist 

 chamber at 28°C. The time of incubation will depend upon the 

 microorganisms and the medium. After 5 to 10 days count the 

 number of colonies on each plate. If the colonies are not too 

 thick, it is well to dot each one with a pen and ink. When the 

 colonies are too thick to count easily, use a hand lens and count- 

 ing plate. 



Reduce all results to number of bacteria in 1 gram of dry soil. 



Exercise 7 

 Number of An^robic Bacteria in the Soil (Optional) 



Prepare a series of dilutions of soil in sterile tap water as 

 previously outhned. Dilutions equal to 1:1,000 and 1:10,000. 



