PREPARATION OF PLATE CULTURES 51 



I. Procedure for Agar Plates. The loop or straight- 

 needle dilution method is valuable as a quick method of 

 obtaining pure cultures when quantitative results are not 

 desired. 



1. Place the glass plate on the leveling stand. 



2. Place the spirit level on the glass plate and make 

 level by means of the leveling screws. 



Note. The plate-leveling stand facilitates the uniform distribution 

 of the medium over the bottom of the Petri dish, but is not necessary 

 for the accomplishment of favorable results. If the desk top is level 

 this apparatus is unnecessary. 



3. Place three sterile Petri dishes, labeled 1, 2 and 3, 

 in a row on the glass plate. 



4. Liquefy three tubes of agar at 100 C. in the water- 

 bath or steam and keep at a temperature of 40 to 

 45 C. 



5. Number the tubes of agar 1, 2 and 3 and flame the 

 plugs. 



6. With the sterilized platinum needle, merely touch 

 the culture and transfer to tube No. 1. 



Note. Hold cultures and plugs while transferring as in Fig. 20, 

 p. 59. 



7. Distribute the microorganisms through the medium 

 with the needle. 



8. Transfer one loopful from tube No. 1 to tube No. 2 

 and mix with the needle, as in 7. 



9. Slightly raise the cover of Petri dish No. 1. Intro- 

 duce the flamed mouth of tube No. 1 and pour the melted 

 agar into the plate; remove the mouth of the tube, and 

 replace the cover of the Petri dish. If the medium has 

 not entirely covered the bottom of the plate, tilt slightly 

 in different directions to distribute evenly. 



Note. Passing the Petri dish several times through the flame just 

 previous to pouring the plate will aid greatly in preventing the forma- 

 tion of condensation water on the cover. 



